Agronomic Potential and Limitations of Factory-Derived Tea Waste in Kale Cultivation Under Drought Stress

Abstract

Plant-derived wastes are increasingly explored as organic matter sources for sustainable agriculture. Tea waste, a by-product of industrial tea processing, is often regarded as an environmental pollutant, yet its potential for agricultural use remains conditional and requires careful evaluation. This study examined the effects of factory-derived tea waste on kale (Brassica oleracea var. acephala) under drought stress. Plants were grown in soils amended with 5% or 10% tea waste and subjected to mild (75% field capacity) and moderate (50% field capacity) water deficits, compared with full irrigation (100% field capacity). Fifteen morphological and physiological parameters were assessed, and data were analyzed using principal component analysis (PCA) and correlation heatmaps to identify trait associations and stress markers. Drought stress significantly reduced all growth and yield traits, with stronger effects under more severe water deficit. Tea waste generally exacerbated stress impacts, increasing damage indices, reducing plant height, and lowering chlorophyll values. However, 10% tea waste under non-stress conditions increased plant and root dry weights without negatively affecting other traits, suggesting a partial nutrient contribution. In contrast, 5% tea waste aggravated stress effects, likely due to phenolic and caffeine toxicity. Overall, raw tea waste was found to be unsuitable for kale production under drought conditions. To harness its potential, bioactive compounds must be degraded or removed, and the material stabilized through composting or biochar conversion for safe integration into drought-resilient systems.

1. Introduction

Kale (Brassica oleracea var. acephala) is a widely cultivated leafy vegetable valued for its rich phytochemical profile and associated health benefits [1]. Originating in the Eastern Mediterranean, it is one of the oldest domesticated morphotypes of B. oleracea and remains an important crop worldwide [2]. In addition to its nutritional value, which includes glucosinolates, fiber, minerals and vitamins [3,4,5], kale exhibits considerable genetic diversity across cultivars, reflected in variation in leaf color, morphology, and flavor [6,7,8]. Because it is grown in temperate, subtropical, and tropical regions, understanding its resilience to water stress is a global priority. Recent bibliometric analyses further underscore the increasing research focus on drought stress, particularly in physiological and antioxidant response pathways [9].

Optimal growth of kale occurs at temperatures below 22 °C, while temperatures above 25 °C inhibit leaf development and reduce yield [10]. Water management is critical, as irrigation frequency and volume strongly influence biomass production, metabolism, and nutritional quality. Kale shows measurable responses to drought (changes in metabolome, leaf physiology), making it a good model leafy vegetable for studying amendment effects under water stress [11]. Stressful conditions such as drought or seasonal heat reduce chlorophyll and carotenoid levels in kale, and may also alter mineral composition—typically decreasing nitrogen, potassium, and calcium concentrations while increasing sodium and proline accumulation as adaptive responses [12,13]. In kale, drought stress reduces growth, physiological activity, and the accumulation of primary and secondary metabolites, with biomass losses exceeding 10% within two weeks of water deficit [14,15]. Greenhouse experiments in cabbage maintained at 80–60% of field capacity demonstrated clear reductions in plant height, stem diameter, leaf number, leaf area, shoot biomass, photosynthesis, stomatal conductance, transpiration, and chlorophyll content [16]. Likewise, most Brassica seeds germinate effectively at 50–75% field capacity [17], indicating that these moisture levels represent physiologically relevant thresholds for early development. On this basis, irrigation at 75% field capacity can be regarded as a mild drought stress, whereas 50% field capacity constitutes a moderate stress level that causes measurable reductions in growth and physiology without inducing irreversible damage.

Drought and desertification represent major threats to agriculture. Rising evaporative demand coupled with insufficient soil moisture intensifies agricultural and ecological drought, while reduced river flow and surface storage contribute to hydrological drought [18]. Many production regions already face more frequent and severe droughts that reduceyields across Africa, Asia, and Europe, with root crops and vegetables particularly vulnerable [19].

Improving soil water-holding capacity is a practical strategy to alleviate drought stress. Soil organic matter enhances aggregate stability, porosity, and infiltration, and organic amendments improve soil structure and microbial activity, thereby increasing resilience to water deficit [20,21]. Higher soil organic carbon reduces bulk density and can raise infiltration rates by up to three times, restoring critical soil functions in degraded soils [22]. Plant-derived carbon inputs promote macroaggregate formation through microbial and plant mucilage that binds mineral particles [23]. The use of organic residues as soil conditioners supports circular agriculture by reducing waste, reusing resources, and producing high-value inputs, while also providing safe disposal pathways [24,25,26]. Studies on leafy vegetables illustrate that drought reduces yield and nutrient content, but soil amendment (or fertility) can modulate these effects [15,27]. However, the behavior of organic amendments varies with composition and interactions with soil minerals, requiring crop- and site-specific evaluation [28]. Among these residues, tea waste represents an especially abundant yet underutilized resource with potential to improve soil properties under stress conditions.

Factory-derived tea waste has recently attracted attention as an organic amendment because of its high organic matter, phenolic compounds, and mineral composition. Previous studies have highlighted its potential for sustainable valorization [29] and reviewed its bioactive properties relevant to agriculture [30]. Experimental evidence indicates that tea-waste-derived biochar can reduce phytotoxicity and improve seed germination in garden cress (Lepidium sativum L.) [31]. Composting or incorporating tea waste into soil has been reported to enhance soil organic matter, aggregation, and nutrient availability, thereby improving growth and yield in pepper and maize [32,33]. It may also contribute to salinity mitigation by increasing cation exchange capacity and soil moisture retention [34,35]. Despite these findings, its potential effects on leafy vegetables under drought conditions remain largely unexplored. This knowledge gap is particularly relevant given the dual challenges of crop productivity and waste management in modern agriculture.

This study examined the effects of tea-waste amendments on kale under controlled water-deficit conditions. By testing different incorporation levels and irrigation regimes, we aimed to assess whether tea waste can be used as a sustainable soil amendment to improve drought resilience in kale cultivation. The objective was to quantify the main and interactive effects of drought severity (100, 75, and 50% field capacity), tea-waste dose (0, 5, and 10% w/w), and cultivar on kale growth and physiological traits. We hypothesized that: (H1) drought would reduce biomass and physiological performance; (H2) a 10% tea-waste amendment would partly offset drought effects through its contribution of nutrients and organic matter; (H3) cultivars would differ in their response patterns; and (H4) interactions between tea waste and drought stress would exert stronger effects than single factors alone.

2. Materials and Methods

2.1. Plant Material

Two kale (Brassica oleracea var. acephala) cultivars were used: ‘Karadeniz Yaprak’ (Naz Seed Agricultural Production Livestock Industry and Trade Co. Ltd., Ankara, Türkiye; available at www.naztohumculuk.com.tr; VK1) and ‘KEЙЛ’ (Semenabulgaria Company, Sofia, Bulgaria; available at www.semenabulgaria.com; VK2). Karadeniz Yaprak has flat, broad, and waxy leaves, whereas KEЙЛ is curly-leaved with smaller, serrated blades. These cultivars were chosen because they represent contrasting morphotypes that are widely cultivated in different production systems. Leaf structure and morphology may influence physiological responses such as transpiration, light interception, and water-use efficiency, making them relevant for evaluating drought × amendment interactions. Although no detailed drought-resistance profiles are available for these specific varieties, this study provides the first comparative assessment of their performance under controlled water-deficit conditions.

2.2. Tea Waste and Soil Characteristics

Tea waste was obtained from a private tea processing factory in Rize Province, Türkiye. This material, consisting of leaf residues, fibers, and fine particles removed during the processing of green tea into black tea, was left to decompose naturally under outdoor conditions for eight months at the Faculty of Agriculture, Recep Tayyip Erdoğan University, Rize, Türkiye. A representative image is shown in Figure 1, and its basic properties are summarized in

Table 1. Physical and chemical properties of factory-derived tea waste applied as an organic amendment in kale cultivation.

Property	Value
pH	5.57
EC (dS m−1)	0.102
Total N (%)	2.19
P (ppm)	395.6
K (ppm)	5792
Organic matter (%)	47
Water holding capacity (%)	96.02

. The physical and chemical characteristics of the soil used in the experiment are shown in

Table 2. Physical and chemical properties of soil used in the experiment.

Property	Value
Texture class	Clayey
EC (dS m−1, saturated paste)	0.62
pH (saturated paste)	6.93
CaCO3 (%)	1.80
Total N (%)	0.09
P (ppm)	4.18
K (ppm)	31.7
Organic matter (%)	2.58

.

2.3. Seedling Production

Seeds of each kale cultivar were germinated in trays containing a 1:1 (v/v) mixture of peat and perlite. A balanced NPK fertilizer (20–20–20) was supplied during the seedling stage to support uniform early growth. When plants reached the three-to-four true-leaf stage, they were transplanted into pots filled with the designated soil–tea-waste mixtures, with one seedling planted per pot.

Nitrogen fertilization was supplied using diammonium phosphate (DAP, (NH₄)₂HPO₄) and urea (46% N, (NH₂)₂CO) (İGSAŞ-Istanbul Fertilizer Industry Inc., Istanbul, Türkiye; available at www.igsas.com.tr). The total nitrogen input per pot was standardized according to the soil mass to ensure uniform nutrient availability. Before transplanting, each pot received 0.35 g of DAP as a basal application. Urea was applied at a total rate of 0.45 g per plant, divided into two equal portions: the first incorporated into the soil together with the basal fertilizer at planting, and the second applied to the soil surface (as a top dressing) one week prior to initiation the drought-stress treatment. Based on the 2 L pot volume, this corresponds to an estimated field-equivalent rate of approximately 270 kg N ha⁻¹ (≈63 kg N ha⁻¹ from DAP and 207 kg N ha⁻¹ from urea), assuming an equivalent soil depth of 0–20 cm. This conversion is presented solely to contextualize the pot-scale fertilization and does not imply field-scale recommendations.

2.4. Pot Experiment and Drought Stress Application

Soil and tea waste mixtures were prepared at 0% (TW0, control), 5% (TW1), and 10% (TW2) tea waste (w/w) and filled into 2 L polyethylene pots (17 × 13.3 cm). One seedling was planted per pot. Drought stress was applied two weeks after transplanting, following the procedure described by Kıran et al. [36]. The field capacity of the pots was determined gravimetrically by saturating and draining two randomly selected pots per treatment. Irrigation regimes were imposed as follows:

DS0: 100% field capacity (well-watered control)

DS1: 75% of field capacity (mild stress)

DS2: 50% of field capacity (moderate stress)

A randomized complete block design was employed, with five replicates per treatment and five plants in each replicate. The experiment followed a factorial arrangement with two kale varieties (VK1 and VK2), three tea waste levels (0%, 5%, 10%), and three drought stress levels corresponding to 100%, 75%, and 50% of field capacity (DS0, DS1, and D2, respectively). Each treatment combination consisted of five individual plants, resulting in total of 90 plants (2 varieties × 3 tea waste levels × 3 drought stress levels × 5 plants per treatment). Morphological and physiological parameters were recorded individually for each plant to ensure precise replication. A randomized complete block design (RCBD) was chosen because all factors were randomized at the pot level, and no hierarchical restriction necessitated a split-plot layout.

Irrigation at 100%, 75%, and 50% of field capacity represented well-watered, mild, and moderate drought stress levels, commonly used in Brassica studies to impose realistic deficits without causing mortality [16,17]. Drought treatments were maintained for 21 days, with irrigation based on the water requirements of the control group. Stress was applied for 21 days to allow physiological adjustment and measurable growth responses. Pilot tests confirmed that these conditions produced consistent and distinguishable effects without irreversible damage. The factorial treatment structure, including the two kale varieties, three tea waste levels, and three drought stress regimes, is summarized in

Table 3. Factorial structure of kale variety (VK), tea waste level (TW), and drought stress regime (DS).

Variety (VK)	Tea Waste (TW, %)	Irrigation Regime (RI)
Karadeniz Yaprak (VK1)	0 (TW0)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)
	5 (TW1)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)
	10 (TW2)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)
KEЙЛ (VK2)	0 (TW0)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)
	5 (TW1)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)
	10 (TW2)	DS0: Control (100% field capacity) DS1: Light stress (75% field capacity) DS2: Moderate stress (50% field capacity)

Varieties: Karadeniz Yaprak (VK1) and KEЙЛ (VK2); TW: tea waste amendment incorporated into soil at 0% (TW0), 5% (TW1), and 10% (TW2) (w/w); DS: drought stress imposed at 100% field capacity (DS0, control), 75% field capacity (DS1, mild stress), and 50% field capacity (DS2, moderate stress).

, while a schematic representation of the experimental layout is provided in Figure 2.

2.5. Observations and Measurements

Following 21 days of stress exposure, trait evaluations were conducted. Damage index (0–5): Visual scoring of drought damage was performed according to the method described by Kıran et al. [37] and Kuşvuran et al. [38]. The scoring criteria were: 0 = plants unaffected by drought stress; 1 = slight slowdown in growth; 2 = onset of wilting in lower leaves; 3 = curling and wilting of upper leaves; 4 = severe wilting and yellowing of leaves with drying at leaf margins; 5 = complete wilting and drying of lower leaves.

Plant height (cm) was measured from the root collar up to the shoot apex. Leaf thickness (mm) was measured from the fourth true leaf using a digital caliper (Bacolis Digital Caliper, Stainless Hardened, Generic, Hong Kong, China; precision of 0.01 mm). Leaf length (cm) was measured from the leaf base to the tip. Leaf number was recorded as the total number of true leaves per plant. Chlorophyll content was determined using a SPAD-502 chlorophyll meter (Konica Minolta Inc., Tokyo, Japan; product information available at www. konicaminolta.com) on the third true leaf of three plants per replicate, and mean values were calculated.

Leaf weight (g) was determined from three leaves per replicate and weighed on a precision scale (GX-600, A&D Company, Ltd., Tokyo, Japan) with internal calibration and a readability of 0.001 g. Leaf area (cm²) was quantified from scanned images (HP Scanjet G2410, HP Inc., Palo Alto, CA, USA; available at www.hp.com) using WinDIAS 3.2 software (Delta-T Devices Ltd., Cambridge, UK; product information available at www.delta-t.co.uk). Relative water content (RWC, %) was calculated according to Smart and Bingham [39] using fresh, turgid, and dry weights (samples were dried at 85 °C for 24 h).

Plant fresh weight (g plant⁻¹) was determined by harvesting and weighing aboveground biomass, and plant dry weight (g plant⁻¹) was recorded after drying the same samples at 65 °C to constant weight. Roots were carefully removed from the pots and washed over a 1 mm mesh sieve to remove adhering soil particles without damaging fine roots. Root length (cm) and root diameter (cm) were measured with a digital caliper. Root fresh weight (g plant⁻¹) was recorded immediately after washing, and root dry weight (g plant⁻¹) was measured after drying samples at 65 °C to constant weight. The drying process of the samples was carried out in an oven (MMM Medcenter Einrichtungen GmbH, Planegg/Munich, Germany; model ECOCELL 55 L).

2.6. Statistical Analyses

Data were analyzed using JMP Pro 13.0 software (SAS Institute Inc., Cary, NC, USA; software information available at https://www.jmp.com/en/home). Prior to analysis, the data were checked for normality (Shapiro–Wilk test) and homogeneity of variances (Levene’s test) to ensure the validity of parametric assumptions. A factorial analysis of variance (ANOVA) was performed to evaluate the effects of variety, tea waste rate, and drought stress, as well as their interactions. When significant differences were detected, mean separations were conducted using the least significant difference (LSD) test at p < 0.05.

Because several traits exhibited significant three-way interactions (Variety × Tea waste × Drought stress), simple two-way means were presented to aid interpretation of specific factor combinations. These tables are not intended as independent analyses but serve to illustrate component patterns within the overall three-factor interaction.

To explore multivariate relationships among traits, principal component analysis (PCA) was carried out using R Studio (version 4.3.1; RStudio PBC, Boston, MA, USA; software information available at https://posit.co). The PCA was based on standardized (z-score) data to account for differences in measurement scales. This exploratory, dimension-reduction approach was used to identify the major sources of variation among treatments, detect correlated groups of morphological and physiological traits, and highlight key indicators contributing most to overall variability. Correlation patterns were further visualized through polar heatmaps and hierarchical clustering.

3. Results

Analysis of variance (

Table 4. Variance analysis of kale traits under factorial combinations of variety, tea waste amendment, and drought stress.

Sources of Variation	df	DI	PH	LT	LL	LN	SPAD	LW	LA	RWC	PFW	PDW	RL	RD	RFW	RDW
VK	1	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**
TW	2	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**
DS	2	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**
VK × TW	2	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**
VK × DS	2	**	*	**	*	**	**	**	**	**	ns	ns	**	*	**	ns
TW × DS	4	**	**	**	**	**	**	*	**	*	**	**	**	**	**	*
VK × TW × DS	4	**	**	**	**	*	**	**	**	**	ns	*	**	ns	**	ns
Error	68	0.0026	0.236	0.0005	0.0243	0.1369	0.7464	0.0101	0.58	0.181	1.008	0.035	0.194	0.037	0.027	0.005
CV (%)		4.39	3.32	4.14	2.07	4.82	1.86	4.29	1.40	0.55	8.89	11.30	1.610	4.23	5.16	12.71

Note: ns, non-significant; * p < 0.05, significant; ** p < 0.01, highly significant. VK: kale variety (VK1 = Karadeniz Yaprak, VK2 = KEЙЛ); TW: tea waste amendment (TW0 = soil only, TW1 = soil + 5%, TW2 = soil + 10%); DS: drought stress (DS0 = 100% field capacity, DS1 = 75% field capacity, DS2 = 50% field capacity). DI: damage index; PH: plant height; LT: leaf thickness; LL: leaf length; LN: leaf number; SPAD: chlorophyll content; LW: leaf weight; LA: leaf area; RWC: relative water content; PFW: plant fresh weight; PDW: plant dry weight; RL: root length; RD: root diameter; RFW: root fresh weight; RDW: root dry weight.

) revealed that variety, tea waste level, drought stress, and their interactions significantly affected most measured parameters, including damage index, plant height, leaf thickness, leaf length, leaf number, chlorophyll content (SPAD), leaf weight, leaf area, relative water content, root length, and root dry weight.

Several parameters exhibited significant three-way interactions (variety × tea waste × drought stress), confirming that the effects of tea-waste amendment and drought level were depended on cultivar. Accordingly, the main interpretations are based on this three-factor interaction (

Table 5. Interaction effects of variety, tea waste level, and drought stress on morphological and physiological traits of kale.

Variety	TW (%)	DS	DI	PH (cm)	LT (mm)	LL (cm)	LN (Number Plant−1)	SPAD	LW (g)	LA (cm2)	RWC (%)	PDW (g Plant−1)	RL (cm)	RFW (g Plant−1)
VK1	TW0	DS0	0.00 ± 0.00 l	21.67 ± 1.01 a	0.200 ± 0.01 b	12.44 ± 0.26 a	8.60 ± 0.12 bc	50.30 ± 0.48 bc	3.62 ± 0.08 b	98.6 ± 0.90 a	92.36 ± 0.32 a	2.84 ± 0.18 a	35.36 ± 0.46 a	7.64 ± 0.29 a
		DS1	0.34 ± 0.05 k	18.98 ± 0.39 c	0.188 ± 0.01 cd	10.20 ± 0.23 b	8.04 ± 0.36 def	50.36 ± 0.69 bc	3.18 ± 0.13 c	81.80 ± 1.55 b	71.54 ± 0.40 j	2.06 ± 0.32 cd	31.28 ± 0.41 b	3.90 ± 0.21 cd
		DS2	1.10 ± 0.10 h	16.48 ± 0.48 e	0.176 ± 0.01 e	8.08 ± 0.08 d	8.38 ± 0.29 bcd	52.60 ± 0.63 a	2.60 ± 0.07 e	58.88 ± 0.48 g	60.22 ± 0.27 o	1.80 ± 0.16 ef	29.72 ± 0.54 e	3.76 ± 0.15 d
	TW1	DS0	0.72 ± 0.04 i	17.90 ± 0.22 d	0.140 ± 0.01 j	9.10 ± 0.10 c	7.78 ± 0.22 efgh	40.04 ± 1.05 h	3.04 ± 0.11 d	70.56 ± 0.34 d	84.41 ± 0.27 e	1.84 ± 0.18 def	30.50 ± 0.43 cd	3.46 ± 0.09 e
		DS1	1.46 ± 0.05 e	15.86 ± 0.68 f	0.142 ± 0.01 ij	7.52 ± 0.08 e	7.60 ± 0.20 fgh	40.78 ± 0.72 h	2.68 ± 0.08 e	66.74 ± 0.87 e	77.57 ± 0.14 h	1.54 ± 0.05 gh	28.70 ± 0.63 f	2.86 ± 0.18 g
		DS2	1.74 ± 0.05 c	15.22 ± 0.34 g	0.156 ± 0.01 gh	7.26 ± 0.05 f	7.02 ± 0.08 i	43.00 ± 0.83 fg	2.20 ± 0.12 g	61.28 ± 0.88 f	70.46 ± 0.38 k	1.18 ± 0.15 ij	26.90 ± 0.29 g	2.50 ± 0.12 h
	TW2	DS0	1.04 ± 0.05 h	19.90 ± 0.31 b	0.198 ± 0.01 b	7.92 ± 0.08 d	8.80 ± 0.25 b	42.32 ± 1.19 g	4.18 ± 0.13 a	97.66 ± 0.63 a	86.52 ± 0.34 d	2.98 ± 0.26 a	35.78 ± 0.30 a	4.88 ± 0.39 b
		DS1	1.36 ± 0.05 fg	18.54 ± 0.40 c	0.160 ± 0.01 fg	7.52 ± 0.08 e	8.04 ± 0.58 def	42.96 ± 1.09 fg	3.10 ± 0.10 cd	73.64 ± 0.76 c	78.56 ± 0.36 g	2.58 ± 0.13 b	25.28 ± 0.24 h	4.00 ± 0.10 c
		DS2	1.74 ± 0.05 c	17.62 ± 0.33 d	0.150 ± 0.01 hi	7.08 ± 0.08 fg	7.54 ± 0.18 gh	49.30 ± 0.71 cd	2.98 ± 0.08 d	70.10 ± 0.65 d	65.33 ± 0.36 l	2.00 ± 0.16 cde	24.58 ± 0.22 i	3.38 ± 0.13 e
VK2	TW0	DS0	0.00 ± 0.00 l	13.64 ± 0.56 h	0.224 ± 0.01 a	6.94 ± 0.09 g	8.58 ± 0.10 bc	50.42 ± 1.39 b	2.38 ± 0.08 f	42.40 ± 0.76 j	91.78 ± 0.60 b	1.66 ± 0.27 fg	27.44 ± 0.42 g	3.14 ± 0.05 f
		DS1	1.42 ± 0.04 ef	8.80 ± 0.42 l	0.220 ± 0.01 a	5.28 ± 0.19 j	6.24 ± 0.19 kj	49.06 ± 0.90 d	1.02 ± 0.13 k	23.96 ± 0.50 n	82.74 ± 0.56 f	0.70 ± 0.12 lm	21.50 ± 0.57 k	2.36 ± 0.11 h
		DS2	1.96 ± 0.05 a	8.62 ± 0.46 l	0.200 ± 0.01 b	4.56 ± 0.09 k	5.82 ± 0.18 k	50.54 ± 0.61 b	0.80 ± 0.10 l	22.08 ± 0.59 o	63.30 ± 0.35 m	0.60 ± 0.10 m	17.16 ± 0.47 m	1.98 ± 0.13 i
	TW1	DS0	1.34 ± 0.05 g	12.12 ± 0.19 i	0.204 ± 0.01 b	8.04 ± 0.11 d	8.24 ± 0.18 cde	39.90 ± 1.01 h	2.64 ± 0.05 e	48.92 ± 0.73 h	90.11 ± 0.49 c	1.28 ± 0.16 i	30.30 ± 0.51 d	3.14 ± 0.05 f
		DS1	1.44 ± 0.05 e	11.62 ± 0.38 i	0.196 ± 0.01 bc	6.34 ± 0.34 i	8.00 ± 0.27 defg	49.04 ± 0.58 d	1.28 ± 0.08 j	27.78 ± 0.54 m	78.01 ± 0.42 h	1.32 ± 0.15 hi	28.48 ± 0.42 f	2.40 ± 0.07 h
		DS2	1.64 ± 0.05 d	9.52 ± 0.29 k	0.186 ± 0.01 d	5.16 ± 0.05 j	6.42 ± 0.30 j	43.60 ± 0.79 f	1.02 ± 0.13 k	25.52 ± 0.53 n	61.96 ± 0.61 n	0.92 ± 0.08 kl	25.64 ± 0.49 h	1.98 ± 0.13 i
	TW2	DS0	0.48 ± 0.04 j	15.24 ± 0.69 g	0.204 ± 0.01 b	8.10 ± 0.10 d	9.42 ± 0.27 a	46.20 ± 0.86 e	2.42 ± 0.08 f	46.72 ± 0.80 i	90.56 ± 0.46 c	2.16 ± 0.25 c	30.86 ± 0.65 bc	3.08 ± 0.13 f
		DS1	1.44 ± 0.05 e	11.52 ± 0.41 i	0.220 ± 0.01 a	6.94 ± 0.09 g	7.36 ± 0.18 hi	45.50 ± 0.68 e	1.66 ± 0.05 h	35.98 ± 0.37 k	84.83 ± 0.44 e	1.30 ± 0.20 i	22.68 ± 0.40 j	1.66 ± 0.09 j
		DS2	1.86 ± 0.05 b	10.46 ± 0.67 j	0.168 ± 0.01 ef	6.62 ± 0.26 h	6.42 ± 0.30 j	48.70 ± 0.78 d	1.42 ± 0.08 i	29.26 ± 0.83 l	72.85 ± 0.55 i	1.00 ± 0.16 jk	19.58 ± 0.29 l	1.36 ± 0.09 k
LSD %5			0.065	0.614	0.023	0.197	0.467	1.090	0.127	0.959	0.537	0.236	0.555	0.208

Statistically significant means have been grouped according to the LSD (5%) test. Differences between means denoted by different letters are significant (p < 0.05). VK: kale variety (VK1 = Karadeniz Yaprak, VK2 = KEЙЛ); TW: tea waste amendment (TW0 = soil only, TW1 = soil + 5% tea waste, TW2 = soil + 10% tea waste); DS: drought stress (DS0 = 100% field capacity, DS1 = 75% field capacity, DS2 = 50% field capacity); DI: damage index; PH: plant height; LT: leaf thickness; LL: leaf length; LN: leaf number; SPAD: chlorophyll content; LW: leaf weight; LA: leaf area; RWC: relative water content; PDW: plant dry weight; RL: root length; RFW: root fresh weight.

). For transparency and clarity, detailed two-factor means illustrating simple effects within this interaction are presented in Supplementary Tables S1–S3.

For plant fresh weight, no significant effects were detected for the variety × drought stress or three-way interactions; however, both variety × tea waste and tea waste × drought stress interactions were significant. Similarly, plant dry weight was affected by all three main factors and by the variety × tea waste, tea waste × drought stress, and three-way interactions, while the variety × drought stress interaction was not significant. Root diameter responded significantly to most main factors and two-way interactions, except for the three-way combination. In contrast, root dry weight was unaffected by the variety × drought stress and three-way interactions but showed significant differences under variety × tea waste and tea waste × drought stress effects. Detailed ANOVA results for all parameters and interaction terms are presented in Table 4.

3.1. Growth and Biomass

Drought stress significantly suppressed vegetative growth in both kale cultivars, though the extent varied withtea-waste level (Table 5). Overall, plant height, leaf area, and biomass decreased progressively with increasing drought severity, confirming the high sensitivity of kale to moisture limitation. The significant three-way interaction (Variety × Tea Waste × Drought Stress) demonstrated that the effect of tea-waste amendment on growth depended on both cultivar and stress intensity.

In VK1, height reductions were most pronounced under mild to moderate stress (DS1–DS2) combined with 5% tea waste (TW1), whereas VK2 was more sensitive in unamended soil (TW0). In VK1 under DS2, plant height was 16.48 cm in TW0, decreased to 15.22 cm in TW1, and increased again to 17.62 cm in TW2, while in VK2 the corresponding values were 8.62, 9.52, and 10.46 cm, indicating that tea-waste addition did not effectively mitigate drought and in some cases exacerbated reductions. Under severe drought (DS2), plant height declined by approximately 20–30% relative to the control, while 10% tea waste (TW2) slightly alleviated this reduction.

Leaf-related traits showed comparable trends. Leaf length, thickness, and number declined as water availability decreased, and tea-waste supplementation only partially alleviated losses. Under DS2, VK1 leaf length was 8.08 cm (TW0), 7.26 cm (TW1), and 7.08 cm (TW2), while VK2 reached 4.56 cm (TW0), 5.16 cm (TW1), and 6.62 cm (TW2), indicating that 10% tea waste (TW2) slightly reducedstress-induced losses. Under DS2, VK1 leaf thickness was 0.176 mm (TW0), 0.142 mm (TW1), and 0.150 mm (TW2), confirming that moderate tea-waste levels accentuated stress effects. VK2 maintained relatively higher values, decreasing from 0.200 mm (TW0) to 0.168 mm (TW2). In VK1 under DS2, leaf number remained around 8.08 in TW0, but decreased to 7.26 in TW1 and 7.08 in TW2, while in VK2 values declined from 5.82 (TW0) to 6.42 (TW1–TW2), confirming that tea waste had little mitigating effect. Under DS2, VK1 leaf area was 58.88 cm² (TW0), 61.28 cm² (TW1), and 70.10 cm² (TW2), indicating a modest improvement with increasing tea-waste level. VK2 showed a similar trend, rising from 63.30 cm² (TW0) to 61.96 cm² (TW1) and 72.85 cm² (TW2). These results suggest that the 10% tea-waste amendment slightly reduced leaf-area losses under drought compared with unamended soil. The 10% amendment marginally enhanced leaf expansion in VK2 but not in VK1. Leaf weight and area followed comparable patterns, reaching their maximum values under well-watered conditions (DS0) and lowest under DS2.

Dry-matter accumulation also declined significantly under drought. VK1 consistently maintained higher dry weights than VK2, suggesting stronger inherent vigor. Under DS2, VK1 plant dry weight was 1.80 g (TW0), 1.18 g (TW1), and 2.00 g (TW2), whereas VK2 recorded 0.60 g (TW0), 0.92 g (TW1), and 1.00 g (TW2), indicating that 10% tea waste partially mitigated drought-induced biomass loss. Although the 10% tea-waste level compensated partially for biomass loss under DS2, overall growth suppression under drought remained evident. Detailed two-factor means illustrating simple effects within this three-factor interaction are presented in Supplementary Tables S1–S3.

3.2. Water Status and Pigments

Drought stress intensified visible injury and adversely affected leaf water relations, with tea waste showing variable effects depending on dose and cultivar (Table 5). Under DS2, the damage index (DI) increased from 1.10 in TW0 to 1.74 in TW1 and 1.74 in TW2 for VK1, while in VK2 it rose from 1.96 in TW0 to 1.64 in TW1 and 1.86 in TW2, showing that elevated tea waste tended to amplify visible stress symptoms. The damage index (DI) rose steadily with drought intensity, most sharply in VK2, and was often aggravated by higher tea-waste levels.

Chlorophyll content (SPAD) generally decreased under drought stress but occasionally increased under TW2, suggesting that nutrient release from higher amendment rates may have partially supported pigment retention. Values rose under DS2 in TW1–TW2 for both cultivars (Table 5). In VK1 at DS2, SPAD was 52.60 in TW0 compared with 43.00 in TW1 and 49.30 in TW2, while VK2 maintained values between 50.54 (TW0) and 48.70 (TW2), suggesting that tea waste sometimes reduced chlorophyll loss under stress. Nevertheless, the highest SPAD readings were recorded in control plants without tea waste.

Relative water content (RWC) decreased markedly under DS2 in all treatments, demonstrating reduced leaf hydration under water deficit. VK2 maintained comparatively higher RWC than VK1 at moderate stress, suggesting better water-retention capacity. The highest values occurred in DS0 for both cultivars, with VK2 maintaining relatively higher values under DS1–DS2 in TW1. In VK1, RWC declined to 60.22% in TW0, 70.46% in TW1, and 65.33% in TW2, while VK2 dropped to 63.30% in TW0, 61.96% in TW1, and 72.85% in TW2, demonstrating inconsistent effects of tea waste under drought. However, higher tea-waste levels did not consistently improve water status and occasionally aggravated drought impacts.

3.3. Root System Traits

Root characteristics mirrored aboveground responses (Table 5). Root length and diameter decreased significantly under drought, particularly at 50% FC (DS2). Under DS2, VK1 root length declined to 29.72 cm (TW0), 26.90 cm (TW1), and 24.58 cm (TW2), while VK2 decreased to 17.16 cm (TW0), 25.64 cm (TW1), and 19.58 cm (TW2), showing cultivar-specific responses. Under DS2, VK1 root diameter was 0.176 mm (TW0), 0.156 mm (TW1), and 0.150 mm (TW2), while VK2 decreased from 0.200 mm (TW0) to 0.186 mm (TW1) and 0.168 mm (TW2), indicating a stronger reduction in VK2.

Root fresh and dry weights were similarly affected. Both cultivars exhibited pronounced reductions under DS2, reflecting constrained biomass allocation to belowground organs. Under DS2, VK1 root fresh weight declined to 3.76 g (TW0), 2.50 g (TW1), and 3.38 g (TW2), whereas VK2 decreased from 1.98 g (TW0) to 2.40 g (TW1) and 1.36 g (TW2). Under DS2, VK1 root dry weight declined to 1.80 g (TW0), 1.18 g (TW1), and 2.00 g (TW2), whereas VK2 decreased to 0.60 g (TW0), 0.92 g (TW1), and 1.00 g (TW2). The 10% tea-waste treatment slightly increased root dry weight under control conditions but had minimal benefit during stress.

3.4. Multivariate Patterns (PCA, Correlations)

To assess the reliability of the parameters used to evaluate the effects of mild (DS1) and moderate (DS2) drought stress in soils amended with tea waste, a principal component analysis (PCA) was conducted. Multivariate analyses revealed distinct associations among morphological and physiological traits of kale under tea waste and drought stress. The polar heatmap with dendrogram (Figure 3) displayed two major trait clusters. The analysis showed that the 15 measured traits formed into a two-factor structure that explained 78.3% of the total variance. The first factor accounted for 65.4%, and the second for 12.9%, confirming the robustness of the factorial design in capturing treatment effects. Growth-related parameters such as plant height (PH), leaf area (LA), leaf weight (LW), plant fresh weight (PFW), plant dry weight (PDW), root dry weight (RDW), and root diameter (RD) grouped together, indicating their strong interdependence in determining biomass production. Conversely, relative water content (RWC), leaf length (LL), leaf number (LN), leaf thickness (LT), and root fresh weight (RFW) formed a separate cluster, reflecting physiological adjustments under stress conditions. The damage index (DI) and chlorophyll content (SPAD) were associated as a distinct pair, highlighting their role as stress indicator traits.

The correlation heatmap (Figure 4) supported these findings, showing strong positive associations (r > 0.90) among PH, LA, LW, PFW, and PDW, as well as similarly strong correlations between root traits and aboveground biomass. Relative water content (RWC) was moderately correlated with LL and LN, confirming their functional linkage as observed in the dendrogram. The damage index (DI) exhibited strong negative correlations with nearly all growth and biomass traits, validating its role as a direct measure of stress impact. In contrast, SPAD displayed only weak associations with growth traits, instead clustering with LT and LN as part of a distinct physiological response group.

4. Discussion

4.1. Drought Stress Effects on Kale Growth and Biomass

Drought stress significantly reduced kale biomass, in agreement with previous reports on Brassica oleracea [10,15]. Both mild (75% field capacity) and moderate (50% field capacity) water deficits decreased plant height, leaf area, and fresh and dry biomass, though the magnitude of reduction varied between varieties. Karadeniz Yaprak exhibited comparatively smaller yield losses than KEЙЛ, suggesting stronger adaptive mechanisms such as osmotic adjustment, stomatal regulation, and deeper rooting capacity. Genotype-specific responses of kale to water deficit have similarly been documented in Brassica oleracea accessions [11,40], where differences in antioxidant capacity and water-use efficiency determined drought resilience.

The findings of Barickman et al. [14], who reported a 22.5% biomass increase under higher irrigation thresholds, likely reflect differences in irrigation scheduling and genotypic behavior. Collectively, our findings confirm that even mild drought substantially reduces biomass in kale and emphasize the critical role of varietal selection in mitigating yield losses under water-limited conditions.

4.2. Interaction Effects of Drought Stress and Tea Waste Amendment

Tea-waste addition modified drought responses in kale, showing a clear dose-dependent pattern. Under full irrigation (DS0), 10% tea waste increased leaf area and fresh weight, indicating improved soil fertility and structure. The addition of tea waste did not enhance root elongation under stress but improved root length and thickness under non-stress conditions, suggesting that organic amendment primarily benefited growth when soil moisture was adequate. In contrast, 5% tea waste reduced pigment stability and biomass, particularly under water deficit conditions. This dual response reflects the chemical and physical nature of tea residues. At low rates, residual phenolics and caffeine may immobilize nitrogen and damage membranes, intensifying oxidative stress [41,42]. At 10%, the higher organic matter and exchangeable cations (K, Ca, Mg) improved soil aggregation, water retention, and nutrient uptake, thereby partially alleviating drought effects [29,43].

Physiologically, drought and low-dose residues acted synergistically to reduce chlorophyll concentration and relative water content, whereas the 10% amendment stabilized pigments and turgor. Similar patterns were reported by Barickman et al. [14], linking drought and phenolic stress to pigment degradation. These findings indicate that the negative effects of 5% tea waste arise from phenolic toxicity, whereas 10% level promotes osmotic and nutritional balance through organic-matter enrichment. Overall, tea waste acts as a context-dependent amendment: raw, low doses may aggravate stress, while higher or composted forms can enhance soil–plant interactions. Future work should quantify phenolic degradation and nutrient dynamics to establish safe and effective application thresholds.

Because variety, amendment level, and drought intensity interacted significantly for most traits, the discussion hereafter focuses to patterns within the combined three-way interaction rather than to isolated two-factor effects.

4.3. Physiological and Biochemical Responses Under Drought and Tea Waste Amendment

Drought stress alters pigments and secondary metabolism in kale, consistent with previous reports in Brassica species [14]. Water deficit reduced chlorophyll and carotenoid contents, while promoting accumulation of stress-related metabolites such as glucosinolates and phenolics. Tea waste addition under drought further amplified these changes, particularly at 5%, likely due to phenolic interference and caffeine-induced inhibition of photosystem activity [41,42].

By contrast, 10% tea waste under moderate drought partially restored pigment and biomass levels, reflecting the positive role of organic matter and mineral supplementation in supporting photosynthetic capacity. Enhanced soil moisture retention and micronutrient availability likely stabilized chlorophyll and osmotic balance, mitigating drought-induced oxidative stress. This aligns with Ekbiç et al. [43], who demonstrated that composting reduces phytotoxicity, and Debnath et al. [29], who highlighted the significance of organic residue processing for safe agricultural application. Overall, these responses underline the importance of balancing organic matter benefits and phenolic toxicity. The 5% amendment likely represented a transitional threshold where phenolic load exceeded decomposition benefits, whereas the 10% level supplied sufficient nutrients to counter drought-induced metabolic disturbances. Future work quantifying phenolic degradation dynamics, soil enzyme activity, and mineral fluxes would help clarify the mechanisms through which tea waste amendments influence plant physiology under combined abiotic stresses.

4.4. Trait Interrelationships Revealed by PCA and Correlation Heatmap

PCA was not used to infer causal relationships but rather to summarize multivariate patterns and trait associations. Principal component analysis (PCA) revealed two major trait clusters explaining 78.3% of the total variance. Biomass-related traits grouped closely, while water relation and leaf traits formed a second cluster, and SPAD and damage index emerged as stress indicators. Similar clustering has been reported in Brassica oleracea, where PCA distinguished tolerant from sensitive accessions [40], and in kale, where root traits and osmolytes contributed to resilience under combined stress [11]. Comparable findings in wheat confirm PCA as a robust tool for detecting interdependent traits and identifying reliable markers of drought tolerance [44].

In our study, PCA provided an integrated view of how drought stress and tea waste levels shaped trait coordination in kale. As a multivariate exploratory tool, it summarized multivariate patterns without implying causality, revealing correlated groups most responsive to water deficit and organic amendment.

Correlation analysis showed strong positive associations among growth and biomass traits, reflecting their coordinated role in yield formation. RWC correlated moderately with leaf number and length, while the damage index was negatively associated with nearly all growth traits, confirming its reliability as a stress marker. SPAD was weakly related to biomass, clustering instead with leaf thickness and number, suggesting a separate physiological response. Similar trait interdependencies have been reported in Brassica oleracea [11,40] and other Brassica species, where pigments often decouple from biomass under drought [45,46].

4.5. Role of Organic Amendments in Mitigating Drought Stress

Organic matter contributes to improved soil aggregation, enhances nutrient cycling, and increases water-holding capacity, thereby supporting plant tolerance to drought. Biochar, vermicompost, and rice husks amendments have shown beneficial effects [47]. In kale, biochar substitution for organic fertilizers improved yield and soil fertility [48], and farm-waste compost enhanced biomass and chlorophyll content [49]. In our study, tea waste acted as a dose-dependent amendment: 10% partly mitigated drought stress by buffering toxicity and stabilizing pH [50,51], whereas 5% aggravated stress, likely due to exposure to allelochemicals.

4.6. Composition and Variability of Tea Waste

Tea waste composition varies with cultivar, processing, harvest, and season [52]. It contains catechins, gallic acid, caffeine, and other polyphenols, which contribute to strong antioxidant activity [41,42] but may act as phytotoxins in soils. This variability helps explain why low application aggravated stress, while 10% partly buffered toxicity through added organic matter and nutrient release. Thus, the agronomic performance of tea waste is highly context-dependent, reflecting both composition and application rate.

4.7. Valorization Potential of Tea Waste

As the world’s most consumed beverage, tea generates substantial processing by-products that pose environmental concerns while offering opportunities for resource utilization [30]. Processing is essential to reduce toxicity and unlock agronomic value. Yıldırım et al. [33] showed that composted tea waste enriched with organic fertilizers enhanced seedling growth and chlorophyll in maize. Similarly, Kang et al. [48] and Thepsilvisut et al. [49] reported positive effects of biochar and compost in kale. These findings support the view that raw factory-derived tea waste is unsuitable under drought stress, but composting or biochar conversion can transform it into a valuable soil conditioner within sustainable agriculture.

4.8. Comparative Stress Physiology with Other Crops

Species respond differently to drought. For example, Cebeci [53] compared eggplant genotypes with their wild relatives under gradually increased drought stress and found that wild relatives often retained higher biomass, relative water content, and superior physiological traits under 50–75% water deficit, highlighting both intra- and inter-species variation in drought tolerance. Moreover, Kıran and Baysal Furtana [36] reported that eggplant exposed to combined drought and salinity exhibited increased chlorophyll and antioxidant activity, whereas in our study kale subjected to drought with tea waste showed reduced pigment stability. Similarly, Azotobacter inoculation in eggplant improved antioxidant enzymes and proline accumulation, thereby buffering oxidative damage [54]. Collectively, these comparisons suggest that while unprocessed tea waste may intensify stress in kale, biostimulants can mitigate drought injury in other crops, underscoring the importance of species- and amendment-specific responses.

4.9. Agronomic Potential and Limitations of Tea Waste Under Drought Stress

This study highlights both the promise and the challenges of using factory-derived tea waste to enhance kale performance under drought stress. While higher application rates (10%) partly mitigated water deficit effects by improving soil structure and nutrient supply, lower doses (5%) intensified stress symptoms, likely due to phenolic toxicity and temporary nitrogen immobilization. The dose dependent outcomes demonstrate that the influence of tea waste operates through both chemical (phenolic) and physical-nutritional (organic matter and mineral) pathways, which jointly determine plant physiological balance under stress.

Overall, drought stress consistently reduced all morphological and physiological parameters, although the magnitude of reduction varied among treatments. The variety Karadeniz Yaprak (VK1) exhibited greater resilience, and the 10% tea-waste treatment occasionally **alleviated—but not prevented—**stress-induced losses.

Varietal differences (Karadeniz Yaprak vs. KEЙЛ) further underline the genetic basis of stress response, as contrasting relative water content and pigment stability suggest distinct drought adaptation mechanisms. These observations are consistent with Bauer et al. [11] and Ben Ammar et al. [40], who reported cultivar-specific responses and identified indices such as the Stress Tolerance Index (STI) for effective screening B. oleracea.

From an agronomic perspective, factory-derived tea waste cannot be safely applied in raw form under drought conditions; however, composting or biochar conversion can reduce phenolic load, enhance nutrient release, and improve soil health. Future research should: (i) evaluate diverse kale genotypes under combined drought × amendment interactions, (ii) employ integrative stress indices such as STI, and (iii) investigate physiological markers including chlorophyll stability and antioxidant defense. Combining genotype selection with optimized processing of agro-industrial residues represents a sustainable strategy to improve drought resilience and close nutrient cycles in vegetable production systems.

5. Conclusions

Drought stress in kale affected physiological performance primarily through reductions in chlorophyll stability, leaf water status, and biomass accumulation. The integration of tea-waste amendment modified these responses in a dose-dependent manner, influencing both physiological and morphological traits. At 5%, residual phenolics and transient nitrogen immobilization intensified oxidative and osmotic stress, thereby aggravating growth inhibition. In contrast, the 10% application improved water retention, maintained relative water content, and partially restored pigment stability and dry-matter production, likely due to increased organic matter and exchangeable mineral availability.

These outcomes demonstrate that drought and organic amendment interact through complementary chemical (phenolic) and physical–nutritional (mineral–organic matter) mechanisms that determine plant resilience. Raw tea waste is unsuitable under stress conditions due to its high phenolic load and nitrogen-binding capacity; however, when applied at higher rates or after suitable composting or processing, it can serve as a sustainable soil conditioner, enhancing physiological stability and yield performance of kale under limited water availability. The observed outcomes represent integrated responses of the variety × tea waste × drought stress interaction, confirming that cultivar selection and optimized amendment rates are critical for balancing stress tolerance with sustainable nutrient management.