Nursery Cultivation Strategies for a Widespread Mangrove (Kandelia obovata Sheue & al.): Evaluating the Influence of Salinity, Growth Media, and Genealogy

Abstract

Mangrove plant seedling cultivation is crucial for the protection, management, and restoration of the mangrove ecosystem. In this study, we focused on Kandelia obovata Sheue & al., a typical mangrove, and evaluated nursery cultivation with different combinations of three salinity levels (S1: 0 ppt, S2: 10 ppt, and S3: 20 ppt), three genealogies (EZD, JX, and YZ), and five growth media (M1: 100% loess, M2: 100% sandy, M3: 50% loess + 50% sandy, M4: 40% loess + 40% sandy + 20% peat, and M5: 40% loess + 40% sandy + 20% coir), by measuring the growth parameters such as mortality rate, seedling height, seedling diameter, and biomass partition. These growth indexes were significantly affected by salinity and medium, and genealogies also had significant effects on mortality rate and biomass accumulation. S2 or S3 both had lower mortality and higher growth indexes than S1. M1 was the medium that increased seedling height, diameter, and biomass the most and had the lowest death rate. EZD and JX were also at higher levels than YZ in these indicators, but the difference between them was not obvious. S3, M1, and EZD consistently performed well in fuzzy evaluation and quality assessment (Dickson quality index: 1.179, 1.478, and 1.089, respectively). Furthermore, combinations involving these treatments also produced highly favorable results. This indicates that the quality of seedlings produced under these conditions was high. These results furnish both a theoretical and practical foundation for advancing nursery cultivation techniques and germplasm breeding of K. obovata in mangroves.

1. Introduction

Mangroves are unique wetland forests that thrive in the coastal intertidal zone. Mangroves have long served as natural barriers, defending coastal areas against storms and huge waves. For example, a six-year-old mangrove forest about 100 m wide can withstand the impact of a super typhoon [1]. Mangroves have an ecological service value of preventing coastal erosion, raw material supply, regulating water resources, climate regulation, and air quality maintenance [2], with an average value of USD 4185 per hectare per year [3]. However, with accelerated anthropogenic processes and climate fluctuations, the extent of the mangrove range degenerated at a visible rate [4,5]. At the same time, mangroves are increasingly recognized for their role in providing food, coastal conservation, and biodiversity reserves [6] and serving as a large carbon bank [7]. As a result, the number of individuals, organizations, and countries involved in the conservation of mangrove forests is increasing, and the efforts in research in the field of mangrove forests are increasing. Therefore, in order to cope with the risk of mangrove degradation and disappearance, artificial intervention is urgent. Meanwhile, in order to meet the afforestation and restoration needs of mangroves, it is necessary to develop cultivation technology and breeding improvement. Cultivating mangrove seedlings is crucial to support mangrove ecosystem protection, management, and recovery activities, as well as for scientifically and judiciously planting mangrove plants to reverse the gradual degradation of ecological function.

The production of mangrove plant germplasm and seedlings is the basis of all mangrove ecological restoration projects [8]. Mangrove germplasm is the repository of mangrove genetic resources and acts as the guarantor and source of mangrove genetic diversity [9]. Under natural conditions, the diversity of plant germplasm is generally screened by environmental heterogeneity, resulting in the distribution and abundance of different genotypes [10,11]. Mangroves are limited by climatic conditions, possessing thin species resources. Therefore, the conservation of genetic diversity of mangrove species needs to be considered from different regional environments, including provenance, genealogy, or even individual.

Cultivating seedlings is one of the ways to increase large-scale plant production, playing an important role in the yield and quality of seedlings, germplasm resource preservation, and cost savings. Due to the particularity of mangroves, cultivating mangrove seedlings is a challenging task. The germination, growth, and maturation process of mangrove seeds need to take into account many aspects, including growth media, water salinity, and seed quality [12,13]. The growth media is a crucial factor in seedling cultivation, which can provide the basic conditions for seedling growth and development, and address the issue of damage or death due to poor site conditions in the young age. Many studies have shown that different soil substrates have different effects on seedling growth, and overall growth quality [14,15,16,17]. However, the majority of these studies are found in terrestrial species, but there are few reports on true mangrove species.

Additionally, salinity is also one of the important factors to be considered in mangrove seedling cultivation. Mangroves are renowned plant communities growing in “drinking seawater”. Although seawater can produce salt stress on plants, mangrove plants have evolved unique salt management strategies, such as excluding, secreting, and accumulating, that are well adapted to the hypersaline environment [18,19,20]. In fact, access to fresh water can also boost mangrove productivity [21,22]. Many believe that salt water is an ecological advantage for mangroves, rather than a physiological need [23,24,25]. Contrarily, some people argue that mangroves cannot grow in fresh water for a long time and that mangrove plants store enough salt in their bodies for growth [26,27,28]. Therefore, the cultivation of mangrove seedlings needs to take into account the salinity of the water.

Kandelia obovata Sheue & al. is the most widely distributed typical true mangrove species and is also one of the most commonly used mangrove species in afforestation and restoration [29,30]. For instance, in the high-latitude ecological restoration model, the replacement of Spartina alterniflora Loisel. by K. obovata as the main mangrove species showed promising ecological benefits, as demonstrated by the establishment of a stable community [31]. In previous studies, K. obovata was considered to adapt at 5~25 ppt [32,33], while some scholars recognized its adaptability in fresh water [34]. K. obovata thrives in muddy site types [35], and other scholars propose that sandy soil can also be suitable for K. obovata cultivation [36]. However, there is still limited research documentation available in this particular field of study. In terms of germplasm mining, there have been instances demonstrating growth variations in K. obovata based on latitude or provenance [13,37]. However, there is still a dearth of research exploring the genetic diversity within the genealogies of K. obovata.

Nurseries can produce high-quality mangrove seedlings to avoid the low survival rate caused by a severe habitat environment [38]. However, as far as we know, there is little specific research on the cultivation conditions of mangrove nurseries. Therefore, we chose the K. obovata species and used nursery experiments to find suitable breeding conditions. The main purpose of our work was to evaluate the nursery conditions suitable for producing better growth of K. obovata seedlings. Based on this purpose, we make the assumption that K. obovata may grow in a suitable nursery medium, with the influence of various salinities and germplasm. We conducted interactive experiments and highlighted suitable conditions by evaluating the effects of salinity, growth media, and genealogy for K. obovata cultivation. The results will offer a theoretical and practical foundation for the nursery cultivation technology and germplasm breeding of K. obovata.

2. Materials and Methods

2.1. Experimental Site and Environment

The study was conducted at the nursery base of Guangxi Forestry Research Institute (108°21′24″ E, 22°55′1″ N), designated as a dry land facility nursery [8]. The area experiences a subtropical monsoon climate with abundant light, heat, and year-round rainfall. Throughout the experiment, the average monthly temperature was 22.9 °C, the highest monthly temperature reached 36 °C, and the lowest monthly temperature was 5 °C.

2.2. Plant Material

The plant materials originate from Dianbai District, Maoming City, Guangdong Province (Figure 1, 110°1′37″ E, 21°30′11″ N), where the salinity ranges around 10.3 ppt, with irregular semidiurnal tides. The climate type falls under the tropical monsoon climate category, featuring an average annual temperature of 23.5 °C, a temperature range of 19.6 °C annually, and an average annual precipitation of 1909 mm. In March 2022, three representative maternal trees (i.e., three genealogies) from the local K. obovata populations, named Jiu Xiao (JX), Yi Zhong (YZ), and Er ZhouDao (EZD), were selected (Figure 1). A distance of over 30 m between these genealogies ensured genetic independence.

2.3. Experimental Design and Treatments

A three-factor (salinity, growth media, and genealogy; 3 × 5 × 3) experimental design was utilized in this study. Each seedling bag (size 30 cm × 20 cm) contained one propagule, and 12 plants were repeated in each treatment, resulting in 540 plants.

Salt water was prepared by mixing solarized sea salt and fresh water, creating three salinity levels: S1 (0 ppt), S2 (10 ppt), and S3 (20 ppt). Salinity was measured using a pen-type salinometer (Model 5250, Sanxin Instrument Factory, Shanghai, China). The salt water was stored in containers, and monitoring occurred weekly. Given that the soil in mangrove growth environments is predominantly sandy or sticky [39], this experiment focused on five growth media: M1 (100% loess), M2 (100% sandy), M3 (50% loess + 50% sandy), M4 (40% loess + 40% sandy + 20% peat), and M5 (40% loess + 40% sandy + 20% coir). According to 2.2, propagules of three genealogies (EZD, JX, and YZ) were classified and the propagules without pests were selected, washed, and sowed.

2.4. Measurements

2.4.1. Properties of the Media

In May 2022, cutting rings with a volume of 100 cm³ and a weight of W0 (including bottom net and top cover) were used for sampling randomly, and the process was repeated 3 times. Then weights were recorded in the laboratory after soaking for 6 h (W1), standing for 12 h (W2), and drying in a 105 °C oven (DHG9140A, YiHeng Scientific Instrument Co., Ltd., Shanghai, China) until a constant weight (W3). Soil bulk density (BD), total porosity (TP), capillary porosity (CP), and non-capillary porosity (NCP) were calculated using Formulas (1)–(4). The media pH was determined by the potentiometric method, and total N, P, and K were measured using the elemental analyzer (Elementar EL, Elementar Analysensysteme GmbH, Hanau, Germany), colorimetry, and flame photometer (WGH6431, LiChen Technology Co., Ltd., Linyi, China), respectively. The results are shown in

Table 1. Physical and chemical properties of the growth media.

	M1	M2	M3	M4	M5
BD/g·cm−3	1.46 ± 0.01 ab	1.49 ± 0.02 a	1.50 ± 0.03 a	1.28 ± 0.05 bc	1.25 ± 0.06 c
TP/%	43.40 ± 0.45 b	42.32 ± 0.85 b	44.07 ± 1.28 b	45.50 ± 0.87 b	53.06 ± 0.69 a
CP/%	37.93 ± 0.12 b	33.43 ± 1.14 b	37.22 ± 1.31 b	37.00 ± 0.77 b	43.52 ± 1.04 a
NCP/%	5.48 ± 0.38 c	8.88 ± 0.32 a	6.85 ± 0.16 bc	8.50 ± 0.66 ab	9.53 ± 0.41 a
PH	6.72 ± 0.05 c	8.97 ± 0.39 a	8.51 ± 0.21 ab	7.83 ± 0.2 bc	7.67 ± 0.24 bc
P/g·kg−1	0.24 ± 0.03 bc	0.52 ± 0.04 a	0.18 ± 0.01 c	0.33 ± 0.03 b	0.26 ± 0.02 bc
K/g·kg−1	6.44 ± 0.03 a	3.68 ± 0.69 b	5.65 ± 0.45 ab	5.05 ± 0.37 ab	5.64 ± 0.37 ab
N/g·kg−1	0.47 ± 0.03 bc	0.38 ± 0.01 c	0.42 ± 0.01 bc	0.51 ± 0.01 ab	0.60 ± 0.02 a

Data are given Mean ± Standard error (SE). Different letters indicate significant differences among different growth mediums (p < 0.05). M1:100% loess, M2: 100% sandy, M3: 50% loess + 50% sandy, M4: 40% loess + 40% sandy + 20% peat, M5: 40% loess + 40% sandy + 20% coir; BD: soil bulk density, TP: total porosity, CP: capillary porosity, NCP: non-capillary porosity.

.

$$\mathrm{BD}(\mathrm{g}·{\mathrm{cm}}^{-3})=\frac{\mathrm{W}3-\mathrm{W}0}{100};$$

(1)

$$\mathrm{TP}(\%)=\frac{\mathrm{W}1-\mathrm{W}3}{100}\times 100;$$

(2)

$$\mathrm{CP}(\%)=\frac{\mathrm{W}2-\mathrm{W}3}{100}\times 100;$$

(3)

$$\mathrm{NCP}(\%)=\frac{\mathrm{W}1-\mathrm{W}2}{100}\times 100.$$

(4)

2.4.2. Seedling Height, Diameter and Mortality Rate

After sowing in April 2022, the seedling height (H) was measured every 30 days to determine the length from the upper end to the top end of the stem. A death plant was recorded if the whole shoot was withered or necrotic. From June, the seedling basal diameter (D) was measured every 30 days from the stem at the upper end of the propagule. The plant was recorded as dead if the stem was completely necrotic. The calculation formula for mortality is as follows: mortality (%) = number of dead plants/total number of plants × 100. All the measurements were finished in April 2023. Considering the monthly average temperature changes during various seasons and throughout the experiment, our attention is directed to specific periods: T1 (from April 2022 to July 2022), T2 (from July 2022 to October 2022), T3 (from October 2022 to January 2023), and T4 (from January 2023 to April 2023).

2.4.3. Biomass

In April 2023, three plants were randomly selected for each treatment. The attached soil was carefully washed off as Figure A1, and the plants were separated into roots, stems, and leaves. These plant parts were placed into envelopes and dried in a 105 °C oven for 30 min, followed by further drying at 80 °C until a constant weight was achieved. After cooling in the oven to room temperature, the dry biomass of plant parts was weighed by a 1/100 electronic balance.

2.4.4. Data Processing and Analysis

The fuzzy comprehensive evaluation method of membership index (MI) (5) and Dickson quality index (DQI) (6) were used to evaluate the seedlings comprehensively. The formula for calculating values was as follows:

$$\mathrm{U}\left({\mathrm{X}}_{\mathrm{i}}\right)=\frac{{\mathrm{X}}_{\mathrm{i}}-{\mathrm{X}}_{\min }}{{\mathrm{X}}_{\max }-{\mathrm{X}}_{\min }};$$

(5)

$$\mathrm{DQI}=\frac{\mathrm{Total} \mathrm{biomass}}{\frac{\mathrm{H}}{\mathrm{D}}+\frac{\mathrm{Shoot} \mathrm{biomass}}{\mathrm{Root} \mathrm{biomass}}}.$$

(6)

where X_i represents the determined value of a treatment index; X_min is the minimum value of a specific processing index; and X_max is the maximum value of that processing index (5). Shoot biomass includes the sum of leaf biomass and stem biomass (6). Both values for each treatment index were calculated within groups, and the mean for different treatment groups was then sorted. A higher mean value indicates better performance.

Both the mean and standard error (SE) were calculated. The one-way analysis of variance (ANOVA) was employed to assess statistical significance. In the case of statistically significant main effects, the Tukey test was applied for multiple comparisons. All reported differences were deemed statistically significant at a threshold of p < 0.05. Data collation, statistical analysis, and figure production were conducted using Excel 2016 (Microsoft Inc., Redmond, DC, USA), SPSS version 26 (SPSS Inc., Chicago, IL, USA), and Origin 2016 (OriginLab Inc., Northampton, MA, USA), respectively.

3. Results

3.1. Changes in Seedling Mortality Rate

Salinity (Figure 2a), growth media (Figure 2b), and genealogy (Figure 2c) all had significant effects on the mortality rate (p < 0.05). It can be observed that the death rate increased obviously from July 2022. After one year of observation, the lowest mortality rate was recorded on S2 (17.22%± 6.38%), M1 (4.63% ± 2.02%), and EZD (15.00% ± 5.22%). Conversely, the rate of death increased the most on S1 (29.44% ± 5.07%), M2 (53.71% ± 3.43%), and YZ (28.89% ± 6.80%). Moreover, salinity between S2 and S3 was a little different and both were significantly lower than S1. The medium with a high sand concentration (M2, M3) displayed a significantly higher death rate, where adding peat (M4) or coir (M5) obviously reduced the mortality rate. The various mortality rates between genealogies were relatively stable and followed a specific order (EZD > JX > YZ).

3.2. Seedling Height and Diameter

Variations in growing medium and salinities had a significant influence on the height (Figure 3a) and basal diameter (Figure 3b) of seedlings. A salinity condition of 20 ppt (S3) promoted the height, which reached the highest value at 14.37 cm ± 0.34 cm, and basal diameter in 10 ppt (S2) reached the highest value at 5.08 mm ± 0.04 mm. Salt water had more advantages than fresh water (S2 > S3 > S1). Media had a greater effect on seedling growth (

Table 2. Results of full-effect ANOVA for Kandelia obovata Sheue & al. seedling height and diameter.

Source	F Value
	Height				Diameter
	T1	T2	T3	T4	T1	T2	T3	T4
Salinity (S)	6.119 **	15.657 **	15.724 **	10.863 **	39.433 **	31.214 **	20.735 **	12.347 **
Growth media (M)	42.357 **	39.142 **	26.678 **	17.055 **	54.099 **	35.813 **	24.792 **	15.700 **
Genealogy (G)	15.552 **	2.420	0.985	0.256	21.285 **	7.057 **	2.602	1.415
S × M	5.754 **	5.006 **	5.937 **	5.197 **	2.907 **	1.354	1.742	2.112 *
S × G	1.879	0.404	0.416	0.276	3.968 **	3.948 **	5.191 **	3.388 *
M × G	1.578	3.990 **	3.081 **	2.650 **	2.274 *	3.282 **	2.040 *	1.530
S × M × G	1.497	1.86 *	1.909 *	1.852 *	1.838 *	1.066	0.784	0.660

* and ** represent significant at 0.05 and 0.01, respectively. T1: from April 2022 to July 2022, T2: from July 2022 to October 2022, T3: from October 2022 to January 2023, and T4: from January 2023 to April 2023.

). Loess (M1) was the medium that increased seedling height the most (15.35 cm ± 0.42 cm), ordered by M1 > M4 > M3 > M5 > M2. Similar variations existed for basal diameter (5.29 mm ± 0.04 mm; M1 > M4 > M3 > M5 > M2). JX was the genealogy that grew the best, and the height and diameter were 13.98 cm ± 0.41 cm and 5.05 mm ± 0.63 mm, respectively. However, the genealogy effect on seedling height and diameter was not obvious (p > 0.05).

In the univariate analysis, both salinity and growth media demonstrated highly significant effects on the seedling height and basal diameter of K. obovata (Table 2), and the influence of growth media was higher than salinity and genealogy, which were consistently observed across different seasons. While genealogy only had a significant effect during the early stages of growth. In the context of multi-factor interaction, the interaction between salinity and growth media exerted the most significant effect on seedling height, while the interaction between salinity and genealogy had the greatest impact on basal diameter. This pattern remained consistent across seasonal observations. Seedling height showed significant differences in the interaction of salinity, media, and genealogy at T2, T3, and T4, but not at T1, which was the opposite of basal diameter. In addition, interactions with growth media primarily influenced seedling height, while interactions with genealogy predominantly affected basal diameter.

3.3. Biomass Accumulation

The total biomass was affected by salinity, growth media, and genealogy and reached the highest value at S3 (4.31 ± 0.30 g), M1 (5.59 ± 0.47 g), and EZD (3.87 ± 0.28 g), respectively (Figure 4). The biomass of roots, stems, and leaves exhibited increased with rising salinity. Biomass in S2 and S3 was significantly higher compared with S1. Under the influence of various media, the biomass of K. obovata seedlings followed the order M2, M5, M3, M4, and M1 in ascending order, consistently observed across different plant parts. Genealogies between EZD and YZ exhibited significant differences in the biomass of roots and leaves (order: EZD > JX > YZ), while stem biomass did not differ significantly.

In the analysis of the one-way ANOVA, salinity exhibited the most significant impact on stem biomass accumulation (F = 38.60), while growth media exerted the greatest influence on root (F = 48.93), leaf (F = 27.18), and total biomass (F = 54.75) accumulation (

Table 3. Results of full effect ANOVA for Kandelia obovata Sheue & al. biomass.

Source	F Value
	Root	Stem	Leaf	Total
Salinity (S)	38.544 **	38.604 **	13.288 **	40.357 **
Growth media (M)	48.929 **	34.996 **	27.183 **	54.754 **
Genealogy (G)	3.871 *	0.943	3.621 *	4.267 *
S × M	5.152 **	4.167 **	4.404 **	6.262 **
M × G	3.266 **	1.479	1.419	2.802 **
S × G	2.297	0.969	0.836	1.895
S × M × G	1.379	0.888	1.259	1.087

* and ** represent significant at p < 0.05 and p < 0.01, respectively.

). The interaction between salinity and growth media attained the highest value of F, and this consistency was observed across all parts of the biomass. Salinity and growth media emerged as the primary factors influencing the biomass accumulation of K. obovata in multifactor interaction. There was no significant effect of the three-factor interaction on the biomass accumulation of K. obovata.

3.4. Difference of the Media Properties and Correlation Analysis

As shown in Table 1. TP, CP, and total N showed no significant difference between M1 and M2. The aeration (NCP), pH, and total P of the media were significantly increased by M2, while the total K was decreased (p < 0.05) in comparison to M1. Compared with M3, the peat (M4) or coir (M5) additions obviously decreased BD, and M5 had considerably higher porosity levels than M4 and M3.

Correlation analysis results indicated a significant negative correlation between seedling growth and NCP, pH of the media, and a significant positive correlation between seedling growth and K content. The mortality rate of seedlings was significantly positively correlated with pH, while negatively correlated with the content of K and N. All significant correlation coefficients were above 0.5 (Figure 5).

3.5. Evaluate the Influence of Growth Media, Salinity, Genealogy, and Their Combination

The MI and DQI were employed to assess the seedling effect of each treatment combination, and a similar pattern was observed for the evaluation results (

Table 4. Membership index (MI) and Dickson quality index (DQI) evaluations for different treatment combinations.

Treatment		MI					DQI
Salinity	Media	EZD	JX	YZ	Average	Rank	EZD	JX	YZ	Average	Rank
S1	M1	0.548	0.390	0.330	0.423	9	1.364	0.700	0.632	0.899	9
	M2	0.125	0.115	0.174	0.138	15	0.480	0.279	0.382	0.380	15
	M3	0.280	0.219	0.097	0.198	13	0.966	0.524	0.393	0.628	13
	M4	0.572	0.423	0.515	0.504	6	1.159	0.971	1.282	1.137	6
	M5	0.221	0.227	0.206	0.218	12	0.583	0.580	1.123	0.762	11
S2	M1	0.923	0.878	0.784	0.862	1	1.862	2.123	1.694	1.893	1
	M2	0.179	0.074	0.197	0.150	14	0.424	0.687	0.602	0.571	14
	M3	0.560	0.390	0.465	0.471	7	1.211	1.161	0.936	1.103	7
	M4	0.484	0.664	0.483	0.544	5	1.097	1.716	1.212	1.342	5
	M5	0.427	0.489	0.436	0.451	8	1.362	0.691	0.798	0.950	8
S3	M1	0.769	0.932	0.636	0.779	2	1.607	1.931	1.391	1.643	2
	M2	0.252	0.240	0.180	0.224	11	0.603	1.051	0.604	0.753	12
	M3	0.690	0.547	0.484	0.574	4	1.577	1.308	1.160	1.348	4
	M4	0.471	0.773	0.528	0.591	3	1.282	1.719	1.131	1.377	3
	M5	0.378	0.428	0.432	0.413	10	0.762	0.667	0.901	0.777	10
Average		0.459	0.453	0.396			1.089	1.074	0.950
Rank		1	2	3			1	2	3

). In terms of fuzzy evaluation (MI), S3, M1, and EZD obtained the highest values of 0.516, 0.688, and 0.459, respectively. Combinations of S2-M1 and S3-M1-JX recorded the highest values of 0.862 and 0.932. Similarly, in quality evaluation (DQI), S3, M1, and EZD also displayed the highest values of 1.179, 1.478, and 1.089, respectively. And the combinations of S2-M1 (1.893) and S2-M1-JX (2.123) achieved the highest values. These results suggest that S3, M1, and EZD have consistently excelled in both relative and absolute evaluation measures. Furthermore, the combinations involving these treatments also yielded highly favorable results.

4. Discussion

Seawater is a special component of the mangrove habitat, with fluctuating salinity levels that influence the growth and development of mangrove plants. While the debate continues regarding whether mangrove plants require or tolerate salt, salinity has undoubtedly emerged as a key factor in cultivating mangrove plants. Earlier investigations have indicated that the optimal salinity for mangrove plants, including K. obovata and Avicennia marina (Forssk.) Vierh., is 50% or less of the standard seawater [40,41,42]. In this study, salt water proved beneficial in reducing the death of seedlings, promoting height and diameter growth, and enhancing biomass accumulation. Concurrently, ANOVA analysis suggested that salinity had a high influence on seedling growth and biomass accumulation. This suggests that the growth of K. obovata seedlings in a freshwater environment was inferior to that in a saltwater environment. According to a study by Tang [34], the survival rate of K. obovata in fresh water was 61.9%, aligning with the finding of 70.6% in this study. Moreover, the growth parameters of 10 ppt and 20 ppt salinities were found to be similar in this study. However, in another study by Liao [32] and Lv [43], 20 ppt was significantly lower than 10 ppt in the observation of seedling height, diameter, and total biomass. In fact, this may be affected by the growth media or other factors. For example, in Table 4, 10 ppt had the highest DQI or MI in loess, while 20 ppt had the highest DQI or MI in sand. Additionally, we found the death of seedlings was obviously increasing during T3 (October to January) under freshwater irrigation. It may be caused by a drop in temperature [44]. The salt content relative to fresh water acts as a natural “antifreeze” and is more advantageous for mangroves. Hence, this study has once again demonstrated that mangrove growth requires salt water to some degree; however, other factors may also have an impact, leading to varying adaptive salinity ranges, which will require additional investigation in future studies.

Growth media plays a pivotal role in plant seedling growth, and an appropriate media can furnish container seedlings with an optimal growth environment [45]. In this study, significant differences among growth media were observed in the growing of K. obovata seedlings. Seedlings exhibited a low death rate and positive growth in loess but the opposite in sand. However, previous studies have suggested that soil with a high sandy content enhances the growth and development of K. obovata seedlings [35,46]. This difference may be attributed to various experimental durations. In our study, the 12-month cultivation may result in K. obovata seedlings lacking nutrition in sand soil (Figure A1). Furthermore, the incorporation of the organic media decreased soil bulk density and reduced the death of K. obovata seedlings. Regarding growth and biomass, the growth performance of seedlings in peat-containing growth media outperforms that of seedlings in coir. Peat has higher mineral nutrient content [47], and peat can be recommended as the preferred choice for cultivating K. obovata with an organic media. Moreover, being grown in a freshwater environment, M4 seedlings possess the highest quality available. It may be attributed to the beneficial effects of nutrient additions, such as soil fertilizer, on the growth of mangrove plants in fresh water [48]. Consequently, in the process of raising K. obovata seedlings, an appropriate growth media can be selected based on diverse conditions and requirements. Moreover, in the correlation analysis between soil properties and seedling mortality, growth, and biomass, significant relationships were found with potassium, pH, and non-capillary porosity. These findings serve as important indicators for the direction of future research work. To put it briefly, plant media is essential for sustaining plant growth, but it also needs the right kind of assistance from other elements. The study’s findings offer a theoretical basis for choosing the right material for K. obovata nursery cultivation. Using conventional growth media for planting mangrove seedlings can reduce the use of mudflat sludge and human activities in mangrove habitats. It still needs some future investigation of the influence of diverse physical and chemical properties, as well as trace elements for planting mangrove seedlings in the growth medium.

Varied populations, encompassing provenances and genealogies, contribute to differences in the growth and development of seeds and seedlings. These variations are typically influenced by genotype, environmental conditions, and other factors. Previous studies have revealed substantial differences in phenotypes among various provenances of K. obovata [13,49]. In this study, as the mortality rate rises, various genealogies display gradual distinctions. Distinct genealogies exhibit varied advantages in basal diameter, seedling height, and biomass, indicating the potential for germplasm mining within K. obovata genealogies. Based on the evaluation results, EZD and JX are superior in growing, possibly because they have stronger parent trees. Additionally, plants may exhibit varying salt tolerance across different provenances [50,51]. In this study, the membership value of genealogy JX is highest at 20 ppt salt water, differing from that of genealogies EZD and YZ at 10 ppt salinity. This suggests potential genetic differences in salinity adaptability among different genealogies. Three genealogies of K. obovata seedlings, on the other hand, were thought to come from the same provenance and have similar initial environmental conditions; all plants flourished in a uniform garden setting. Therefore, the differences in their growth primarily stem from genetic variations among the original genealogy and the interactions between these genetic factors and their growth environment [52]. In summary, when selecting suitable germplasm resources for cultivation, it is essential to consider not only the impact of environmental heterogeneity but also the diversity of genetic factors among individuals. Therefore, this work presented the evaluation for these three genealogies, which might serve as a theoretical reference for local mangrove K. obovata germplasm resources screening and collecting. It is necessary to increase the collection of genealogies number and further studies from the physiological or molecular in subsequent studies.

5. Conclusions

Our study evaluated the strategy of cultivating K. obovata seedlings from the aspects of water salinity, growth medium, and genealogies in order to obtain high-growth and high-quality seedlings. All growth indexes and evaluation results showed that (1) seedlings with 10 ppt or 20 ppt salt water were significantly better than fresh water (0 ppt); (2) The medium of loess produced the best seedlings, though peat added was also a good option; (3) EZD was the genealogy growing the best, followed by JX. Therefore, we recommend using loess as a growth medium (or added peat) in cultivating K. obovata, while fresh water and pure sand should be avoided. In addition, it is imperative that we also emphasize the collection and development of genealogy resources. Generally speaking, these results can be used for mangrove nursery cultivation, providing a theoretical reference for the artificial cultivation of K. obovata and aiding in the identification of potentially valuable germplasm resources. Future research can focus on exploring raising other mangrove species and integrating and optimizing different germplasm resources. This will contribute to the efficient and high-quality development of mangrove nurseries.