Effects of Abiotic Stresses on Horticultural and Cereal Crops at Physiological and Genetic Levels

1. Introduction

Various abiotic stresses (e.g., salinity, heat, cold, drought, waterlogging, and heavy metals) induced by climate change and anthropogenic activities are major constraints for food security, as they can cause significant yield reductions in crops and vegetables [1,2,3]. Among these stresses, salinity is one of the most severe stresses limiting plant productivity due to increased land salinization [4,5,6]. Extreme temperatures can also adversely affect crop growth, development, and yield via physiological and biochemical regulation [7,8]. Photosynthesis is one of the most sensitive processes to abiotic stress due to its high sensitivity to environmental changes. Notably, several abiotic stresses coincidentally occur, especially in natural conditions like fields [9,10]. For instance, heat stress is usually accompanied by drought stress due to increased water evaporation of plants, resulting in combined heat and drought stress [7]. The inhibited photosynthetic rate accompanied by ROS (reactive oxygen species) accumulation are the common responses in plants across multiple stress types. Specific responses of different plant species to various abiotic stresses are observed, which become a major problem especially for horticultural crops, as these crops contain a large number of different species. Moreover, valuable strategies to alleviate the adverse effects of abiotic stress on crops are urgently needed. Our Special Issue not only explores the morphological, physiological, biochemistry, metabolic, and genetic regulation of horticultural and cereal crops responding to abiotic stresses, but also provides potential valuable strategies for mitigating the adverse effects of abiotic stress on plants. This Special Issue enriches knowledge concerning the effects of abiotic stress on various horticultural and cereal crops.

2. Responses of Horticultural and Cereal Crops to Abiotic Stresses

Generally, various abiotic stresses can inhibit plant photosynthesis, induce ROS accumulation, and cause oxygen damage, which are regulated and feedback-regulated by the expression of relevant key genes. This Special Issue mainly focused on different horticultural crops, including not only the model plant-tomato, but also pomegranate, peach, watermelon, Lespedeza, sweet cherry, and spinach, etc. The main findings on the specific horticultural crop species in this Special Issue confirmed the previous results on other plant species.

2.1. Plant Responses to Salinity Stress

SOSs (salt overly sensitive), key genes in SOS pathway, are closely associated with salt tolerance regulation in plants. Huang et al. (2024) characterized the tomato SOS gene family and found that salt stress increased the expression levels of most SOS genes in the leaves of ‘LA0716’ (Solanum pennellii L.), ‘LA2093’ (Solanum pimpinellifolium L.), and ‘Heinz 1706’ (Solanum lycopersicon L.) (contribution 1). In addition, the expression levels of SOS1-1, SOS1-2, SOS2-2, SOS3-3, SOS4-1, and SOS5-2 in leaves of salt-tolerant tomato (‘LA0516’ and ‘LA1598’) were higher than that of salt-sensitive tomato (‘LA1698’ and ‘LA0012’) in response to salinity (contribution 1). Tang et al. (2024) demonstrated that change trends of SOD (superoxide dismutase), POD (peroxidase), proline, and total soluble content were different in pomegranate (Punica granatum) between low- and high-salinity stress after three days (contribution 2). Transcription factors (TFs) played a vital role in plant defense against stress by regulating expression of relevant genes [11]. Except for physiological parameters, Tang et al. (2024) identified 6571 DEGs (differentially expressed genes) containing 374 TFs (e.g., AP2/ERF, HD-ZIP, MYB, and WRKY) in pomegranate leaves at low (0.5%) and high (0.8%) salt for one and two days using transcriptome sequencing (contribution 2).

2.2. Plant Responses to Waterlogging and Water Deficit

Zhang et al. (2024) analyzed plasma membrane H⁺-ATPase family genes (PPA) and detected expression of key gene in peach (Prunus persica), showing that waterlogging significantly increased the expression of PPA1, PPA2, and PPA5 but decreased the expression of PPA15, PPA19, and PPA26 (contribution 3). Yin et al. (2023) showed that there was significant correlation between leaf H₂O₂ and malondialdehyde content and tomato plant yield, which could be applied to predict tomato plant production during waterlogging stress [12]. Clarifying the specific effects of water on plants at different stages, especially at anthesis and fruiting stages, is important for crop production, as the plants at the reproductive stage were usually more sensitive to environmental changes. In this Special Issue, two articles elucidated the morphological and physiological responses of plants to drought during the reproductive period. Silva et al. (2024) found that short-term drought mainly reduced photosynthesis but did not affect yield by treating watermelon (Citrulus lanatus Thumb. Mansf.) at the fruiting stage with drought for four and eight days (contribution 4). Yasin et al. (2024) investigated the growth and morphological parameters of three maize genotypes (‘P-3011w’, ‘P-3092’, ‘iku20’) at pre-flowering stage under drought stress (40% soil moisture content) (contribution 5). Similarly, drought adversely affected photosystem II (PSII) as shown by decreased photosynthetic chlorophyll fluorescence and the net photosynthetic rate, with ‘iku20’ as the most drought-tolerant genotype (contribution 5).

2.3. Responses of Plants to Heavy Metal Stress

Aluminum (Al) is one of the major heavy metals hindering plant growth, especially in the pollution area [13]. To clarify the tolerance mechanism of Lespedeza root to Al, Sun et al. (2023) analyzed sugar changes in root systems exposed to Al, where Al-tolerant Lespedeza bicolor could convert sugars into large amounts of organic acids and, thereby, reduce Al toxicity (contribution 6). Moreover, less Al can be explained by less pectin accumulation in cell walls of root tips, contributing to Al-tolerance of Lespedeza bicolor (contribution 6).

3. Valuable Strategies for Mitigating the Negative Effects of Abiotic Stresses

Rapid population growth and abiotic stress caused by extreme weathers threatens global food security [14,15]; thus, the development of new strategies to alleviate abiotic stress damage on plants is necessary and urgently needed. Several studies in this Special Issue attempted the strategies of spraying exogenous additives, applying rational nitrogen (N) fertilizers, and regulating the light environment, etc., to alleviate the adverse effects of abiotic stresses.

Sweet cherries (Prunus avium L.) suffer a significant drop in yield due to spring frosts with the co-occurrence of fruit cracking (contribution 7). Ruiz-Aracil et al. (2024) found that pre-harvest putrescine treatment was an effective way to mitigate susceptibility of sweet cherry buds to frost, which reduced cracking rate and enhanced the frost tolerance of sweet cherries (contribution 7). Pinciroli et al. (2023) found that exogenous salicylic acid attenuated oxidative damage associated with lipid oxides in rice, thereby, improving rice yield and quality characteristics (contribution 8). High nitrate stress (150 mM NO₃⁻) adversely affected the development of spinach (Spinacia oleracea L.), with photosystem damage caused by ROS accumulation, leading to reduced photosynthesis (contribution 9). Fulvic acid (FA) treatment increased biomass accumulation, enhanced chlorophyll content and photosynthetic capacity, improved activities of antioxidant enzymes and decreased ROS accumulation in spinach at high nitrate stress, with the most significant positive effects by 0.15% FA (contribution 9). Potassium phosphite (PP) alleviated the adverse effects of combined water deficit and high light on soybeans via enhancing photosynthetic rate, maintaining cell membranes stability, activating antioxidant enzymes and increasing proline concentrations (contribution 10).

Two studies in our Special Issue examined the mitigation effects of N fertilizer application on tomatoes when recovering from high temperatures. First, Li et al. (2023a) found that suitable N fertilization (2.6 g/plant) alleviated heat damage and improved the heat tolerance of tomato plants (contribution 11). By comparison, excessive N fertilization (3.75 g/plant) aggravated heat damage on tomato plants (contribution 11). Similarly, Li et al. (2023b) demonstrated that different N supply significantly affected chlorophyll a fluorescence curve in tomato plants recovering from high temperatures (contribution 12). Suitable N supply increased the number of PSII-active reaction centers and improved the capacity of electron transfer in tomato leaves during the heat recovery period (contribution 12).

In addition, the light environment, like red/far-red ratio, mediated the plants’ development process [16]. Based on physiological and metabolomic analyses, Miao et al. (2024) showed low far-red light ratio (0.7) could promote the tricarboxylic acid cycle in tomato fruits through metabolite accumulation and, thus, improve fruit quality at salt stress (contribution 13). Ramezani et al. (2023) indicated that 2 h of supplementary blue light with 4 mg/L selenium and iodine enhanced the physiological and metabolic characteristics of fenugreek (Trigonella foenum-gracum L.) (contribution 14). Tola et al. (2024) found that tomato plants with ‘Vivifort and Beaufort’ as rootstocks can be cultivated in a hydroponic system at >6.0 dS m⁻¹ salt concentration with no negatively affecting tomato production quality (contribution 15).

4. Perspectives

In the future, more attention will be needed to study the effects of combined stress, including different abiotic stress combinations and even abiotic and biotic stress combinations, which can be regarded as new stress states. Molecular regulatory mechanisms of plants at both single and combined stress remains to be elucidated. More importantly, the effects of stress priming can be a promising approach to enhance the resilience of crops and vegetables to environmental changes. How stress priming mediates the formation of stress memory and enhances stress tolerance needs further investigation, during which the key regulatory module is expected to be found.