1. Introduction
Cucumber (
Cucumis sativus L.) is an important horticultural crop, since it is one of the major vegetables grown in greenhouses worldwide [
1]. Although cucumber is a thermophilic vegetable crop, it is sensitive to heat stress [
2]. The production of cucumber in greenhouses faces some abiotic stresses, particularly in arid regions, due to excessive application of mineral fertilizers, use of brackish water, soil salinization, and high temperatures in the summer [
3,
4]. These stresses are expected to be intensified because of climate change; under these conditions, the resulting imbalances in plant hormones, nutrition, and physiological disorders may also contribute to an increase in biotic stresses [
5].
Abiotic stresses cause significant losses in global crop production. Several abiotic stresses have been studied in cucumber, including alkalinity stress [
6], drought [
7], salinity [
8,
9], and heat stress [
2,
10]; however, these studies have focused on a single stress, while global environmental changes cause multiple stresses in various combinations [
11,
12]. To the best of our knowledge, no studies have investigated the impact of multiple stresses, in particular, the combined effect of soil salinity and heat stress on the yield of cucumber plants in arid regions.
Plants, as sessile organisms, have gradually developed defense systems to modulate environmental stresses [
13]. In addition, several agricultural practices have contributed to adaptation to environmental stresses [
14]. One of the approaches used in agriculture to overcome abiotic stresses is grafting [
15]. Grafting is considered to be one of the most important agronomic techniques, which could save time and costs in breeding programs [
16]. The most common grafted vegetables include watermelon, melon, tomato, cucumber, pepper, and eggplant [
17]. Grafting of cucumber plants is a common practice, by which the tolerance to stresses of cultivars used as scion are improved using rootstocks that are tolerant to abiotic stresses such as salinity [
8,
18,
19], water stress [
20], or heat stress [
2]. In addition, grafting of cucumber plants may also increase plant vigor and enhance the uptake of water and nutrients [
21].
Soil salinity is considered to be one of the most important abiotic stresses, with a global increase of 26% from 2000 to 2016 in areas affected by soil salinity, resulting in over 1 billion hectares currently affected, mostly situated in arid regions [
22]. In cucumber plants, salinity stress causes a growth reduction, toxicity in the leaves, and increases membrane permeability and electrolyte leakage, and therefore, results in a reduction in both the yield and the quality of fruit [
23,
24,
25]. Heat stress is an important global issue, with over 1.7 billion hectares presently affected [
26], and a steady increase in areas affected due to rising atmospheric temperatures. The increased greenhouse production of cucumbers during the summer in warm arid regions has potentially contributed to increasing the occurrence of high temperature stress. In cucumber plants, heat stress induces important changes in gene expression [
27], as well as negative effects at the physiological level, including a reduction in photosynthetic functions, a reduction in water and nutrient uptake, and increased respiration [
28], which in turn impact growth and yield. In agriculture, combined stresses frequently occur [
29]; in this regard, high temperatures often coexist with soil salinity in several areas worldwide [
30]. Despite the importance of combined salinity and heat stresses in vegetables, there is a lack of studies on cucumber plants’ responses to this combination of stresses and on the potential of grafting for overcoming the impact of stresses on this crop. Apart from the evaluation of growth and development traits, several antioxidant enzymes, such as catalase and peroxidase, play a main role in reducing oxidative damage caused by stress [
31]. In addition, osmolytes such as proline are known to be involved in maintaining the osmotic balance under salinity stress [
32].
The aim of this study was to evaluate the growth, yield, physiological, biochemical, and mineral composition parameters in cucumber plants grafted onto five cucurbit rootstocks grown under combined salinity and heat stresses. The results obtained provide relevant information for improving the production of cucumber under these conditions.
4. Discussion
The establishment of net houses and greenhouses for vegetable production in arid regions of the world is increasing. For example, in Egypt, it is increasing rapidly under a national project to build nearly a million greenhouses as part of the sustainable development strategy of 2030. The intensive cultivation of cucumber plants, and other vegetables, in net houses and greenhouses in these areas is threatened by biotic and abiotic stresses that may lead to a reduction in crop productivity and quality. The production of greenhouse cucumbers in arid environments, especially in the summer, is considered to be a significant challenge, particularly in developing countries, where low-cost structure greenhouses are common. Cultivation under these conditions is often exposed to both saline soil conditions and heat stress [
41,
42].
We conducted a comprehensive evaluation of the response of the grafted net house cucumber plants to salinity and heat stress by measuring vegetative growth parameters, yield, fruit quality, and physiological and biochemical parameters, which included chlorophyll content, catalase and peroxidase enzymatic activities, proline contents, electrolyte leakage, and the mineral composition of leaves in both the grafted and ungrafted cucumber plants. For many traits, significant differences among treatments were observed in both years, while in others, there were differences only in one year. Repeating the experiment in two different years facilitated the identification of stable responses.
The grafting process has a crucial role in ameliorating vegetative growth under salinity and heat stress. For example, salinity stress limits cucumber growth by decreasing the photosynthetic rate and reducing the stomatal conductance, both of which are improved by grafting [
43,
44]. A possible explanation for this may be that the grafted plants are more vigorous and have powerful root systems, resulting in a higher water and nutrient uptake rate, greater leaf area, and higher net assimilation rate of CO
2 as compared with the ungrafted plants [
45]. These results provide further support for the hypothesis that the yield of net house cucumber plants under salinity and heat stress could be improved by grafting [
8].
Stressed plants have different defense strategies; enzymes such as catalase, peroxidase, and superoxide dismutase, and non-enzymatic antioxidants such as proline, ascorbic acid, and glutathione, are amongst the most common defense mechanisms of plants against stresses. Other approaches for improving plant tolerance to stress include genetic engineering, cultivating stress-tolerant/resistant cultivars, exogenous application of soil amendments, mineral nutrients, microbes, osmolytes, and proper agricultural practices such as grafting. The growth of grafted net house cucumber plants is limited by soil properties (mainly pH, cation exchange capacity (CEC), salinity, and the availability of nutrients), temperature, and rootstock characteristics. As we observed, grafted cucumber plants may lead to a higher biomass as compared with ungrafted plants. Concerning the mechanism for nutrient uptake and transport in grafted vegetables such as the cucumber, the mechanism is mainly controlled by communication pathways between rootstock–scion interactions, long-distance signaling, and by the rootstock genotype in grafted plants [
46]. These factors may enhance grafted cucumber tolerance to nutrient toxicity or deficiency stress. We observed that grafting onto some rootstocks activated and increased catalase and peroxidase activities, while electrolyte leakage was reduced, suggesting an adaptation to the stressful conditions. Our results are in agreement with recent studies indicating that grafting can enhance the growth of net house cucumbers under salinity or heat stress [
27,
47], although no study has tested both stresses together.
Chlorophyll allows plants to absorb energy from light by producing essential nutritional compounds through photosynthesis. The availability of sufficient nutrients, water, atmospheric CO
2, and light are required compounds for photosynthetic assimilation. Photosynthesis can be restricted because of water deficit or heat stress in arid environments, which decreases the stomatal conductance and the net photosynthetic rate. The chlorophyll content in cucumber plants has been shown to increase because of grafting under salinity, which also enhances high photosynthetic activity [
43]; these findings are in agreement with our results. Ungrafted cucumber plants can suffer from a reduction in uptake of water and nutrients, which reduces water conductivity, transpiration, and the photosynthesis rate [
48]. This suggests that grafting onto appropriate rootstocks can contribute to higher photosynthetic activity under combined salinity and heat stress.
Under salinity stress, many biological processes in cucumber plants are affected, ranging from photosynthetic capacity to biochemical activity [
8]. These biological activities disrupt the normal metabolism of cucumber leaves and reduce water and nutrient uptake because of oxidative stress, which is the result of excessive Cl
- and Na
+ concentrations. The results we obtained match those observed in earlier studies [
18,
21,
25] that indicated a better physiological and nutritional status in grafted plants. This is probably linked to the stronger root systems in grafted plants, which allow for the greater uptake of water and nutrients, as compared with ungrafted plants under soil salinity stress. In addition, grafting may improve nutrient uptake and translocation from root to shoot, increase leaf water content and photosynthesis capacity, and reduce Na
+ and/or Cl
− in shoots as compared with ungrafted plants under saline conditions [
21]. Cucumber growth is mainly controlled by the interaction of root temperature and nutrients [
49], and therefore, grafting on an appropriate rootstock may contribute to better growth by having a stronger root system.
This study confirmed that the negative impacts of heat stress on net house cucumber growth, as also reported by Ali et al. [
10], which include reductions in vegetative growth, chlorophyll content, antioxidant enzyme activities, yield, and quality, can be mitigated by grafting. The productivity of net house cucumber plants is affected by many factors, such as the environmental conditions (e.g., soil salinity, acidity, temperature, and the bioavailability of nutrients) and the rootstock. Some recent studies have investigated the effects of soil fertility [
50], soil microbial communities [
51], nutrient balance, and changes in soil [
3] on greenhouse cucumber production. Higher temperatures as a result of climate change are projected to increase soil salinization, and therefore, the occurrence of soil-borne diseases in cucumber greenhouses, as expected in environments with increased atmospheric temperature [
36]. Soil salinization may also be accelerated by excessive fertilization and irrigation, which are common practices in greenhouse production systems. Previous studies on the impact of salinity stress on grafted cucumber plants [
8,
18,
21,
52] have confirmed an association between salinity stress and a reduction in most physiological activities of cucumber plants (e.g., photosynthetic pigments activity, chlorophyll content, yield, and quality). Grafting could protect chlorophyll from reactive oxygen species (ROS) under salinity stress, which causes a disruption in the fine chloroplast structure and in chlorophyll stability, resulting in the oxidation of chlorophyll [
8]. Our results reveal that VSS-61 F1 and Ferro, which are interspecific hybrids of
C. maxima ×
C. moschata, are promising rootstocks for the grafting of cucumber plants under combined heat and salinity stress conditions, because they conferred better vegetative growth parameters to the scion, and resulted in a higher marketable fruit yield and good fruit quality. The plants grafted onto rootstock VSS-61 F1 had higher catalase activity and proline content in the leaves, which may contribute to increased tolerance to both stresses [
30], while those grafted on Ferro also had higher levels of silicon in the leaves, which can play a role in better adaptation to the saline stress [
23].