The study revealed that under drought stress there was a substantial decrease in length, fresh weight and dry weight of the roots for both varieties. Research has shown that with sufficient moisture root growth increased and that by going away from the optimal amount of moisture root growth decreased. Asang et al. (1998) reported a decrease in root growth and growth limitations in upper layers of the soil due to water deficit. In low irrigation, less moisture is around the root. This results in mechanical resistance of the soil against root development and, as a result, a reduction in the length and density of the root in common irrigation treatments. With sufficient irrigation, water is more reserved in the root area and the plant by condensing its roots make better use of water. As a result, acceptable use of water in this treatment was increased relative to low irrigation treatments. Plant growth is dependent on the supply of essential carbohydrates to stems and shoot. Factors limiting photosynthesis like light and water, in addition to decreasing plant function also decreases root growth. This is the main reason for the observed difference in the dry weight of the root in different treatments. Plants in dry environment prefer to deposit its photosynthetic production in the root and not in the stems and shoots as the plant can preserve its ability to absorb more amounts of soil water (Assenge et al. 1998). Tomato is susceptible to drought stress, and therefore, when applying stress, its vegetative growth and function decrease. Miguel and Francisco (2007) also reported a reduction in root growth, fresh weight and dry weight in tomatoes, which is comparable to the results of this study.
Plant growth under stress usually depends on roots ability to absorb water from the soil and transferring it to stems (Navarro et al. 2008). The intensity of root growth affects the shoot of a plant. Root length is an index for absorbing water from deep layers of soil (Franco et al. 2011).
Based on this study's results, drought stress caused a reduction in leaf surface in both CaljN3 and SuperstrainB varieties. In Farooqi et al.'s (1999) studies on Martini symbopogon, leaf area decreased in comparison to the control sample. Drought stress decreases leaf growth, and leaf area in a lot of species (Farooq et al. 2009). The production and expansion of leaves are very susceptible to water deficit. The reason for this is the essential need for cellular division and growth to turgor the pressure of cells which stimulating power is water (Pagter et al. 2005). Drought has a profound impact on the growth, production, and quality of the plant. The first impact caused by stress is a loss of turgor pressure, which affects the speed of cell expansion and final cell size. Turgor is a process which is very susceptible to drought stress. Its absence caused by stress will cause a reduction in the speed of growth, reduction of stem length, reduction of leaf expansion, and a reduction in stomata pores (Kumar and Purohit, 2001). Reduction of leaf growth induced by drought stress could be considered an adaptation response. Drought stress restricts leaf area and ultimately transpiration (Sikuku et al. 2010). Results from this study showed that drought stress caused a reduction in height, fresh and dry weight of shoot in both CaljN3 and SuperstrainB varieties. Hoseini et al. (1389), Jaafarabad et al. (1389), Mohammadi et al. (1390), and Dehghan et al. (1394) also reported a reduction in length, fresh and dry weight of shoot in tomato, which is comparable to the results of this study. In Mentha piperita (Simon & Alkire 1993), and Ocimum basilicum (Saleh & Reffat 1997), results similar to our study has been reported.
On the other hand, in the control and under stress conditions, shoot dry weight of in susceptible varieties was lower relative to tolerant varieties, which can be used as an index for selection of susceptible and tolerant varieties. The typical reaction of a plant to drought stress includes reducing stem growth and the size of the whole plant (Munns 2002). A decrease in leaf area causes a reduction in the receival of light and photosynthesis (Ourcut and Nilsen, 2000). Researchers also reported that water stress causes a reduction of shoot dry weight in plants and is reported in Vigna radiate, Cicer arietinum, Glycine max, Medicago sativa and Trifolium subterraneum (Mrema et al. 1997).
A reduction in photosynthesis, increased production of inhibitory substances, and a reduction in the levels of hormones during drought stress are possibly some of the reasons for decreased growth and shoot weight (Hayat & Ahmad, 2007). It is expected that under water deficit conditions, the absorption of nutritional substances decreases, transpiration is reduced which closes the stomata thereby preventing the entrance of carbon dioxide and reducing photosynthesis. These events cause a reduction in the growth and expansion of shoots in the plants. Water deficit causes a reduction of root and shoot growth. The reduction for the shoot is however greater than that of the root (KirnaK 2001).
The level of production of fresh and dry matter of plants has a strong co relation with leaf area and absorbed light. A reduction of each one of these indexes can reduce the fresh and dry weight of the plant. Continuous reduction of water in the soil, causes a reduction of leaf size and surface, and as a result, reduces the dry matter of the shoot (Wanldron et al. 1987).
Anatomical changes can occur in plants under water deficit. Some of these changes include increased lignification or suberin deposition in the cortex, endoderm cells, and cell layers that are near to cortex and medulla. Such changes protect cortex cells from death and drought (Mmostajeran et al. 2008).
In this study vessel diameter decreased for both tomato varieties which is comparable with the results of Claudio et al. (1998) in Vitis vinifera and also Babu et al. (2001) in Oryza sativa.
Reduction of vessel diameter, which is caused by an increase in lignification, shows the adaptability of a plant to stress conditions and the prevention of water wastage. Increased thickness of the transverse wall of vessels and a reduction in the diameter of the vessels, allows water to run through the vessels with greater speed. Other researchers who have studied the effects of drought and salinity stress have also reported a reduction of dermal parenchyma and a reduction of vessel diameter. The development of secondary structure is a kind of immune response of plants against stressful conditions and inappropriate environments. It has been observed that the tonality rate of lignified areas is much lower than that of the control plants, which can be as a result of increased polymerization of the lignin component. The number of vascular elements in the root, leaf and stem of treated plants tended to increase. A decrease can be justified and may serve as a mechanism for more water transportation and lower water loss. The number of layers and root volume of dermis cells in drought stress plants for both varieties increased as compared to the control plants. Cell shriveling was also observed. Dishevelment of dermal parenchyma cells in growing roots under drought conditions is a common observation in plants (Pena- Valdivia et al. 2010).
Tissues placed in water deficit conditions usually demonstrate a decrease in cell size and amount of vascular tissues. Cell wall thickness also increased. Under these conditions, processes corresponding to cell elongation are more vulnerable compared to processes related to cell division (Claudio et al. 1998).
In this study the number of parenchyma cells under the midrib decreased in both tomato varieties. The length of palisade parenchyma cells also decreased, which is comparable with the results of Eydie Najaf Abadi & Enteshari (1389). The space between spongy parenchyma cells seems to be beneficial for the prevention of water wastage. Blade thickness did not show any change in the tomato varieties studied. In a study of the effect of water stress on some varieties of avocado (Americana Persea), it was shown that palisade parenchyma and the total leaf thickness was lower than that of the control plants with an observable 35–45 percent reduction in intercellular space (Chartozoulakis et al. 1999). Reduction in blade thickness, palisade and spongy parenchyma in some species of Acacia auriculiformis under water deficit stress was reported by Liu et al. (2004). A leaf is considered a responsive organ to environmental conditions (Nevo et al. 2000) and among environmental factors that could potentially affect the structure of a leaf, certainly drought stress is one of the most important ones (Nardini et al. 2005). Changes in leaf anatomy in plants under stress could be related to reduced transportation via the stomata. A reduction of leaf expansion could be related to different mechanisms such as reduction in cell division (Granier et al. 2000), hardening of cell wall (Neumann 1995), or reduction of turgor pressure (Bouchabke et al. 2002).
Based on the results of this study, drought stress did not have a significant effect on the expression of the CAT1 gene in both tomato varieties. Relative gene expression was the same in CaLjN3 and SuperstrainB varieties. In some plants, changes in antioxidant enzyme function is a mechanism utilized by the plant to increase plant tolerance against stress. Several reports have documented that drought stress, high temperature and salinity causes an increase in SOD, APX, CAT and GR activity in tolerant wheat genotypes (Sairam et al. 2001). The level of antioxidant enzyme activity and levels of antioxidants during drought stress, is variable between plant species and even varieties of the same species (Bacelar et al. 2006; Bacelar et al. 2007). Changes in expression of the catalase enzyme during stress is dependent on the species. In some species an increase is observed whereas in other species there is a decrease (King et al. 1992).
In this study, expression of the gene CAT1 in drought stressed plants of the CaLjN3 variety did not increase when compared to control plants. Increased expression of the catalase gene in drought stressed tomatoes have been reported, which is comparable with the results of this study (Daneshmand et al. 2014). In rice seedlings water deficit stress has been found to increase the expression of all the enzymes that delete active oxygen, including CAT (Srivalli et al. 2003). Increased catalase function has been reported in Cathar anthus under salinity stress (Jaleel et al. 2007) as well as in Sesamum indicum (Koca et al. 2007). Decreased catalase expression has however been reported in wheat (Herbinger et al. 2002) and Arabidopsis (Jung 2004) undergoing stress.
In this study, we did not observe an increase in CAT1 gene expression in the CaLjN3 variety relative to the SuperstrainB variety despite the CaLjN3 variety being tolerant compared to SuperstrainB. Increased expression of the CAT1 gene has however been observed in drought stressed conditions for the more tolerant variety of Brassica napus, relative to the susceptible variety (Hosseini et al. 2015). The SuperstrainB variety, did not show decreased CAT1 gene expression compared to the control which shows that despite SuperstrainB being a susceptible variety, CAT1 inactivation, under drought stress conditions doesn’t occur due to the inhibition of enzyme synthesis or enzyme inactivation by radical oxygen, peroxide and hydroxyl ions (Hosseini Boldaji et al. 2012).
A study of antioxidant response and photosynthetic behavior of some algae under abiotic stress showed no significant increase of antioxidant enzyme activities, indicating that these antioxidant enzymes from algae persisted for some stress (Li et al. 2016). The impact of some stress on antioxidant systems of alfalfa did not have a significant effect on antioxidant activity and capacity of the alfalfa plant. The outcomes revealed that inoculation did not impose any significant effect on antioxidant activity (SOD, CAT, GPOX), and there was no significant change between inoculated and non-inoculated plants (Hosseinkhani Hezave et al. 2015). A study of the impact of salinity on oxidative stress in two Faba bean varieties did not show a significant effect on SOD activity in plant roots (Gaballah, Maybelle & Gomaa, Abu-Bakr, 2005). Also, in pea plants, APX activity did not show any significant changes under aluminum toxicity (Panda and Matsumoto, 2010).
Some soybean genotypes did not show any significant change in APX activity due to NaCl treatment as compared to the control. CAT and GR activity was not significantly affected by any level of NaCl treatment in some soybean genotypes (Faheema Khan et al., 2009). Study of Stress-Tolerant and Stress-Sensitive Potato Genotypes under stress suggested that the plants responded to potential increases in oxidative stress by altering antioxidant metabolism and activities of key antioxidant enzymes measured. Many other changes in activity were observed under stress. Antioxidant enzyme activities of the stress-sensitive genotype were moderately unaffected by stress treatment. Activities of ascorbate peroxidase, peroxidase, catalase, and superoxide dismutase did not change following heat and drought stress (Ufuk Demirel et al., 2020). Research conducted by Rizhsky et al. with the Double antisense method showed that the plant genome is a highly redundant and dynamic genome. In this research, plants lacking two significant enzymes, ascorbate peroxidase and catalase, stimulate an alternative/redundant defense mechanism that compensates for the lack of APX and CAT. It seems that the plant genome has a high ability to alter the degree of plasticity and will react contrarily to different stressful conditions, namely, lack of APX, lack of CAT, or a lack of both (Rizhsky et al., 2002).
Bioinformatics study of the catalase gene in microarray studies showed no significant difference in catalase gene expression in different tomato plants studied under salinity and drought stress. On the other hand, WGCNA results showed that this gene is in a fully conserved cluster. It does not show any difference between the susceptible and tolerant cultivars. The results of the enrichment gene showed that this gene does not guide any significant cell pathways. Gene hub studies for the catalase gene also indicated that this gene is a non-hub in microarray studies. Other studies show that in tomato drought and salinity treatments, rather than activating the catalase pathway, the cell process activates the SOS pathway of cells, pumps, carriers, and cellular messengers until they have an enzymatic response. Tomatoes seem to go one step further in response to stress oxidation and increased oxygen free radicals, activating enzymes other than catalase. Apparently, in this plant, the fight against oxidative stress begins one step before the enzymes and seeks to expel the stressor by activating proteins, especially channels, pumps and cellular messengers.
BR signaling activation adjusts the expression of genes involved in cell wall biosynthesis and remodeling and cell wall homeostasis through cell expansion in response to environmental stress (Sahni et al., 2016). Cellulose synthase-like protein encoded with SOS genes plays a role in response to osmotic stress tolerance in Arabidopsis (Zhu et al., 2010). The plasma membrane receptor kinase BRASSINOSTERIOID INSENSITIVE 1 (BRI1) after connecting to its co-receptor BAK1 upon successful recognition of BRI1-EMS SUPPRESSOR 1 (BES1) and BRASSINAZOLE-RESISTANT 1 (BZR1) transcription factors leads to expression of BR-responsive genes. BES1/BZR1 is controlled by the GSK3-like kinase BR-INSENSITIVE 2 (BIN2), a vital member of BR-associated signaling (Falkenstein et al., 2000).