Vegetative and physiological responses of purple seedlings to water stress

To investigate the effect of drought stress on the vegetative, physiological, and biochemical characteristics of purple seedlings, an experiment was conducted in the form of a completely randomized design with four levels of irrigation including 75% (mild stress), 50% (medium stress) and 25% (severe stress) of the field capacity (FC) and the control (100% FC) in greenhouse conditions. The results showed that severe drought stress significantly reduced vegetative traits such as seedling height growth (86.8%), the number of leaves, and total fresh and dry biomass by 93%, 46.2%, and 44.8%, respectively, but increased root volumetric growth (1034%). Also, drought stress affected all parameters of gas exchange and photosynthetic pigments. So that at the level of 25% FC, it caused a significant decrease in the rate of photosynthesis (1/20 µmolCo2m-2s-1), transpiration (0/65 mmolm-2s-1), stomatal conductance (106/2 mmolm-2s-1) and mesophilic (0/0088 mmolm-2s-1), intracellular carbon dioxide concentration (134/2 mmolCO2 m-2s-1) and carboxylation efficiency (0/00112 µmolCO2 mol-1H2O). The amount of photosynthetic pigments (chlorophyll a, b, total chlorophyll, and carotenoids) also showed a significant decrease at 25% FC (4.07, 1.96, 6.03 and 1.23 mg g-1 FW, respectively.). Also, drought stress increased the amount of proline and soluble sugar and showed the highest values at the level of 25% FC (respectively, 1.348 µmol g-1 FW of proline and 41.51 mg g-1 FW of soluble sugar). According to the above evaluations, it seems that the purple variety can tolerate drought stress up to 75% FC without causing damage to the plant tissue, but if the amount of water is reduced to the threshold of 25% FC, its growth and performance will be severely affected.