چكيده لاتين :
Introduction: Most plants have developed morphological and physiological mechanisms which allow them to cope with drought stress. Almost all the studies conducted on grapevines (Vitisvinifera L.) responses to drought conditions have focused on physiological responses such as stomatal reactions, photosynthesis and osmotic adjustment, and biochemical responses like carbohydrates and proline. According to these studies, physiological and biochemical responses of grapevines to water stress are quite variable. This variability could be related to cultivar, time of the year, previous water stress level, intensity of stress, and environmental conditions. Osmotic adjustment in terms of compatible solutes accumulation has been considered as an important physiological adaptation for plant to resist drought, which facilitates the extraction of water from dry soils and maintenance of cell turgor, gas exchange and growth in very dry environments. Acting as compatible solutes as well as antioxidants, a significant rise in proline amount was observed in grapevine leaves under water stress conditions, suggesting that this amino acid has a protective role against the formation of excessive reactive oxygen species (ROS). Plants, in order to overcome oxidative stress, have developed enzymatic and non-enzymatic antioxidant defense mechanisms against scavenge ROS.
Materials and Methods: This research was conducted to assess the effect of different levels of irrigation on some characteristics of three cultivars of grapevine (Yaghooti, Bidanesefid and Askari), as a factorial based on a randomized complete block design in two years with four replications. The experiment started in June 21, 2014 and 2015. Water treatments were applied in four levels including: control plant (100% FC), moderate stress (60% FC), severe stress (30% FC) and rewatering treatment after severe stress treatment. Increase height, leaf number, stem diameter, leaf fresh and dry weight, stem dry weight, chlorophyll index,RWC, electrolyte leakage, soluble sugar, antioxidant activity, phenolic compound and proline were measured at the end of the experiment. JMP8 software was used to test the significant differences among the treatments and the interactions. When there were significant differences, means were separated by Tukey HSD test at the probability level p <0.05.
Results and Discussion: Results showed that drought stress had significant effects on most traits at statistical levels. The effect of year on measured traits was not significant differences. The results demonstrate that the three investigated grapevine cultivars showed a clear difference in their response to water stress. Increase of height, leaf number, stem diameter, leaf fresh and dry weight, stem dry weight, chlorophyll index and RWC decreased as soil water content reduced and increased again after rewatering. The greatest increase height (9.12 cm), leaf number (35.12), leaf and stem dry weight (4.92 and 8.41 g respectively) in severe stress was in Yaghooti cultivars. Drought stress induced a significant decrease in leaf relative water content (RWC) of all three cultivars during the drought period. The RWC of Yaghooti (85.49%) was significantly more than that of Askari and Bidanesefid at severe stress. Bidanesefid demonstrate a higher decrease of RWC (12.3%) compared to control. Resistance of plant to drought is related to its ability to maintain high RWC in leaves under stress. Water stress treatments resulted inlower chlorophyll index in all three grapevine cultivars, so that a significant reduction in this variable was observed in stressed grapevine compared to the control. Electrolyte leakage, soluble sugar, antioxidant activity, phenolic compound and proline increased in all the cultivars as drought stress levels increased. Significant differences were observed among cultivars in terms of electrolyte leakage. Water stress treatments caused an increase in electrolyte leakage. Bidanesefid showed greatest electrolyte leakage than the other cultivars in all treatment. Under water deficit, cell membranes undergo some changes such as an increase in permeability and a decrease in selectivity, which can be viewed through the increase in electrolyte leakage. In moderate stress treatment, the highest amount of soluble sugars accumulation was recorded in Askari cultivar but in severe stress treatment, no significant differences were observed among the cultivars. Significant differences were found in the antioxidant activity and proline content among cultivars and watering regimes. Under the severe stress conditions, the highest antioxidant activity (45.3%) and proline accumulation (11.12 µmol g-1 FW) was registered in the Yaghooti cultivar, and no significant differencesexisted among two other cultivars. Plants accumulate compatible solutes, such as soluble sugars and proline, in response to stress to facilitate water uptake.
Conclusion: Of all the cultivars, Yaghooti had the lowest decrease in growth characteristics and RWC and greatest increase in antioxidant activity and proline. In the present study, based on the responses of cultivars to different levels of drought stress, it can be concluded that Yaghooti seems to be a more resistant cultivar to water stress compared to Askari and Bidanesefid cultivars. Based on the results, it can be said that Bidanesefid cultivar has the lowest resistant cultivar to water stress compared to Askari and Yaghooti cultivars. Further research especially under field conditions is needed to support this statement.
