Photosynthetic efficiency and grain yield of two wheat (Triticum aestivum L.) cultivars during drought stress under exogenous application of cytokinin and abscisic acid treatments

Drought stress is a major factor that reduces growth, development and production of plants especially in generative phase. Among crop plants, wheat (Triticum aestivum), which often experiences water-shortage conditions, is an appealing study system because there are so many natural genotypes differing in drought tolerance. Wheat as well as the most cereals is especially threatened by water deficit during flowering and grain filling period in relation to decreasing yield potential (Khanna-Chop and Selote., 2007). Phytohormones play an important role in plants tolerance of abiotic stresses by mediating a wide range of adaptive responses although some of them act differently. Abscisic acid (ABA) and cytokinin (CK) are two antagonist hormones that play useful roles when stresses as drought appear. But, recent data implied that abscisic acid (ABA)/CK ratios in xylem sap are important for stress signaling. Therefore, this experiment was established to study and compare the influence of exogenous treatments of individual CK and ABA and also CK and ABA interaction on flag leaves relative water content (LRWC), chlorophyll florescence and gas exchange in related to yield were studied in Triticum aestivum cv. Pishgam (drought-tolerant) and cv.MV-17 (drought-sensitive) under irrigation and drought conditions after post-anthesis stage.
The influence of CK, ABA and their combination CK/ABA on LRWC, photochemical efficiency and gas exchange in Triticum aestivum cv. Pishgam (drought-tolerant) and cv.MV-17 (drought-sensitive) flag leaves and yield under irrigation and drought conditions during the grain-filling period. Factorial experiment was based on based on split plot in a randomized complete block design with three replications. Chlorophyll fluorescence as photochemical efficiency was assayed by OS1-FL, a pulse modulated fluorometer (OptiScience Corporation, Tyngsboro, MA). Gas exchange traits were determined at flowering using a portable gas exchange measuring system (Li 6400, Li-Cor, USA). For yield assay, 60 plants were selected randomly to assess grain weight 1000 and grain yield per plant. One-way ANOVA was applied to determine the significance of the results between different treatments and then Duncan multiple range tests (p Drought stress decreased LRWC in Pishgam cv. more than MV-17, significantly. Among hormonal treatments, the interaction of CK and ABA increased LRWC in both cultivars, significantly more than other hormonal treatments during drought stress. Chlorophyll florescence results showed that ФPSII and qP decreased and NPQ increased significantly in sensitive cv. more than tolerant cv. when irrigation was interrupted. CK and ABA interaction increased Fv/Fm, ФPSII and NPQ more than other hormonal treatments. Also CK/ABA application caused the significant increment in qP in Pishgam cv. Stomatal conductance (gs), net photosynthesis (A), transpiration (E) and Mesophyll conductance (MC) decreased in both cultivars when drought occurred while sub-stomatal CO2 concentration (Ci) and photosynthetic water use efficiency (PWUE) increased. These changes were more significant in sensitive cv. than tolerant cv. CK and ABA interaction increased Ci, A and PWUE in both cv. more than other hormonal treatments, while ABA treatment was the most effective hormonal treatment in reduction of gs, E and MC under drought condition. Yield reduced in both cultivars when drought stress appeared. Yield reduction was more in sensitive cv. CK and ABA combination was the most effective treatment for increment of yield, especially in sensitive cv. under drought stress. More Physiologic damages and yield reduction under drought exposure in susceptible cultivars were reviewed in the previous study (Chaves et al., 2003). CK effects on chlorophyll stability, protein synthesis in chloroplast and ABA effects on reduction of oxidative damage in photosynthesis system under drought and salt stress are reasons for production efficiency in plants (Pospisilova, 2003; Pospisilova et al., 2005). óAlso assesment of correlation coefficient showed the most significant relation between NPQ, A and PWUE with 1000-grain weight and grain yield.
It seems that CK and ABA combination is the best treatment in related to improvement of chlorophyll florescence, photosynthesis and yield efficiency in both cultivars under drought stress. Also, MV-17 (sensitive cv.) could use the hormonal treatment better than Pishgam (tolerant cv.) to improve photosynthesis and yield stability.