Response Of Different Photoprotective Pathways To Drought Stress In Tomato Leaves Deficit In Xanthophylls Biosynthesis

The 21 th century is embarrassed with drought and water deficit is becoming increasingly severe in agricultural production. It causes large yield loss. Drought has also great impact on both economy development and agricultural production. Therefore, intense study has been carried out to elucidate the resistance or adaptation mechanism of plants to drought and breed varieties with resistance to drought. In this study, we investigated the response of several photoprotection pathways in tomato leaves to drought stress under different light levels and discussed the role of xanthophylls in non-photochemical quenching (NPQ) and the compensatory acclimatized photoprotection mechanisms. Furthermore, we also compared the differences of tomato Ailsa Craig and its Xa deficit in lutein (L) in their response to drought stress. The results are as follows:1. The role of several photoprotection pathways in protection from drought stress was investigated in tomato leaves at different light levels. Under high light conditions, drought stress significantly decreased electron flux for photosynthetic carbon reduction (J_c) and electron flux for photorespiratory carbon oxidation (J_o). After a transient increase, electron flux to water-water cycle decreased gradually, suggesting that the protective role of water-water cycle was limited. In contrast, non-photochemical quenching (NPQ) increased significantly during drought stress and was the important photoprotection mechanism during drought stress under high light conditions. Under low light, drought stress also significantly decreased J_c and J_o, but increased J_a. However, no significant increase of NPQ was detected during drought stress under low light conditions. Under both high and low light conditions, drought stress increased the activities of SOD, APX and CAT.2. To probe the role of xanthophylls in non-photochemical quenching (NPQ) and the compensatory acclimatized photoprotection mechanisms, a tomato (Lycopersicon esculentum Mill.) Xa mutant with deficit in lutein (L) and neoxanthin (N) was used. The Xa mutant showed an accumulation of antheraxanthin (A) and an increased degree of de-epoxidation state [(A+Z)/(V+A+Z)]. However, the Xa mutant showed significantly compromised NPQ. Meanwhile, the Xa mutant displayed the decreases of chlorophyll content and the increases of Chla/b ratio, consequently causing a lower capacity for light harvesting. However, the Xa mutant had an unaffected CO₂ assimilation rate (Pn) and a much higher stomatal conductance (Gs). Moreover, the Xa mutant exhibited the decreases in J_o and J_a, thus reducing the amount of ROS damage to the photosynthetic apparatus. The Xa mutant also showed the decreases in flux of light-dependent thermal dissipation (Jnpq) and total flux of energy dissipation (J_PSII+J_NPQ+J_f,D)- Importantly, the Xa mutant displayed higher activities of antioxidant enzymes. All these results suggested some compensatory acclimatized mechanisms of photoprotection operated properly in lack of NPQ and xanthophylls. 3. Tomato Ailsa Craig and its Xa mutant plants were also compared for their difference in phtotoprotection under drought conditions. Drought resulted in significant increases in the levels of L and N in the Xa mutant, followed by the increases in NPQ. Meanwhile, the increases of L and N also resulted in increases of capacity for light harvesting in the Xa mutant. During drought stress, the increase in light harvesting as well as high G_S and Ci resulted in an increased Pn in the Xa mutant. Accompanied by the increase in Pn, the Xa mutant exhibited increased J_c but lowed J_a during drought stress, together with increased activities of antioxidant enzymes. It is likely that different photoprotection pathways operated coordinately under drought conditions to compensate the lack of xanthophylls and NPQ, thus enhancing its resistance to drought stress in the Xa mutant.