The impact of <Emphasis Type="Italic">trans</Emphasis>-zeatin <Emphasis Type="Italic">O</Emphasis>-glucosyltransferase gene over-expression in tobacco on pigment content and gas exchange

The responses of tobacco plants over-expressing trans-zeatin O-glucosyltransferase gene under constitutive or senescence-inducible promoter (35S:ZOG1 and SAG12:ZOG1) and of wild type (WT) plants to water stress and subsequent rehydration were compared. In plants sufficiently supplied with water, both transgenics have higher net photosynthetic rate (P_N) in upper and middle leaves and higher stomatal conductance (g_s) in middle leaves than WT. Water use efficiency (WUE = P_N/E) was higher in both transgenics than in WT. During prolonged water stress, both P_N and E declined to a similar extent in both transgenics and WT plants. However, 7 d after rehydration P_N in SAG:ZOG (upper and middle leaves) and 35S:ZOG (upper leaves) was higher than that in WT plants. Increased content of endogenous CKs in 35S:ZOG plants did not prevent their response to ABA application and the results obtained did not support concept of CK antagonism of ABA-induced stomatal closure. The chlorophyll (Chl) a+b content was mostly higher in both transgenics than in WT. During water stress and subsequent rehydration it remained unchanged in upper leaves, decreased slightly in middle leaves only of WT, while rapidly in lower leaves. Total degradation of Chl, carotenoids and xanthophyll cycle pigments (XCP) was found under severe water stress in lower leaves. Carotenoid and XCP contents in middle and upper leaves mostly increased during development of water stress and decreased after rehydration. While β-carotene content was mostly higher in WT, neoxanthin content was higher in transgenics especially in 35S:ZOG under severe stress and after rehydration. The higher content of XCP and degree of their deepoxidation were usually found in upper and middle leaves than in lower leaves with exception of SAG:ZOG plants during mild water stress.