Rubisco gene expression and photosynthetic characteristics of cucumber seedlings in response to water deficit

Photosynthesis is one of the most sensitive physiological processes to water deficits. Here, cucumber (Cucumis sativus L.) seedlings with 6–7 leaves were deprived of water to study photosynthetic responses to water stress. The results showed that, a decrease in stomatal conductance occurred early in response to water deficit and may prevent excessive dehydration and improve water use efficiency in cucumber plants. Then the CO2 assimilation rate (PN) declined gradually during dehydration and was significantly affected after 7–8 days of water deficit in relation to well-watered plants. Meanwhile, relative expressions of rbcL and rbcS were increased at first and then declined rapidly to reach minimum in the 8th day of dehydration during water deficit. Moreover, the polypeptide contents of Rubisco large (RBCL) and small (RBCS) subunits, the Rubisco initial activities and activation state, decreased significantly in the 8th day of dehydration compared to control plants. After 2 days of re-watering, the above parameters increased rapidly, and reached near the level of the control. These results indicated that, in addition to the lower CO2 available for carboxylation (in consequence of the decreased stomatal conductance), Rubisco activity and amount contributed to limit photosynthesis in dehydrated cucumber plants.