The relationship between CO2 assimilation, photosynthetic electron transport and water-water cycle in chill-exposed cucumber leaves under low light and subsequent recovery

The effects of chilling under low light (9/7degreesC, 100 mumol m(-2) s(-1)) on the photosynthetic and antioxidant capacities and subsequent recovery were examined in two (one tolerant and one sensitive) cucumber genotypes. Chilling resulted in an irreversible inhibition of net CO2 assimilation and growth for the sensitive genotype, which was accompanied by decreases in the maximum velocity of RuBP carboxylation by Rubisco (V-cmax), the capacity for ribulose-1,5-bisphosphate regeneration (J(max)), Rubisco content and activity, and the quantum efficiency of photosystem II, in the absence of any stomatal limitation of CO2 supply or inorganic phosphate limitation. In contrast, CO2 assimilation for the tolerant genotype fully recovered after chill. The chill-induced decrease in the proportion of electron flux for photosynthetic carbon reduction was mostly compensated by an O-2-dependent alternative electron flux driven by the water-water cycle, especially in the sensitive genotype. Compared with the tolerant genotype, the sensitive genotype after chill showed reduced capacity for scavenging reactive oxygen species and increased accumulation of reactive oxygen species. The balance between O-2-dependent alternative electron flux and the capacity for scavenging reactive oxygen species in response to chill plays a major role in determining the tolerance of cucumber leaves to this stress factor. It is concluded that the water-water cycle operates at high rates when CO2 assimilation is restricted in cucumber leaves subjected to chill and low light conditions.