Abstract

Two cucumber genotypes, S404 and S1 with low light-sensitivity and low light-tolerance, respectively were used to investigate the oxygen consumption rate of photosystem I, the oxygen evolution rate of photosystem II, cab transcript levels, and activities of enzymes involved in photosynthetic carbon reduction cycle. The results show that short term (24 h) / long term (five and 10 days) low light stress had significant effect on PSII activities while PSI’s effect was not significant. Under the low light stress, S1 cab gene transcript levels were quickly recovered while S404 cabgene transcript levels were slowly recovered. The total dry mass and leaf area of S1 was lower than S404. Low light treatment decreased C₃ photosynthetic carbon cycle enzyme activities involved in ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco) carboxylation and fructose-1,6-bisphosphatase (FBPase), and increased C₄photosynthetic carbon cycle enzyme activities involved in nicotinamide adenine dinucleotide phosphate malate dehydrogenase (NADP-MDH). The NADP-MDH activity in S1 leaves increased significantly compared to S404. These observations suggest that S1 photosynthetic capacity is higher than S404 under low light conditions. Photosynthetic C₄-microcycle possibly would have played a role in low light stress. Therefore, the transcript levels of cab and the involvement of NADP-MDH in low light-resistance need further research.

Key words: Low light, oxygen consumption rate, oxygen evolution rate, cab gene, NADP-MDH.