Levels of endogenous sugars, transcripts of rbcS and rbcL, and of RuBisCO protein in senescing sunflower leaves

Abstract

Sugar content, transcript levels of rbcS and rbcL, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) content were measured in the first leaves of sunflower plants grown under high light and low nitrogen supply. After the peak level of the photosynthetic activity was attained, photosynthates accumulated, and the photosynthetic activity decreased. When these plants were supplied with high nitrogen, the sugar content decreased, and the transcript level of rbcS increased. Subsequently, the amount of RuBisCO increased. To decrease the leaf sugar level without changing the nitrogen supply, we applied DCMU, an inhibitor of photosynthesis, to the first leaves of the plants. This treatment also increased the transcript level of rbcS and suppressed the decrease in RuBisCO. These results indicate that sugar repression of rbcS may play a role in the early phases of leaf senescence under nitrogen deficiency.