PARENT SESSION

Symposium S4B Sugar signalling: Photosynthesis, hormones and development
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 210A
Chair: Dan Bush
Co-Chair: Mathew Paul

The differences between sink and source leaves in carbohydrate repression of photosynthesis. Takao Araya^*,1, Ko Noguchi, Ichiro Terashima, ¹ Department of biology, Graduate School of Science, Osaka university, 1-1 Machikaneyama-cho, Toyonaka, Osaka, Japan

ABSTRACT- Accumulation of non-structural carbohydrates in leaves causes repression of photosynthesis. However, effects of carbohydrates must be different in sink leaves, sugar consumers, and source leaves, sugar exporters. We thus examined differences in the carbohydrate repression of photosynthesis at various developmental stages. Bean plants (Phaseolus vulgaris L.) were grown at PPFD of 360 mol m^-2 s^-1. In order to increase the carbohydrate content of the primary leaves, we supplied 20 mM sucrose solutions to the bean roots everyday for 5 days at various developmental stages of primary leaves (sugar-treatment). we measured the maximum photosynthetic rate per leaf area (Amax), Rubisco content. We also measured free phosphate content, and the internal conductance for CO₂ diffusion from the intercellular spaces to chloroplast (gi) to assess limitations of photosynthesis by cytosolic phosphate availability and by CO₂ diffusion. The starch contents of the sugar-treated leaves were always 2-3 times higher than those of the control leaves. At the young and senesced stages, Amax did not differ between the sugar-treated and control leaves. However, Amax of expanding sugar-treated leaves was about 30% lower than that of control leaves. The Rubisco content of the leaves showed the same trend. Orthophosphate content of the leaf was unaffected by sugar treatment. Although gi was negatively correlated with starch content, the decrease in gi was not marked. These results indicate that carbohydrate repression of photosynthesis is significant in the expanding leaves, but not in the young and senescing leaves and that the decrease in Rubisco content would be the main cause of the photosynthetic depression by carbohydrate. This growth-stage dependent difference in sugar-repression effect is unclear. A repressor of sugar signalling could be active in the sink leaves while it could disappear on sink-source transition. Ecological significance of the growth stage dependent is also discussed.

KEY WORDS: Sugar repression, Internal conductance (gi), Bean (Phaseolus vulgaris L.), Leaf development