Regulation of photosynthesis by sugars in sugarcane leaves

In sugarcane, increased sink demand has previously been shown to result in increased photosynthetic rates that are correlated with a reduction in leaf hexose concentrations. To establish whether sink limitation of photosynthesis is a result of sugar accumulation in the leaf, excision and cold-girdling techniques were used to modify leaf sugar concentrations in pot-grown sugarcane. In excised leaves that were preincubated in darkness for 3h, sucrose accumulation was reduced but accumulated again upon transfer to the light, while hexose concentrations remained lower than in controls (7.7 micromol mg(-1)FW versus 18.6 micromol mg(-1)FW hexose in controls). These results were associated with a 66% and 59% increase in photosynthetic assimilation (A) and electron transport rate (ETR), respectively, compared to controls maintained in the light. Similar increases in photosynthesis were observed when dark-treated leaves were supplied with 5mM sorbitol, but not when supplied with 5mM sucrose. Further analyses of (14)C-labeled sugars indicated rapid turnover between sucrose and hexose. Cold-girdling (5 degrees C) increased sucrose and hexose levels and resulted in a decline of photosynthetic rates over 5d (48% and 35% decline in assimilation rate and ETR, respectively). These sugar-induced changes in photosynthesis were independent of changes in stomatal conductance. This study demonstrates that the down-regulation of photosynthesis in response to culm sugar accumulation reported previously could be due to the knock-on effect of accumulation of sugar in leaf tissue, and supports the contention that hexose, rather than sucrose, is responsible for the modulation of photosynthetic activity.