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STARCH CONVERSION TO SUCROSE AT NIGHT.   S.E. Weise, and T.D. Sharkey. Dept of Botany and Wisconsin Center for Space Automation and Robotics, Univ of Wisconsin-Madison.

During photosynthesis fixed carbon is partitioned either as starch in the chloroplast or as sucrose in the cytosol. Transitory starch is synthesized in chloroplasts during the day and is broken down at night and exported to the cytosol to make sucrose. The pathway for transitory starch breakdown, export from the chloroplast, and sucrose synthesis at night is currently unknown. We have isolated chloroplasts from bean leaves one hour after onset of the dark portion of the photoperiod and analyzed the sugars and sugar phosphates that were exported  into the surrounding medium after a 2.5 hour incubation in the dark. We found that maltose and/or another glucose containing disaccharide was exported at a rate of 58 ± 11 nmol mg^-1 chl hr^-1, and glucose was exported at a rate of 8 ± 4 nmol mg^-1 chl hr^-1. We found no G6P, F6P, or Fructose present. We observed a small amount of G1P present continually (10 ± 3  M). Exogenous glucose seems to have an inhibitory effect on maltose export while exogenous ATP seems to have the opposite effect, stimulating maltose export. When chloroplasts are incubated with radio labeled glucose a small amount of the labeled glucose is found to be incorporated into the exported maltose. From these results and earlier work by other groups we present a hypothesized pathway for transitory starch breakdown at night. This work is the first step in a larger project to better understand plant carbohydrate metabolism regulation in day vs. night. This will provide insight into engineering continuos photoperiod tolerance, which will be of great importance in future Advance Life Support Systems.