Source physiology and assimilate transport: the interaction of sucrose metabolism, starch storage and phloem export in source leaves and the effects on sugar status in phloem

Abstract

Phloem loading of sucrose is decisive for the speed of mass flow, because sucrose is the dominant solutein the sieve tube sap of nearly all plant species. The export rate of carbon is linearly correlated to the concentration of sucrose in green leaves. Saturation of export was not observed, because surplus of assimilates is converted to starch, a process which is regulated by the sucrose level in the cytosol. Consequently, an increase of sucrose synthesis by overexpression of SPS did not enhance carbon export (at least under normal ambient conditions). Saturation of sucrose export could be observed only in experimental systems, where sucrose was fed directly to the phloem (e.g. in Ricinus seedling) or where constraints on transport activity were imposed by genetic manipulation either on the transporters (e.g. in sucrose transporter antisense plants) or on the path of sucrose (e.g. in plants trans ormed with TMV movement protein, or by incubation in salts). The balance between carbon storage and carbon export is subject to adaptation to meet growth requirements under special circumstances. For example, in a starch-deficient mutant, the day time export rate is nearly doubled compared to wild type plants. Furthermore, plants under short day illumination greatly accelerated starch storage compared to plants under long day illumination (a modulation which persists even a few days after a shift to long day conditions). Plants with a higher assimilation rate due to elevated ambient CO2 increase the nightly carbon export rate, whereas the export rate in day time rate appeared to work at its upper limit. The overall efficiency of sucrose export and incorporation into biomass is ca 0.65, which is close to the theoretical value of 0.75. Sucrose transport along the phloem strands is modulated according to the input at the source, but the individual phloem strands show also partial coordination with respect to sucrose concentrations (as revealed by NMR-imaging), especially obvious after physical interruption of some vascular bundles.