Non-structural carbon reserves, like starch, low molecular weight sugars and storage lipids, are often used as indicators for the net C balance of trees, assuming that tissue concentrations increase at times of oversupply of photoassimilates, and decrease during periods of insufficient photosynthesis. However, up to date our understanding of the physiological regulation of C storage in trees is very limited. Despite the detailed molecular knowledge of C storage regulation at the leaf level (at least for non-tree model organisms like Arabidopsis), the mechanisms of C reserve formation and degradation in woody organs, as the main site of C storage in trees, remains largely unknown. If C reserves are used to predict the C supply of trees, several questions need to be clarified: 1) Does C storage in trees resemble a 'passive' accumulation/degradation of reserve compounds driven by C source-sink-activities, or an actively regulated process competing with other C-sinks, like growth? 2) Which are the minimum tissue concentrations of C reserves found in different tree organs at lethal C starvation? 3) Is the continuous presence of a minimum C reserve pool necessary for specific physiological processes like the hydraulic conductivity of vessels? In this presentation I will summarize results from several experiments that have been conducted at our lab over the last years to specifically answer the above questions.
Results/Conclusions
Two experiments that induced C limitation in broadleaved tree saplings, by either continuous deep shade or low CO2, indicated that C storage is largely 'passively' driven by the tree's C source-sink balance during the first half of the growing season, but is probably increased 'actively' in competition with other C sinks during the second half of the season. In contrast to previous findings, lethal drought treatments that started in mid-season (when C reserves where at a minimum in C limited saplings) did not lead to shorter survival times in C limited saplings. Inducing C starvation by continuous, complete darkness revealed that C reserves (including low molecular weight sugars) declined to almost zero within three to six weeks in all organs of two-year-old broadleaved tree saplings. Surprisingly, these saplings continued to survive for up to 12 weeks under darkness, indicating that vital tree functions can be maintained with no starch and extremely low sugar concentrations for several weeks. The implications of these experimental results for our understanding of C reserve functions in trees will be discussed.