The role of leaves’ non-structural carbohydrates (NSC), in addition to being an energetic source, is still elusive. We tested two common hypotheses which are inconsistent with one another. The first is that NSCs can serve for osmotic adjustment, and their increase enables stomatal opening despite the daily and seasonal reduction in leaf water potential (Ψleaf). The second is that increase in NSCs is a sign of excessive assimilation to the sink demand and thus signals gas exchange downregulation.
To explore these questions, we monitored the diurnal and seasonal dynamics of gas exchange, Ψleaf, osmotic potential, and NSCs of grapevines (Vitis vinifera) through two consecutive growing seasons. Additionally, we dehydrated the vines several times along the season to increase the variability in leaf NSC content and to examine its contribution to leaf osmolarity under stress conditions.
We found that the daily accumulation of soluble sugars constitutes approx.—50% of the daily osmotic adjustment (0.2 MPa), enabling the vines to maintain turgor under low Ψleaf. At the same time, the importance of NSCs as osmolytes decreased as the season progressed, and they did not contribute to the osmotic adjustment under drought conditions. Additionally, there was no correlation between NSCs and gas exchange, implying that NSCs are not the signal for photosynthesis restriction.