The authors measured total non-structural carbohydrates (NSC) and soluble sugars (SS) in roots and stems during drought and after an autumn rain pulse in plants exhibiting leaf loss and in undefoliated ones. Lloret and colleagues note that previous authors have said stored non-structural carbohydrates are never fully depleted under average conditions because a certain concentration of soluble sugars is required to sustain immediate plant functions such as osmoregulation, transport and signalling. If a drought lasts for long enough, this should be visible as a decline in NSC reserves.
The team measured the effect of drought on a plant by examining leaf loss, leading to a dieback of the canopy. They compared plants that showed loss of more than half of their canopy against plants that showed of loss of less than a quarter of the canopy. Using the canopy cover, to categorise the plants, they could then sample them to see what carbohydrates the plants had stored.
The scientists found that during drought, NSC concentrations were overall lower in stems and roots of plants experiencing leaf loss, while SS decreases were smaller. Roots had higher NSC concentrations than stems. However, after seasonal rain, SS increased, while NSC did not. Lloret and colleagues say: “This suggests that newly assimilated C after seasonal pulses of rain was insufficient to meet C demand for new tissue growth after a prolonged drought.” The carbohydrates that could have been stored were instead being put to use building new leaves.
Regarding the SS they added: “The significant increase in SS after the autumn rain does suggest a general increase in physiological activity, as sugars were mobilized for growth and metabolic demands.”
Drought is an increasing concern as climate changes, with the potential for rainless periods to be both more frequent and longer. What Lloret and colleagues show is that even with rain, the effects of drought can continue to build up in plants. They conclude: “Persistent long-term drought, even when interrupted by occasional rain pulses, could eventually deplete NSC stocks to the point that canopy recovery is no longer possible.”