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Abstract

Reduced carbon (C) assimilation during prolonged drought forces trees to rely on stored C to
maintain vital processes like respiration. It has been shown, however, that the use of
carbohydrates, a major C storage pool and apparently the main respiratory substrate in plants,
strongly declines with decreasing plant hydration. Yet no empirical evidence has been produced
to what degree other C storage compounds like lipids and proteins may fuel respiration during
drought.
We exposed young scots pine trees to C limitation using either drought or shading and
assessed respiratory substrate use by monitoring the respiratory quotient, d13C of respired CO2
and concentrations of the major storage compounds, that is, carbohydrates, lipids and amino
acids.
Only shaded trees shifted from carbohydrate-dominated to lipid-dominated respiration and
showed progressive carbohydrate depletion. In drought trees, the fraction of carbohydrates
used in respiration did not decline but respiration rates were strongly reduced. The lower
consumption and potentially allocation from other organs may have caused initial carbohydrate
content to remain constant during the experiment.
Our results suggest that respiratory substrates other than carbohydrates are used under
carbohydrate limitation but not during drought. Thus, respiratory substrate shift cannot provide an efficient means to counterbalance C limitation under natural drought.