Pinus sylvestris switches respiration substrates under shading but not during drought

Abstract

SummaryReduced 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, d13 C of respired CO 2 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.

Research frontiers for improving our understanding of drought‐induced tree and forest mortality

“…To systematically examine the role of tree C metabolism during drought mortality, we suggest investigating whole‐tree C dynamics to identify the onset of a negative C balance during severe drought and in combination with high temperatures and high VPD. Such investigations must also address C storage mobilization and transport, regulation and remobilization of NSCs or alternative reserve compounds such as lipids and proteins during drought (Zhao et al ., ; Fischer et al ., ).…”

Understanding the roles of nonstructural carbohydrates in forest trees – from what we can measure to what we want to know

“…In combination with elevated temperature, however, drought leads to higher cumulative respiratory fluxes in leaves compared to drought only, causing a greater whole‐plant C loss and potentially starvation (Adams et al ., ). Moreover, drought seems to hamper not only NSC mobilization, but also other catabolic processes, resulting in reduced use of alternative respiration substrates like lipids or proteins (Fischer et al ., ).…”

“…() demonstrated large unexplained biases in NSC concentration measurements with only some uncertainty related to different extraction and quantification procedures. In addition, trees may meet metabolic needs by shifting from carbohydrates to other reserve compounds such as lipids, or by recycling existing metabolites like proteins (Fischer et al ., ) and both are rarely measured in tree ecophysiological studies. Such problems severely limit the usefulness of any metric derived from concentrations measurements for mass‐balance approaches for whole‐plant C balance estimation, for example, by comparison with measures of net photosynthesis and respiration fluxes (Hartmann et al ., ).…”

“…Fischer et al . () used such measures of RQ and 13 C in respired CO 2 to show that trees under C limitation (from shading) switched from progressively declining carbohydrates to stored lipids to fuel respiration but this was not observed during drought. The observed decreasing NSC content in shaded trees during the experiment would likely have been interpreted as a preferential use of NSC in respiration, if alternative respiration substrate content, RQ and δ 13 C of respired CO 2 had not also been assessed along with NSC content.…”

Living on next to nothing: tree seedlings can survive weeks with very low carbohydrate concentrations

“…Exposing trees to darkness is a straightforward method that allows NSC concentrations at C limitation to be monitored. Previous deep‐shading experiments have shown significant decreases in tissue NSC concentrations in seedlings, but often C limitation was not severe enough to cause mortality (Veneklaas & den Ouden, ; Fischer et al ., ; Maguire & Kobe, ). Several studies that employed complete darkening or extreme deep shade quantified NSC concentrations at tree mortality, but only for a single species and often not for all tissues separately (Marshall & Waring, ; Piper et al ., ; Sevanto et al ., ; Piper & Fajardo, ; Wiley et al ., ).…”