Summary
Both desiccation and depleted carbohydrate reserves have been suggested as causes of fine-root (≤2 mm in diameter) mortality in trees. In this study, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings were subjected to four combinations of shading and watering to determine whether shading increases drough-induced root mortality and, if so, whether this effect is due to reduced levels of carbohydrate reserves or increased susceptibility to desiccation. Two correlated measures of root mortality (counting root tips and weighing roots) showed that significantly more fine roots died only when seedlings were both shaded and unwatered. Concentrations of suberin, a compound synthesized by plant roots to control desiccation, were unaffected by any combination of shading and watering; however, carbohydrate reserves were nearly exhausted in the shaded and unwatered treatment — the treatment with highest root mortality. Water stress may have increased root mortality indirectly by increasing root temperature and maintenance respiration and by inhibiting photosynthate transport to the root system, but massive die-off in response to drought was apparent only when starch and sugar reserves were nearly depleted. Drought cannot be considered directly responsible for death of fine roots. Instead, a root's ability to continue to respire, which in turn depends on the status of its starch and sugar reserves, seems to be the primary physiological control of fine-root mortality.
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Marshall, J.D. Drought and shade interact to cause fine-root mortality in Douglas-fir seedlings. Plant Soil 91, 51–60 (1986). https://doi.org/10.1007/BF02181818
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DOI: https://doi.org/10.1007/BF02181818