A carbohydrate quandary.

Abstract

Carbon (C) is central to the flow and storage of mass and energy in plants and their ecosystems, but our understanding of C dynamics in plants is still developing. Trees and many plants have ∼50% C (45–60% of the dry mass, Thomas and Martin 2012), and a large portion of this C is in the form of carbohydrates, one of the four main macromolecules comprising plants. Carbohydrates are traditionally classified as either recalcitrant, ‘structural’ components of cell walls and other tissues, or as labile, ‘non-structural’ pools, although this dichotomy is often not so clear ( Hoch 2007). Non-structural carbohydrates (NSCs) can comprise a large portion of a tree’s C and the dry mass (up to ∼50 and ∼25%, respectively, e.g., Hoch et al. 2003), but they can also be scarce due to tight linkages of NSC to C uptake, NSC utilization and active regulation of pools. Several contemporary concepts in ecophysiology center on NSC as a potentially important balance point that links environmental stressors and physiological responses to whole-plant outcomes. Prominent examples include the growth-limitation hypothesis that low temperatures inhibit C allocation to structural growth more than they limit C uptake, leading to greater NSC ( Korner 1998); and the C-starvation hypothesis that drought kills plants through a loss of C uptake and NSC depletion more so than direct hydraulic failure ( McDowell and Sevanto 2010). As formulated, these and many other theories implicitly assume that NSCs are a discrete pool that does not intergrade with structural carbohydrates, and most importantly can be measured. In this issue, Quentin et al. (2015) present an ‘inconvenient truth’ for research addressing these theories that has important ramifications for tree physiology specifically, and plant and ecosystem sciences more generally. Quentin et al. (2015) sent the same set of five plant tissues to 29 NSC laboratories in 13 countries, and found large disagreement among the laboratories in their estimate of NSC content of the samples. Their findings suggest that we currently do not know which technique or laboratory could be most relied upon to develop plant tissue NSC standards needed for accuracy assessment.