The mechanisms of carbon starvation: how, when, or does it even occur at all?

Recent observations of increasing vegetation mortality events appear to be a result of changing climate, in particular, an increase in the frequency, length and intensity of droughts (e.g. Allen et al., 2010). The threat of widespread increases in future mortality has rekindled interest in the mechanisms of plant mortality and survival because we do not yet understand them well enough to confidently model future vegetation dynamics (Sitch et al., 2008). In this issue of New Phytologist, Sala et al. (pp. 274–281) provide a viewpoint on the ‘carbon (C) starvation hypothesis’ (McDowell et al., 2008). Their viewpoint is invaluable for stimulating our field to explicitly refine our definitions and identify the key experiments needed to understand mechanisms of vegetation survival and mortality. Two important conclusions of their paper were that mortality can occur at nonzero carbohydrate levels and that careful experiments focused on the explicit mechanisms of C starvation, as well as on partitioning the roles of hydraulic failure and C starvation, are needed to understand the physiological underpinnings of how plants die. We applaud these conclusions, and agree that hasty acceptance of any hypothesis before adequate testing is foolish. In this commentary, we highlight some of the valuable ideas from Sala et al. and provide additional comments that we hope will prompt careful future tests on the mechanisms of plant mortality. ‘The paucity of studies that quantified mortality forces scientists to use data from nonmortality studies to develop hypotheses … we do this at the risk of confusing stress responses with mortality mechanisms.’ When the C-starvation hypothesis was proposed (McDowell et al., 2008), it represented an attempt to summarize and interpret the existing literature on vegetation mortality, of which there was a wealth of indirect studies, but a paucity of true, mechanistic tests. The original formulation of the hypothesis suggested that stomatal closure minimizes hydraulic failure during drought, causing photosynthetic C uptake to decline to low levels, thereby promoting carbon starvation as carbohydrate demand continues for maintenance of metabolism and defense. The plant either starves outright, or succumbs to attack by insects or pathogens, whichever occurs first. By contrast, failure to maintain xylem water tension lower than its cavitation threshold results in embolisms, which, if unrepaired, can eventually lead to widespread hydraulic failure, desiccation and mortality. We hoped that the C-starvation and hydraulic failure hypotheses would generate discussion and new ideas; and indeed, as summarized by Sala et al., active discussion is taking place. A primary conclusion from the discussion is that we need clarification of the various mechanisms by which C starvation can occur, if it occurs at all. Plants maintain metabolism through respiratory processes that consume carbohydrates, and in doing so their C budgets must obey the law of conservation of energy, that is, respiration (mols per plant) = photosynthesis + carbohydrate storage − growth. Therefore, if not all carbohydrates are available for metabolism during drought, this will accelerate C starvation by reducing the storage pool available for respiratory metabolism. The current evidence is mixed regarding metabolic limitations to utilize carbohydrates. We reviewed the four carbohydrate studies cited by Sala et al. that included plant mortality, as opposed to publications in which seasonal carbohydrate analyses were performed on plants that did not die (representing 4 out of 16 (or 25%) of the studies on carbohydrate cited by Sala et al.), and agree that no clear pattern of carbohydrate content and mortality emerges. Mortality at nonzero carbohydrates could simply be a result of mortality via other mechanisms, such as hydraulic failure. However, mortality at nonzero carbohydrate contents could also be a result of C starvation because of the increased use of sugars for osmotic balance during drought. These sugars may be unavailable for other metabolic maintenance processes (Chaves et al., 2003; Bartels N. G. McDowell Sala et al.). Consistent with this, a classic paper by Marshall Sala et al.); however, we do this at the risk of confusing stress responses with mortality mechanisms. Particular to C starvation, the literature on the carbohydrate patterns of plants that did not die may not be evidence against C starvation. In fact, these data, along with the widespread evidence that plants minimize C loss and maximize C gain during drought (reviewed by McDowell et al., 2008; Sala et al.; and many others), support the critical role of carbohydrate balance in avoiding mortality. During drought, carbohydrates accumulate because growth declines faster than photosynthesis (reviewed by N. G. McDowell Gibon et al., 2009). The carbohydrate concentration of tissues should only decline when the availability of C from photosynthesis plus storage does not equal C consumption to maintain metabolism (Marshall N. G. McDowell Hölttäet al., 2009). Does phloem transport matter to drought survival? Some studies show that photosynthesis declines faster than assimilate transport during drought (Sung Fig. 1), but others indicate the opposite (citations in Sung and citations in Sala et al.). Finally, failure of phloem transport could be important if autophagy, or breakdown and recycling of cellular contents, is an important mechanism to mobilize resources to avoid mortality (Munne-Bosch thus, transport may be particularly critical during the final stages of survival. While phloem transport failure is unlikely to kill plants directly, it could play a critical role in promoting C starvation or other negative consequences that lead to mortality. Because phloem transport failure may have different impacts on sink and source organs, analyzing the sugar contents in different tissues up to the point of mortality will be critical in future experiments. Photosynthesis and 14C loss from leaves (primarily translocation with a small amount lost to respiration, Hofstra, 1967) as a function of leaf water potential for sorghum plants. Data were reproduced from Fig. 3 in Sung photosynthesis, open circles. We applaud the goal of Sala and colleagues of furthering our understanding of vegetation mortality. A number of key questions have arisen from the current discussion. What fraction of stored carbohydrates is truly available to respiratory metabolism? What is the role of starch and other sugars in survival mechanisms, including phloem function? How do these carbohydrates vary at the whole plant level and across taxa? What is the interaction of hydraulics, metabolism and phloem transport? What is the global response of the above processes and their drought-dependence leading up to, and including, the point of mortality? We suggest that future research regarding mortality clearly distinguishes evidence from plants that actually die versus plants that are stressed but survive. The limited research that has been conducted in an appropriate manner to investigate mortality mechanisms (i.e. Marshall Adams et al., 2009) has yielded results consistent with C starvation, as formulated by both McDowell et al. (2008) and Sala et al., but they do not necessarily ‘prove’ C starvation as the mechanism of mortality. Unfortunately, no studies have tested the C-starvation or hydraulic-failure hypotheses, in any of their forms, and sufficiently concluded that other mechanisms are not interacting or driving mortality. To improve our understanding of how plants die, new experiments explicitly designed to partition the different mechanisms need to be conducted. This manuscript benefited from valuable discussions with many colleagues. NGM and SS were supported during the writing of this paper by the US Department of Energy, Office of Science (BER) and by LANL-LDRD.