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 & Sunkar, 2005; N. G. McDowell & J. Amthor, unpublished; Sala et al.). Consistent with this, a classic paper by Marshall & Waring (1985) demonstrated that shaded trees consumed all of their starch pools and subsequently died (J. D. Marshall, pers. comm.), but their sugar pools remained well above zero. Elevated temperatures accelerated depletion of the starch pools and subsequent mortality, consistent with the recent results from Adams et al. (2009). Given the current available evidence, it is quite possible that not all carbohydrates can be utilized, particularly during drought.
The paucity of studies that have quantified mortality forces scientists to use data from nonmortality studies to develop hypotheses (i.e. McDowell et al., 2008; 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 & J. Amthor, unpublished). This is driven not only by mass balance, but also through feedforward signaling in response to carbohydrate availability that down-regulates growth and respiration and up-regulates storage in direct response to depletion of photosynthate and starch (Smith & Stitt, 2007; 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 & Waring, 1985; N. G. McDowell & J. Amthor, unpublished). Therefore, the primary driver of C starvation – declining photosynthesis – actually drives increased allocation to storage carbohydrates during the early phases of stress.
Similar to an inability to utilize carbohydrates at the cellular level, phloem transport failure during drought is also likely to exacerbate C starvation. As stated by Sala et al., this is a critical, but under-studied, question. It is likely that drought can reduce phloem transport via multiple mechanisms, such as lowering carbohydrate loading and unloading (including reduced sink activity), or by lowering phloem conductance by increasing sap viscosity (Chaves et al., 2003; Hölttäet al., 2009). Does phloem transport matter to drought survival? Some studies show that photosynthesis declines faster than assimilate transport during drought (Sung & Kreig, 1979; Fig. 1), but others indicate the opposite (citations in Sung & Kreig, 1979). These studies, however, do not achieve the limit of lethal drought stress and therefore the patterns could change abruptly before mortality (Fig. 1). For example, if photosynthesis fell to near zero (as occurs in piñon pine trees, McDowell et al., 2008) or phloem transport suddenly ceased at −3.0 MPa (as suggested in Fig. 1), then the risk of C starvation would increase. Phloem transport failure could also facilitate drought-induced mortality if the movement of stored carbohydrates, nutrients, or metabolic signals is critical for survival (if storage within local organs is insufficient to outlast drought), or if transport is needed to reduce foliar osmotic stress (Bartels & Sunkar, 2005). Relatively large carbohydrate reserves may have been stored before the late stages of drought (i.e. citations in Sala et al.), but the ability to extract them from storage and to transport them varies widely (Chaves et al., 2003; 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 & Alegre, 2004); 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.
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 & Waring, 1985; 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.