Journal of Experimental Botany, Vol. 65, No. 7, pp. 1751–1759, 2014 doi:10.1093/jxb/ert467  Advance Access publication 15 January, 2014

Review paper

Phloem transport and drought Sanna Sevanto* Earth and Environmental Sciences Division, Los Alamos National Laboratory, Bikini Atoll Road MS J495, Los Alamos, NM 87545, USA * To whom correspondence should be addressed. E-mail: [email protected]

Abstract Drought challenges plant water uptake and the vascular system. In the xylem it causes embolism that impairs water transport from the soil to the leaves and, if uncontrolled, may even lead to plant mortality via hydraulic failure. What happens in the phloem, however, is less clear because measuring phloem transport is still a significant challenge to plant science. In all vascular plants, phloem and xylem tissues are located next to each other, and there is clear evidence that these tissues exchange water. Therefore, drought should also lead to water shortage in the phloem. In this review, theories used in phloem transport models have been applied to drought conditions, with the goal of shedding light on how phloem transport failure might occur. The review revealed that phloem failure could occur either because of viscosity build-up at the source sites or by a failure to maintain phloem water status and cell turgor. Which one of these dominates depends on the hydraulic permeability of phloem conduit walls. Impermeable walls will lead to viscosity build-up affecting flow rates, while permeable walls make the plant more susceptible to phloem turgor failure. Current empirical evidence suggests that phloem failure resulting from phloem turgor collapse is the more likely mechanism at least in relatively isohydric plants. Key words:  Carbohydrate transport, carbon starvation, hydraulic failure, Münch flow, semi-permeable conduit, tree mortality.

Introduction Recent forest mortality events (Allen et al., 2010), combined with climate predictions of increasing drought severity and frequency in many areas (Allison et  al., 2009), have motivated a new focus area of plant mortality mechanisms during drought (McDowell et al., 2008; Adams et al., 2009; Sala et al., 2010; McDowell, 2011; Zeppel et al., 2011; Anderegg et  al., 2012b, Mitchell et  al., 2013). The theoretical frame work for this focus area was set up by McDowell et al. (2008) based on our understanding of plant water relations. During drought plants close their stomata to avoid excessive water loss that could lead to hydraulic failure or wilting (Sperry, 1986; Sperry et al., 1998; Cochard et al., 2002; Meinzer et al., 2009), but closing of stomata inhibits photosynthesis leading to negative carbon balance which could cause mortality if drought persists and carbon availability becomes insufficient (carbon starvation; McDowell et al., 2008; McDowell, 2011). The timing of stomatal closure varies with drought severity and species-specific variation in xylem vulnerability

to cavitation (Sperry, 2000; Brodribb and Holbrook, 2003). Relatively isohydric (dehydration avoiding) plants (Tardieu and Simmoneau, 1998) close their stomata early during drought, thus being theoretically more likely to die of carbon starvation, while relatively anisohydric (dehydration tolerating) plants may be more susceptible to hydraulic failure. This relatively simple theoretical framework is very useful for categorizing and quantifying observations but, because of its simplicity, it has provoked some criticism centred around how exactly the mortality process progresses (McDowell and Sevanto, 2010; Sala et al., 2010), how that affects plant survival time, and how we define the ultimate mortality mechanisms (Anderegg et  al., 2012a; Sevanto et  al., 2013). This criticism has, for the first time, brought phloem transport during drought into focus (McDowell and Sevanto, 2010; Sala et al., 2010). Cessation of phloem transport could be the key to understanding the progress of plant mortality because phloem transport failure could promote carbon starvation

© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: [email protected]

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Received 29 August 2013; Revised 2 December 2013; Accepted 4 December 2013

1752 | Sevanto

Classical view and viscosity limitation The currently most accepted theory of phloem transport, the Münch hypothesis (Münch, 1930; see also Knoblauch and Peters, 2010) states that carbohydrates needed for metabolism

and storage travel from source tissues (mature leaves and other photosynthesizing tissues or transient storage) to sink tissues through a conduit system consisting of sieve cells. Solutes loaded into the conduits increase the osmotic potential that attracts water from the surroundings through the semi-permeable cell walls at the sources (Fig. 1). At the sinks, unloading of solutes reduces the osmotic potential allowing water flow back to the surrounding tissues and reducing the hydrostatic pressure in the sieve cells. The pressure difference between the sources and sinks drives the bulk flow of solutes between these tissues. Thus, the phloem transport system consists of three parts: the source, the transport pathway, and the sink. The flow rate depends on the strength of both the source and the sink and the resistance of the pathway between the two ends. The water drawn to the conduits osmotically at the source ultimately originates from the xylem, which builds a close relationship between phloem transport and xylem water status (Thorpe and Minchin, 1996; Hölttä et al., 2009). The higher the xylem water tension (drier the plant), the higher the osmotic potential that is required to pull water from the xylem to the phloem. Thus, maintaining phloem transport in dry conditions requires a strong source. In this classical view of phloem transport, the flow rate in each phloem conduit is described by the Hagen–Poiseuille law where flow rate (J; m s–1) is proportional to the pressure (P; Pa) difference between the source and the sink, which can be approximated as the solute concentration (c; mol m–3)

Fig. 1.  A schematic presentation of the original Münch pressure– flow hypothesis where both the semi-permeable sink and the source areas are in the same water bath, but they are connected to each other via an impermeable transport conduit. Sugar concentration (C1) at the source end is higher than that at the sink end (C2) resulting in a pressure gradient between the source and the sink. This pressure gradient drives carbohydrate transport in the conduit and the flow in the conduit is covered by the Hagen– Poiseuille law (equation 1).

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by prohibiting the redistribution of carbohydrate reserves to starving tissues (McDowell and Sevanto, 2010; Sala et al., 2010). It could also promote hydraulic failure if the relocation of carbohydrates was needed to ensure refilling of embolized conduits. Therefore, resolving how phloem transport might fail during drought, and whether it can recover from drought, are key questions that need to be answered before mechanisms of drought mortality are understood. This task, however, is very challenging because of our sparse, although constantly increasing, knowledge on phloem transport. At the moment, the majority of the existing phloem transport studies have been conducted under well-watered conditions (for a review see van Bel, 2003), and the majority of drought experiments have focused on xylem vulnerability to cavitation, stomatal control, and tissue carbohydrate stores (for a review see Chaves et al., 2003; McDowell et al., 2008; Choat et al., 2012) implicitly assuming that drought does not affect phloem transport (Sala et  al., 2010). Therefore, our empirical evidence on how drought affects phloem transport is very limited and mostly indirect. From the point of view of understanding phloem transport, drought experiments provide a unique setting for testing theories, as the extreme conditions reveal the boundaries of phloem function and have the potential for shedding light on the transport mechanisms and controls. Thus, phloem transport experiments in drought conditions would benefit both fields. As a result of the scarcity of empirical work on phloem transport during drought, this review will focus on the theories behind phloem transport models. It will use the classical Münch hypothesis and the theory on flow in conduits with semi-permeable walls, to develop arguments on how phloem transport could fail during drought and how the failure mechanisms depend on phloem conduit structure. The goal is to make theoretical predictions that could serve as a basis for a new set of experiments about phloem function during drought. The predictions will be discussed in the light of the anisohyric and isohydric stomatal response strategies to drought, because of the interesting contrast in requirements for phloem transport that these strategies reveal. This review will focus on the effects of a rapid, severe drought on phloem transport in a time-scale that does not allow for structural adaptations. It will also focus on woody plants that are most relevant in the context of forest mortality, but have exhibited the largest challenges to the Münch hypothesis with almost absent pressure gradients in the phloem and the long distance between sources and sinks (for a review see Thompson, 2006; Turgeon, 2010). The theories themselves are not restricted to any particular plant type, but they mostly explain flow in a single conduit and, therefore, the effects of sieve plates, sieve areas or other occlusions in the conduits are mostly omitted here.

Phloem transport and drought  |  1753 difference between these locations (R is the gas constant and T temperature; van’t Hoff relation for dilute solutions). The coefficient of proportionality is a conductivity term that depends on conduit size (radius r and length L) and fluid viscosity (µ)



J=

π r4 π r4 ( Psource − Psink ) = RT (csource −csink ) (1) 8Lµ 8Lπ

Semi-permeability of conduits walls The Hagen–Poiseuille law was originally developed to describe laminar flow in cylindrical tubes of impermeable walls (Bird et al., 2002) and the classical view of phloem transport, based on the experiments of Münch (1930), assumes that the walls of the phloem conduits outside the source and sink areas are impermeable. This model has been successfully applied to explain sink priority in plants (Thornley and Johnson, 1990; Minchin et  al., 1993) and the Hagen–Poiseuille law is used in most current phloem transport models for calculating the pressure changes and flow rates in the axial direction (Henton et  al., 2002; Thompson and Holbrook, 2003; Hölttä et  al., 2006, 2009; Jensen et al., 2009). However, from relatively early on there has been discussion about the role of the possible semi-permeability of phloem conduit walls on flow rates and phloem transport (Eschrich et al., 1972; Christy and Ferrier, 1973; Young et al., 1973; Phillips and Dungan, 1993). There also is a growing amount of experimental evidence supporting the view that the walls of sieve cells are semi-permeable allowing water to enter and exit everywhere along the conductive pathway (Minchin and Thorpe, 1987; Knoblauch and Peters, 2010) and that this water exchange is essential for phloem transport (Zimmermann and Brown, 1980). Therefore, even the models that use the Hagen–Poiseuille law for calculating axial pressure gradients, have relaxed the impermeability assumption and allow water exchange in radial direction for the total length of the phloem conduit (Henton et al., 2002; Thompson and Holbrook, 2003; Hölttä et  al., 2006, 2009; Jensen et al., 2009, 2011). Semi-permeability of the conduit walls generally makes phloem transport less vulnerable to increasing viscosity, but more susceptible to changes in xylem water tension than the classical model. If the conduit walls are impermeable, xylem water tension affects transport only at the sink and source regions and once in the transport phloem, the flow is affected only by the geometry of the conduit and properties of the fluid inserted into the conduit at the source. If the viscosity of the fluid is too high for the system (conduit geometry and existing pressure gradient) flow will simply cease. When the

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Based on equation (1), increases in fluid viscosity decrease flow rate. Sucrose is the most commonly transported sugar in the phloem (Thorne and Giaquinta, 1984), but viscosity of a sucrose solution increases exponentially with increasing concentration (Morison, 2002). At ~5 MPa osmotic pressures the viscosity would already be almost 30 times that of pure water (for comparison, milk is three times more viscous than water and castor oil is 1000 times). Xylem water tensions ~5 MPa can easily be reached by relatively anisohydric species, but isohydric species tend to close their stomata limiting xylem water tensions to below ~3 MPa (McDowell et al., 2008; Plaut et  al., 2012). An increase in solute concentration is needed during drought to attract water into the phloem from the drying xylem for transport purposes. Therefore, during drought, increasing viscosity could lead to severe slowing down of phloem transport, or even a complete phloem blockage, especially in anisohydric species (McDowell et al., 2013). Equation (1) also shows that, to compensate for the viscosity increase, plants that are adapted for maintaining phloem transport during drought should have wide conduits (large r). Alternatively, plants could try to increase source–sink concentration gradient by lowering the sink concentration to overcome the effects of viscosity increase, although that is a less effective means than increasing conduit radius because of the strong dependence of the flow rate on the radius to the fourth power. If it is also assumed that phloem conduits are independent of each other, the total sap flux through the phloem can be calculated by simply adding the fluxes in each conduit together (Hölttä et  al., 2009). This leads to a prediction that having a larger phloem (more conduits) would allow higher total transport rates even at high viscosity during drought. Applying this theory to plants showing isohydric and anisohydric strategies suggests that, as long as phloem transport is a prerequisite of photosynthesis, relatively anisohydric plants that can photosynthesize at high xylem water tensions should have wider phloem conduits or larger phloem (or both) compared with relatively isohydric plants. Interestingly, the opposite is true for xylem conduit size. Relatively anisohydric species have smaller xylem conduits than relatively isohydric species to allow xylem operation at high water tensions without cavitation (Domec and Johnson, 2012; McDowell, 2011). Phloem transport rates during drought could still be comparable in these two groups as long as isohydric plants can use carbohydrate stores as sources when photosynthesis declines. Stomatal closure will keep xylem water tensions low enough so that viscosity increase will not be a challenge for transport in these plants. There is evidence that high concentrations of mono- and disaccharides in the leaves may feed back to the stomatal

guard cells inducing stomatal closure (Sheen, 1990; Krapp et al., 1993; Paul and Foyer, 2001), which suggests that maintaining phloem transport may be essential for photosynthesis (Nikinmaa et al., 2013). There are no data available for evaluating whether phloem conduits of anisohydric plants are wider than those of isohydric plants. There is some evidence of two-year old branches of one-seed juniper (Juniperus monosperma; a relatively anisohydric tree) having a larger phloem area relative to the xylem area than similar branches of piñon pine (Pinus edulis; a relatively isohydric tree) (S Sevanto, unpublished data). But more evidence is needed before conclusions can be made. Therefore, these predictions remain to be tested. It is clear, however, that according to this classical view of phloem transport, drought generates a challenge that could lead to phloem transport failure through viscosity build-up at the sources.

1754 | Sevanto semi-permeability of the walls is taken into account, more water can enter the phloem conduits along the entire transport length, diluting the solution and reducing the effects of high viscosity along the way (Phillips and Dungan, 1993). Flow in conduits with semi-permeable walls can be characterized using two dimensionless parameters H and L p (Phillips and Dungan, 1993; called F and R in Thompson and Holbrook, 2003). H describes the relative importance of osmotic forces to viscous, frictional pressure losses in generating the axial flow, and L p is the dimensionless hydraulic permeability of the conduit walls.



Lp =

Φ0r2 (2) µLU

Lp L2 µ r3

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H=

is diluted by the incoming water along the way (Phillips and Dungan, 1993; Eschrich et al., 1972). In conduits with semi-permeable walls, the axial flow is entirely generated by the water potential difference between the conduit and its surroundings. The flow ceases if water exchange through the conduit walls is hindered. In the above discussion, an infinite water pool surrounding the conduits has been assumed. Drought and increasing xylem water

(3)

where Φ0 is the initial osmotic force at the source (here zero concentration at the sink end is assumed), U is the characteristic axial flow velocity, L is the conduit length, and Lp is the hydraulic permeability of the conduit walls. If H is large, radial water flow through the conduit wall is caused by the presence of osmotically active solutes. Small H means that the radial flow is due to viscous pressure drop in the conduit (Phillips and Dungan, 1993). Parameter H, however, cannot simply be used to assess the flow type (osmotic versus viscous), but the influence of L p on the flow, in the form of the product of H and L p , has to be used (Phillips and Dungan, 1993). If that product is large, even if L p was small, osmotic forces will dominate the flow, and the axial solute flux in a conduit will be constant (Haaning et al., 2013; Eschrich et al., 1972). The maximum osmotically obtainable flow rate (v; m s–1) can be calculated as a product of the initial osmotic force at the source (Φ0) (here zero concentration at the sink end is assumed), the hydraulic permeability of the conduit wall (Lp) and the aspect ratio (α; length per radius) of the conduit

vmax _ osmotic = 2Φ0 Lp α (4)

where Φ0 can be estimated, for example, from the van’t Hoff relation (Haaning et al., 2013). If L p is large, but H is so small that the product becomes small, the axial flow is affected by viscous effects on the pressure gradient but, as long as the conduit walls are permeable, the flow velocity and axial pressure profiles differ from those predicted by the Hagen–Poiseuille equation (equation 1) (Phillips and Dungan, 1993). The viscous forces generate an additional pressure drop which speeds up flow and, instead of being constant, the flow rate will actually increase towards the ends of the conduits (Phillips and Dungan, 1993). This is because the water attracted to the conduit pushes the contents of the conduit with increasing flow velocity, which decreases the pressure inside the conduit attracting even more water from the surroundings (Fig.  2). Interestingly, if the viscous effects are strong enough, this radial water cycle between the conduit and its surroundings will occur even if the solution

Fig. 2.  A schematic presentation of flow in a semi-permeable tube. High sugar concentration at the source area (here top) attracts water from the surroundings to the semi-permeable conduit (Πin>Πout) increasing the pressure (Pin) at the sugar front. This pushes water inside the tube and forces it out back to the surroundings below the sugar front creating a pressure gradient (Pin>P) that drives the flow (velocity, v). If the conduit is wide and flow rates small, the flow is solely driven by the osmotic potential difference between the conduit and the surroundings. If the conduit is narrow and the flow rate fast, viscous forces will create an additional pressure gradient that will increase the flow velocity and flow will not be affected by the dilution of the sugar front (Phillips and Dungan, 1993; Jensen et al., 2009).

Phloem transport and drought  |  1755



Mü = 16

µLp L2 r3

(5)

Applied to drought, Mü could be used for estimating a viscosity limit for flow in a single conduit by arguing that axial flow is too slow to be efficient when axial conduit resistance equals radial conduit wall resistance (i.e. Mü=1). This limit is based on the simple argument that having wide conduits (see equation 1) would not benefit the plant if axial conductivity equals radial conductivity (normally they differ by several orders of magnitude (Sevanto et al., 2011)), and thus flow in such system would be very inefficient. This calculation leads to an inverse relationship between viscosity and conduit wall permeability stating that the less permeable the conduit walls are, the more viscous flow the conduits can transport, which is contradictory to the theories presented above. Even if results from the models using this approach are useful and valid in moist conditions, during drought, and at the limit of increasing viscosity this approach does not give reasonable results, and should be avoided. Flow in conduits with semi-permeable walls differs from flow in impermeable conduits in that the apparent hydraulic conductance of the conduits is not proportional to the fourth power of conduit radius as stated by equation (1), the basis for developing equation(5). Instead, flow rates tend

to be higher in conduits with smaller radii (Eschrich et  al., 1972) resulting in a constant axial solute flux (flow rate per conduit area) [see equation (2) and Haaning et  al., 2013], which would reduce the effect of sieve plates or other occlusions along the flow path. Source and sink strength affect the initial flow velocity (Eschrich et al., 1972), and may direct the flow towards the strongest sinks but, unlike in the classical impermeable conduit model, source and sink strength does not drive the flow. The pressure and concentration gradients are solely created by the interaction between the conduit and its surroundings (Young et al., 1973), and therefore the axial flow resistance cannot be treated in the way it is treated when developing equations (1) and (5). During drought, high conduit wall permeability would allow easy water exchange between the tissues, and higher flow rates (see equation 4), but the trade-off would be that, at high wall permeability, the solute concentration changes in the phloem would have to be fast enough to balance the changes in xylem water tension to prevent turgor loss of the cells and to maintain water potential equilibrium with the xylem along the whole pathway (Thompson and Holbrook, 2003). With semi-permeable conduit walls protecting the phloem turgor is critical because the driving force for the flow is based on water exchange between the conduit and its surroundings (Eschrich et  al., 1972; Young et  al., 1973). Therefore, one could argue that relatively anisohydric species with relatively high xylem water tensions need either low conduit wall permeability in the phloem or a very fast and efficient osmoregulation system to keep water from leaking to the xylem. In the first case, the flow would resemble the classical Hagen–Poiseuille flow, and relatively anisohydric plants could reduce viscosity build-up by using solutes such as potassium (Talbott and Zeiger, 1996) that do not increase fluid viscosity. The second case would require a high capacity to mobilize solutes for osmoregulatory needs (e.g. convert storage carbohydrates to smaller molecules for increasing osmotic strength). Conduit wall permeability will thus determine drought impacts on phloem flow and what anatomical and physiological traits are important for the integrity of the system. Low permeability would prevent phloem turgor collapse during drought, but phloem transport rates would generally be lower than with more permeable conduits. This could have consequences on plant growth rate (Van Bel, 1996) and competition. High wall permeability would ensure fast phloem transport and an advantage in growth and competition but, during drought, it could lead to mortality via turgor collapse. Fast osmoregulation could also require active phloem loading, which is less energy-efficient than passive, diffusion-driven, loading. Therefore, having highly permeable conduit walls might prove a more risky strategy especially in environments with regular droughts.

Does phloem transport cease during drought? All terrestrial plants face the trade-off between water loss and carbon gain. Avoiding excessive water loss via stomatal closure

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tension will affect the water pool ultimately leading to shortage of water to support this kind of flow, but increasing fluid viscosity will speed up axial flow rather than hinder it. This is because increasing viscosity increases L p and the importance of the viscous effects on the pressure gradient in the conduit, but it decreases H in equal proportion so that the product of H and L p does not depend on viscosity at all leaving the importance of the osmotic forces unchanged. Drought could also reduce H by decreasing source strength, which would lead to viscous forces dominating the flow, but this does not mean that solute transport would cease as long as there is some kind of a sugar source and enough water in the surrounding tissue. If we imagine that sugars can be loaded to a conduit along the way while the fluid is already initially moving with a certain velocity, the flow rate will increase along the way, although the relative gain in flow velocity resulting from enhanced osmotic water exchange through the walls decreases with increasing initial velocity (Haaning et al., 2013). Therefore, this kind of an action would be most effective in increasing the flow rate close to the sources where flow rates are initially small. The permeability of conduit walls has also been treated by allowing radial water exchange, but using the Hagen–Poiseuille law to drive the flow and calculating the pressure differences in axial direction. This method correctly makes phloem flow more sensitive to xylem water tensions than the impermeable wall model (Hölttä et al., 2009), but it does not take the real effects of conduit wall permeability on the flow rate into account. This can be seen by applying a dimensionless parameter obtained from this approach to drought conditions. This parameter (Mü) resembles L p, the dimensionless conduit wall permeability, but is interpreted as describing the ratio of flow resistance in the axial and radial directions (R in Thompson and Holbrook, 2003; Mü in Jensen et al., 2009, 2011).

1756 | Sevanto

How does phloem failure occur, and is it important for survival? The classical impermeable conduit model predicts that phloem transport could cease during drought even with active sources and sinks because of viscosity blockage in the transport pathway (Hölttä et al., 2009; McDowell and Sevanto, 2010). The permeable conduit wall model, predicts that phloem transport would primarily cease because of reduced source and sink activity or because of reduced water content of the tissue surrounding the phloem conduits. The only experiment to date, that has tested phloem failure during tree mortality used a relatively isohydric tree species (piñon pine; Pinus edulis). In this experiment phloem turgor collapse occurred at ~2 MPa xylem water tension preceding mortality by several weeks, and the timing of phloem turgor collapse predicted both the amount of carbohydrates the trees could utilize and the survival time (Sevanto et  al., 2013). At 2 MPa the viscosity of a sucrose solution is only roughly twice the viscosity of pure water (Morison, 2002), which should not produce any challenge to transport yet. One could thus argue that failing turgor maintenance rather than viscosity build-up caused phloem failure in these trees consistent with the semi-permeable conduit wall model, and that the timing of turgor collapse determined the level of access to stored carbohydrates. Turgor maintenance during drought can be obtained by two means: (i) by increasing solute concentration inside the cells or

(ii) by reducing cell wall permeability (or both). The failure of these would result in water leaking to the xylem, possibly temporarily refilling embolism. This, however, would not be a permanent solution to progressing hydraulic failure because of the limited amount of water available from the phloem tissue. At present, it is not known which one of these mechanisms fails when phloem turgor collapses and why, and at which stage of the collapse turgor can still recover. It is only known that phloem turgor loss can occur even without depletion of carbohydrate reserves (Sevanto et al., 2013), and it seems to be an important predictor of carbohydrate storage use and plant survival. It is possible that plants control cell wall permeability during drought to reduce the risk of phloem turgor failure by, for example, by controlling aquaporins (Secchi and Zwieniecki, 2010; Sevanto et al., 2011). However, an increase in water tension in the xylem could effectively lead to an apparent reduction of conduit wall permeability even if the qualities of the walls did not change. This is because, independent of the model considered, at high xylem water tensions increased solute concentrations in the cells surrounding the phloem conduits are required for maintaining operationality. In the classical model, this is needed to initiate flow and to compensate for viscosity build-up; in the semi-permeable conduit wall model, it is needed to maintain the water pool around the conduits. Solutes reduce the mobility of water molecules (lower the water potential; Nobel, 2005), which means that more energy (higher pressure gradient) is required to pull water out from the cells to produce the same flux with solutes than without solutes (Darcy’s law). Conductivity is usually defined as the factor determining how large a flux one obtains with a certain pressure gradient (see equation 1). Therefore, the magnitude of conductivity also depends on the fluid that was used to measure it. This means that to separate the effects of high solute concentration and low wall permeability and to understand the drought responses of the phloem, one has to use methods that reveal changes in conduit wall properties separate from changes in cellular solute content. Consistently with the increased osmoregulatory demands, many plants show an initial accumulation of sugars in the leaves (Körner, 2003; Sala et al., 2010, and references therein) or the sieve tubes (Cernusak et  al., 2003) under drought. Supported by the observations of reduced turgor difference between the source and sink under water stress (Roberts, 1964; for a review see Crafts, 1968; Ruehr et al., 2009), this has been interpreted as sink strength (growth) reacting to drought earlier and/or more strongly than photosynthesis (Körner, 2003; McDowell and Sevanto, 2010). The accumulation of carbohydrates, however, could also be interpreted as reduced phloem transport rate (Sala et al., 2010). The accumulation tends to occur at the early stages of drought when xylem water tensions are so low that viscosity should not produce too large a challenge for transport. In the semi-permeable conduit wall model, reduced hydraulic permeability between the xylem and the phloem could slow phloem transport down resulting in carbohydrate accumulation, which could then feed back to both photosynthesis (Nikinmaa et al., 2013) and growth, raising the question of which is the cause and which the reaction and how the long-distance signalling in plants works.

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during drought inevitably leads to reduced photosynthesis and carbohydrate production, which reduces source strength. Also, the turgor-related reduction of growth is one of the first drought responses in plants (Hsiao et al., 1976), reducing sink strength. However, there is evidence that both non-woody and woody plants can cycle water and carbohydrates without a photosynthetic source (Tanner and Beevers, 2001; Sevanto et al., 2013). Thus, reduced source or sink activity will not necessarily lead to the complete cessation of phloem transport as long as carbohydrate reserves can be utilized (Sevanto et al., 2013) but, in the absence of a photosynthetic source, access to carbohydrate reserves becomes essential. What determines the extent to which plants can deplete their carbohydrate reserves is currently unclear. Mild drought seems to enhance the access in crops (Rodrigues et al., 1996; Yang et al., 2001), and there is evidence that nitrogen could play a role both in crops (Yang et al., 2000, 2001) and in trees (Millard and Grelet, 2010; Chapin et al., 1990; Millard et al., 2007; Sala et al., 2012). High water availability clearly helps in the utilization of the reserves in trees (Sevanto et al., 2013) but, during extreme drought (no water supply until trees die), trees of the same species in the same environment can lose access to their reserves at very different times (Sevanto et al., 2013). It seems clear both theoretically and experimentally that, if the drought is long and severe enough, phloem transport failure will occur, but when it occurs and where the threshold between mild and severe drought is for different species, and even individuals, is an open question.

Phloem transport and drought  |  1757

Acknowledgements The author wants to thank Dr Matthew Thompson for informative and interesting discussions on the topic as well as Drs Nate McDowell and Chonggang Xu for comments on the manuscript. This work was supported by a LANLLDRD grant.

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One of the key questions brought up by this review is the permeability of the phloem conduits walls, and its impact on phloem transport. Currently, our knowledge on this topic is very limited. Based on what is known about xylem conduit walls (Sperry, 2003), one would expect the conduit walls to be porous, unless they are sealed with a substance like lignin or suberin, or the pores are air filled, in which case the continuum of water would be interrupted by air–water interfaces. For understanding tree mortality mechanisms during drought, and phloem transport in both moist and drought conditions, it would be of high importance to design experiments for measuring hydraulic permeability of phloem conduit walls as well as detecting whether the permeability can change seasonally and with environmental conditions (e.g. xylem water tension). Permeability of the conduit walls could explain the small pressure gradients in the phloem tissue of many trees (Thompson, 2006), and controlling the permeability could be an easy way for a plant to control water cycling between the xylem and the phloem as well as between the sources and sinks (Sevanto et al., 2011). Before more detailed data on the hydraulic permeability of phloem conduit walls are available for a variety of plant species with different strategies of coping with drought, it is impossible to say whether phloem failure due to viscosity build-up can occur. Based on current evidence, phloem transport seems to be susceptible to the maintenance of the water status of the phloem tissue, and phloem failure resulting from turgor collapse seems a more likely phloem failure mechanism than viscosity buildup, although having impermeable conduits could be beneficial for relatively anisohydric plant species adapted to regular droughts. Phloem failure during drought seems to be a mechanism that promotes plant mortality by, at least, affecting access to carbohydrate reserves (Sevanto et  al., 2013), but even if there is evidence that carbohydrates could be necessary for embolism repair (Secchi and Zwieniecki, 2011), there is little evidence that phloem function during drought could prevent hydraulic failure by increasing refilling. This is because without additional water refilling one conduit would result only in the embolization of an adjacent one (Secchi and Zwieniecki, 2010; Sevanto et  al., 2013) and not improving overall xylem conductivity. However, embolism repair could be critical to recovery from drought and recovery of the phloem function from drought might play a key role there. Theoretically, plants with isohydric and anisohydric stomatal control strategies would clearly benefit from different phloem conduit properties, but how much phloem conduit structure differs between species in reality is to be shown by future experiments.

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