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We assessed the effects of irradiance received during growth on the vulnerability of Fagus sylvatica L. xylem vessels to water-stress-induced embolism. The measurements were conducted on (1) potted saplings acclimated for 2 years under 100% and 12% incident global radiation and (2) branches collected from sun-exposed and shaded sides of adult trees. Both experiments yielded similar results. Light-acclimated shoots were less vulnerable to embolism. Xylem water potential levels producing 50% loss of hydraulic conductivity were lower in sun-exposed branches and seedlings than in shade-grown ones (–3·0 versus –2·3 MPa on average). The differences in vulnerability were not correlated with differences in xylem hydraulic conductivity nor vessel diameter. Resistance to cavitation was correlated with transpiration rates, midday xylem and leaf water potentials in adult trees. We concluded that vulnerability to cavitation in Fagus sylvatica may acclimate to contrasting ambient light conditions.
Since the introduction of reliable techniques to measure cavitation events (Tyree & Dixon 1983) and air embolism in xylem (Sperry, Donnelly & Tyree 1988a), considerable progress has been made in the knowledge of species vulnerability to water stress and in the understanding of whole plant water relations. We now have evidence that xylem conduits are exposed to cavitation events and that many woody species are operating very close to the point of xylem dysfunction (Tyree & Sperry 1988). Therefore, it has been suggested that the maintenance of xylem integrity may impose a limitation to maximum plant transpiration rates (Jones & Sutherland 1991; Cochard, Bréda & Granier 1996a; Lu et al. 1996). This hypothesis has recently been verified for a mutant of Populus trichocarpa lacking efficient water loss regulation (Cochard, Ridolfi & Dreyer 1996b).
The mechanism of dehydration-induced xylem embolism has been identified (Sperry & Tyree 1988). Air enters the lumen of water-filled vessels through pores in the pit membrane of the walls when the xylem pressure potential exceeds the maximum capillary pressure sustainable by the pores. Xylem tension, measured by its water potential, Ψxylem, is therefore the triggering variable for embolism induction. Xylem dysfunction can then be characterized by ‘vulnerability curves’ (VC) which represent the changes in embolism level with increasing tension in xylem sap. Xylem VCs have been established for a large number of woody species and we now have a better idea of its variability. VCs were first shown to differ considerably between species (e.g. Sperry, Tyree & Donelly 1988b; Cochard et al. 1991; Cochard 1992), or genotypes (Neufeld et al. 1992; Borghetti et al. 1993). Significant differences were also found between organs of a same plant (Tyree et al. 1993a; Sperry & Saliendra 1994; Hacke & Sauter 1995) or similar plants growing under contrasted soil water conditions (Jackson, Irvine & Grace 1995; Alder, Sperry & Pockman 1996). It has also been shown that petiole vulnerability varied according to their vertical position in a crown of Fraxinus excelsior (Cochard et al. 1997).
However, the question of whether the xylem vulnerability of a tree can acclimate to modifications in ambient conditions remains largely unexplored. For instance, forest tree species experience very contrasting light conditions during their life-time, especially shade-tolerant species that initiate their growth in the understory. As light is a major factor determining plant water relations, the degree of xylem tension and therefore the risk of xylem embolism may change with the growing conditions of the tree. Xylem vulnerability acclimation to new irradiance conditions might then determine tree survival. This may also be an important issue in the context of a global climate change because the increasing likelihood of drought may interact with the effects of light. There is evidence from the literature that xylem conductance is sensitive to light (Schultz & Matthews 1993) but there is no information available on the effect of light on xylem vulnerability to embolism.
The objective of this study was to assess the plasticity of xylem vulnerability in Fagus sylvatica induced by different ambient light conditions. We chose this species because it is known to grow and develop under both full-light and heavily shaded conditions and to maintain large living branches in the shade of the under-canopy. We compared shoot internodes VCs (1) of saplings grown under two different irradiance levels and (2) of branches in different light conditions from the same adult trees. Xylem safety was compared to xylem anatomy and xylem hydraulic conductivity to examine if a gain in hydraulic safety was associated with a loss in hydraulic efficiency. We also analysed leaf gas exchange and leaf water potentials to find out if differences in VCs had some impacts on branch water relations.
MATERIALS AND METHODS
Two mature, 70-year-old, Fagus sylvatica L. trees were chosen in the Arboretum of Amance, near Nancy, in the eastern part of France (48°45' N, 6°20' E, elevation: 255 m). The trees were growing on the periphery of the Arboretum, with southern branches exposed to full sunlight (=light branches), and northern ones heavily shaded by upper crown branches and surrounding trees (= shade branches).
The experiments on potted saplings were conducted in 1994 after the plants were grown for two consecutive years under contrasting light environments. In spring 1993, 80 1-year-old plants were transplanted in 5-L pots containing a mixture of sand and blond peat and fertilizer (Nutricote 100 N/P/K 13/13/13; Nutril, Paris, France). The plants were randomly allocated to each of two treatments: full light (control) and shade. In the shade treatment, the incident irradiance was artificially reduced by shade screens. The microclimate under the screens was continuously monitored for several weeks with a portable weather station and correlated with the nearby weather station of Champenoux. On average, the global radiation (W m–2) for the shade treatment was 0·12, the air temperature (°C) 0·86, and the air vapour pressure deficit (hPa) 0·9 that of the ambient values, respectively. The shade treatment was close to the light environment in Fagus sylvatica forest understory (Aussenac & Ducrey 1977) with the difference that the red/far red ratio was not modified by these neutral screens (data not shown). Plants were well watered throughout the experiment. Six full-light, and seven shade-grown saplings were sampled for hydraulic analysis.
Vulnerability curves (VCs) are plots of degrees of xylem embolism versus Ψxylem that induced the embolism. They are constructed by dehydrating different excised branches to different Ψxylem. Degrees of embolism were assessed as described by Sperry et al. (1988a) by measuring losses of hydraulic conductance caused by air blockages in the xylem conduits of short (2–3 cm) shoot internodes. We established VCs for current-year and previous-years shoot internodes. In August and September 1993, we collected branches from the mature trees in the morning with a 6-m-long pruning pole, enclosed them in an black airtight plastic bag to reduce water loss through transpiration and brought them to the laboratory for hydraulic analysis. In August 1994, VCs were established for saplings. Saplings’ root systems were cleaned from their soil in the nursery and brought to the laboratory as described above. In the laboratory, the samples were bench dehydrated for a few hours to a few days, then enclosed for at least 4 h in an black airtight plastic bag to stop transpiration and remove water potential gradients between the leaves and xylem vessels. Ψxylem was then calculated from the leaf water potential measured on four randomly chosen leaves with a pressure chamber. Xylem tension was then returned to zero by immersing the branches in tap water for 30 min before hydraulic analysis. When the saplings were large enough, one or two branches were cut under tap water from the main axis and the remainder of the saplings allowed to dehydrate anew to lower Ψxylem values. After rehydration, eight to 15 shoot internodes were excised under water from different growth units of each branch. The initial hydraulic conductivity Kinit (mmol m s–1 MPa–1) was measured by forcing distilled water with 5 kPa pressure difference through each sample and measuring the resulting flow rate (mmol s–1) with five decimal place analytic balances connected to a computer. Air embolism was then removed by successive 0·1 MPa water pressurization until the conductivity no longer increased (Kmax). The percentage loss of hydraulic conductivity (P) was then calculated as:
The following sigmoid function was fitted to the experimental vulnerability curves:
where P0 is the native state of embolism, Ψ50P is the Ψxylem that induces 50 P and s is a slope parameter. Equation 2 was inverted to compute the value of Ψxylem for which P increased by 10% (Ψ10P). Ψ10P was a rough estimate of the point of runaway embolism, i.e. a threshold water potential below which any slight increase in leaf transpiration will lead to 100 P and shoot desiccation (Tyree & Sperry 1988).
To characterize the xylem hydraulic efficiency to water transport (Tyree & Ewers 1991), we assessed xylem anatomy (see below), segment cross-section area specific hydraulic conductivity (Ks, mol m–1 s–1 MPa–1) and leaf area specific hydraulic conductivity (Kl, mmol m–1 s–1 MPa–1). Ks was computed by dividing Kmax by the segment cross-section (m2, bark not included). Ks is an integrative parameter determined by vessel diameter and vessel number. Kl was calculated by dividing Kmax by total leaf area distal to xylem internodes (m2, one side counted).
Water potential and stomatal conductance
Minimum midday leaf water potentials (Ψleaf) of the adult trees were assessed with a portable pressure chamber (PMS, Coravalis, Oregon, USA) on two sunny days during the summers of 1993 and 1997. Midday xylem water potentials (Ψxylem) were estimated in 1997 by measuring the midday water potential of leaves that were previously enclosed in an aluminium foil early in the morning. During the 1994 growing season, we measured sapling midday leaf stomatal conductance, gs (mmol s–1 m–2), and transpiration rate, E (mmol s–1 m–2) with a portable porometer (Li-Cor 1600; Lincoln, Nebraska, USA). This was measured on four plants per treatment on 7 d. In 1997, we measured gs and E for adult trees on leaves randomly taken from representative sunlit and shaded branches.
Vessel diameters were measured for saplings and adult branches on four representative 1-year-old shoots. Thin cross-sections were obtained by hand with a fresh razor blade and observed with a light microscope (400x). On each cross-section we chose a sector delimited by two radial rays and measured at least 50 conduits larger than 10 μm with an micrometric ocular (resolution one μm). For each conduit, we measured the minimum and maximum lumen diameters and computed their geometrical means.
Shade- and full-light-grown branches differed in their anatomy and water relations (Table 1). Leaf mass per unit area (LMA) was higher for leaves grown under high irradiance, but the LMA values for light-acclimated saplings were much lower compared with light-acclimated adult branches. Midday leaf (Ψleaf) and xylem (Ψxylem) water potential values were significantly lower for full-light branches than shaded ones. Lower water potential values were associated with higher stomatal conductance and transpiration rates in adult trees. However, saplings of both treatments exhibited similar stomatal conductance (gs) values.
Table 1. . Anatomy and water relations of shade and light grown Fagus branches or saplings. Leaf characteristics included leaf mass per unit area (LMA) and individual leaf area. E represents leaf transpiration rates, gs stomatal conductance to vapor water and Ψxylem and Ψleaf, xylem and leaf water potentials, respectively. Data are means ± SE with n the number of replicates. Water potential values were not available for saplings
On mature trees, branches developed under full-light conditions were clearly less vulnerable to water-stress-induced embolism (Fig. 1a & b and Table 2). The difference in Ψ50P and Ψ10P was about 0·8 MPa for both branch age classes and highly significant. Older branches (Fig. 1b) exhibited a higher degree of native embolism (about 40%) because of an incomplete removal of embolism accumulated during the previous winter (Cochard & Lemoine, unpublished). The effect of irradiance levels on xylem embolism was experimentally confirmed on saplings grown in a nursery. Higher light intensity corresponded to lower vulnerability to embolism. However, the differences in Ψ50P between treatments were lower (about 0·4 MPa) and for older shoots the differences were not significant. Overall, a linear relationship (r2 = 0·97) was found between Ψ50P of current year shoots and LMA (see also Fig. 5b).
Table 2. . Xylem vulnerability to embolism. Logistic functions (see Eqn 2 in the text) were fitted to experimental P versus xylem water potential (Ψxylem) curves. Parameters of the functions were: (1) the native state percentage loss of conductivity (P0) (2) Ψxylem for 50% increase of P above P0 (Ψ50P) and (3) s, a slope parameter. Ψxylem below which P increased 10% above P0 (Ψ10P), was then computed as a theoretical value for catastrophic xylem dysfunction. We distinguished 1-year-old shoots from older shoots because P0 values were very different. Values are given ± SE. Data having a letter in common are not significantly different at P = 0·01
On Fig. 2 we expressed the hydraulic conductivity after embolism dissolution (Kmax) as a function of stem diameter for saplings and adult trees. The relationships were linear (on a log-log scale) but slopes were greater than two suggesting that segment cross-section area specific hydraulic conductivity (Ks) was increasing with stem diameter. For a given segment diameter, Kmax was significantly higher for the internodes of shaded saplings (compare open and closed symbols on Fig. 2 lower panel) but Kmax were identical for different branches in the same mature tree (Fig. 2 upper panel). Within each treatment, leaf specific conductivities (Kl) were significantly higher for current year apical shoots than older shoot internodes (Fig. 3, compare different symbols with same colour). For a given adult tree stem diameter, Kl values were much higher for branches acclimated to full light (Fig. 3 upper, open versus closed symbols) but irradiance had no effect on Kl for saplings (Fig. 3 lower). The frequency distribution of vessel diameters were comparable in 1-year-old shade and light branches of adult trees (Fig. 4). In saplings, the vessels were wider, especially in the full-light treatment. The fact that Ks was lower in full-light saplings although vessels were wider was probably caused by a lower vessel density or a higher production of late wood in this treatment.
On Fig. 5 we expressed Ψ50P as a function of the mean Ks for each treatment. This graph expresses a safety versus efficiency trade-off for xylem vessels. Within the same branch system, there was a tendency for an increase in vulnerability with segment age and therefore Ks (compare data connected by a dotted line) but the increase in xylem vulnerability under low light conditions was not correlated with changes in Ks (compare data with same symbol and different colours).
In this study we showed that the irradiance level received during growth had strong impacts on the xylem vulnerability to water-stress-induced embolism in Fagus sylvatica. Branches developed under shaded conditions were more vulnerable than sun-exposed ones. These patterns were reproduced experimentally by growing saplings under two different light regimes. However, saplings acclimated to full irradiance levels were more vulnerable than branches from adult trees. These saplings also exhibited lower leaf LMA values so we do not know if these patterns were age-related or due to differences in growth conditions between 1993 and 1994.
Differences in xylem vulnerability were correlated with the differences in Ψleaf and Ψxylem measured at midday during sunny days for the two types of adult branches. Ψ-values were higher in the shade because the air evaporative demand, and thus transpiration rates, were much lower. The most negative Ψleaf values were very close to the Ψ10P values for both shaded and full-light branches. However, it did not signify that xylem conduits were operating close to their point of dysfunction because Ψxylem was much higher than Ψleaf. This result occurred because, in Fagus (Tyree & Cheung 1977; Cochard & Lemoine, unpublished) and other species (Tyree et al. 1993b), most of the branch resistance to the sap pathway is extra-vascular and located in the leaf blades. Under adequate water availability, most of the leaf water potential drop will then be in the leaf blade itself. However, when soil resistances are high and transpiration rates low because of a soil drought, Ψleaf may match Ψxylem more closely and Ψxylem is more likely to approach Ψ10P (Cochard et al. 1996a; Lu et al. 1996).
Our results suggest that within a crown, branch xylem vulnerability correlates with branch leaf transpiration rates and prevailing xylem water potentials. We were surprised to find comparable and high gs and E-values (and stomatal densities, data not shown) for saplings acclimated to shade or light because these parameters were very different for the branches of adult trees. However, individual leaf and total branch leaf areas were much higher for shaded tree branches than saplings so that transpiration rate may not have been very different for branches versus saplings of comparable xylem areas. Schultz & Matthews (1993) suggest that in grapevine acclimated to low light, the xylem transport capacity was uncoupled from leaf area. This was the case only for adult F. sylvatica branches in this study, where shade grown branches exhibited lower Kl values. However, the xylem transport capacity was uncoupled to a lesser extent from water loss capacities because leaf E and gs were greatly reduced in the shade. Uncoupling leaf area from leaf transpiration rate might be a strategy to maximize light interception and water use efficiency in the shade.
Low xylem vulnerability might be advantageous for full-light branches because they experience higher and more variable transpiration rates than shaded branches, which can result in lower Ψleaf and Ψxylem. The lower susceptibility to cavitation was not correlated with a decrease in xylem hydraulic efficiency (Fig. 5). There is no tradeoff of hydraulic efficiency for vulnerability to embolism in Fagus branches acclimated to different light conditions. Tyree, Davis & Cochard (1994) came to the same conclusion when comparing the hydraulic properties of many woody species. A lower efficiency would have enhanced the water potential drop along the sap pathway which would have lessened the gain in cavitation resistance. A benefit associated with an increase in the vulnerability for shaded branches is less clear. However, because xylem conduits can acclimate to new environmental conditions probably only over the long-term (until new vessels are formed), we can predict that shade-grown branches or saplings suddenly exposed to full light (after forest thinning for instance) may experience xylem embolisms if transpiration rates are not efficiently regulated (Tognetti, Michelozzi & Borghetti 1994).
We know from other studies that, within a species, plants acclimated to different habitats may differ in their vulnerability. For instance, Alder et al. (1996) found that root xylem in Acer grandidentatum was more vulnerable for plants in a wetter site. However, vulnerability to drought-induced embolism of Rhodendendron maximum was not affected by the light environment (Lipp & Nilsen 1997). Cochard et al. (1997) established that in Fraxinus excelsior, the leaf rachis from upper crown branches was more vulnerable in comparison with lower crown branches. In their study, the decrease in xylem vulnerability correlated with a decrease in Ψxylem induced by higher xylem resistances in lower branches. Altogether, these results indicate that vulnerability to cavitation can exhibit a high degree of plasticity and can respond to light (present study), soil water availability (Alder et al. 1996) or vertical position in a crown (Cochard et al. 1997). These variations can actually be as large as between species or genotypes. From a methodological point of view, this suggests that, for comparisons of VCs of species or genotypes, plants must have experienced strictly similar ambient conditions during growth.
An important issue of this work is the mechanism by which xylem vulnerability acclimates to irradiance levels. The mechanism of water-stress-induced embolism has been largely documented (Sperry & Tyree 1988; Cochard, Cruiziat & Tyree 1992; Jarbeau, Ewers & Davis 1995) and, according to the current understanding of the phenomenon, differences in vulnerability originate from size differences in inter-conduits pit pores. This explains why xylem vulnerability was clearly not related to vessel size in this study and many others (see Tyree et al. (1994) for a review). However, the mechanism by which the light environment could induce microanatomical modifications in vessel walls remains to be explained. Because mature vessels and tracheids contain no cytoplasm, the pit pore dimensions are probably determined during conduit maturation. This suggests that when vessels differentiate during spring, the size of the pores in the pit membrane is already adapted to the Ψxylem the vessel will experience days or weeks after. It is well-known that light availability can affect plant growth through feedback mechanisms involving water transport or internal carbon balance (Kaufmann 1990). Shoot morphogenesis is also known to be changed by a modification in the far red : red ratio (Ritchie 1997). In the present study we found that under neutral shade (saplings) there were differences in the diameter distribution of vessels, whereas under natural shade and thus altered far red : red ratio there were no differences. So the possibility exists that light quality or quantity may have an effect on pit membrane structure.
In conclusion, our study suggests that xylem conduits of Fagus sylvatica grown under different light conditions are functionally adapted to contrasted xylem water potentials and transpiration rates. The high plasticity we found in Fagus correlates with its shade tolerance and its ability to maintain living branches in different light climates. We believe that these findings represent a new step in the understanding of tree–water relations. Studies on plant cavitation have already revealed that xylem dysfunction was a critical phenomenon and plant water loss must adjust to plant water transport vulnerability (Jones & Sutherland 1991; Cochard et al. 1996a). We now have evidence for an adaptation of xylem vulnerability to changing water loss intensity. The change in vulnerability with growing conditions might be an important parameter to consider in order to understand plant adaptation to its habitat.
We thank T. Améglio, P. Cruiziat, F. Ewers, A. Granier, R. Huc and two anonymous referees for helpful reviews of the manuscript. We are grateful to J.C. Hervé for assistance with the statistics.
New address: INRA-PIAF, Domaine de Crouël, 63039 Clermont-Ferrand, France.