X-ray microtomography observations of xylem embolism in stems of Laurus nobilis are consistent with hydraulic measurements of percentage loss of conductance

Drought-induced xylem embolism is a serious threat to plant survival under future climate scenarios. Hence, accurate quantification of species-specific vulnerability to xylem embolism is a key to predict the impact of climate change on vegetation. Low-cost hydraulic measurements of embolism rate have been suggested to be prone to artefacts, thus requiring validation by direct visualization of the functional status of xylem conduits using nondestructive imaging techniques, such as X-ray microtomography (microCT). We measured the percentage loss of conductance (PLC) of excised stems of Laurus nobilis (laurel) dehydrated to different xylem pressures, and compared results with direct observation of gas-filled vs water-filled conduits at a synchrotron-based microCT facility using a phase contrast imaging modality. Theoretical PLC calculated on the basis of microCT observations in stems of laurel dehydrated to different xylem pressures overall were in agreement with hydraulic measurements, revealing that this species suffers a 50% loss of xylem hydraulic conductance at xylem pressures averaging 3.5 MPa. Our data support the validity of estimates of xylem vulnerability to embolism based on classical hydraulic techniques. We discuss possible causes of discrepancies between data gathered in this study and those of recent independent reports on laurel hydraulics

Water stress-induced modifications of leaf hydraulic architecture in sunflower: co-ordination with gas exchange.

The hydraulic architecture, water relationships, and gas exchange of leaves of sunflower plants, grown under different levels of water stress, were measured. Plants were either irrigated with tap water (controls) or with PEG600 solutions with osmotic potential of -0.4 and -0.8 MPa (PEG04 and PEG08 plants, respectively). Mature leaves were measured for hydraulic resistance (R-leaf) before and after making several cuts across minor veins, thus getting the hydraulic resistance of the venation system (R-venation). R-leaf was nearly the same in controls and PEG04 plants but it was reduced by about 30% in PEG08 plants. On the contrary, R-venation was lowest in controls and increased in PEG04 and PEG08 plants as a likely result of reduction in the diameter of the veins' conduits. As a consequence, the contribution of R-venation to the overall R-leaf markedly increased from controls to PEG08 plants. Leaf conductance to water vapour (g(L)) was highest in controls and significantly lower in PEG04 and PEG08 plants. Moreover, g(L) was correlated to R-venation and to leaf water potential (Psi(leaf)) with highly significant linear relationships. It is concluded that water stress has an important effect on the hydraulic construction of leaves. This, in turn, might prove to be a crucial factor in plant-water relationships and gas exchange under water stress conditions

Hydraulic efficiency of the leaf venation system in sun- and shade-adapted species

We tested the hypothesis that leaf hydraulics is correlated with the light adaptation of different plant species and specifically that the hydraulic resistance of the leaf venation (R-venation) is lower in sun-than in shade-adapted species. R-venation was measured in six sun- and six shade-adapted species with a high-pressure flow meter (HPFM). The number of conduits at the proximal third of the midrib was counted and the diameter of the widest conduits together with vein density were measured. R-venation was higher in shade species than in sun species and it was negatively correlated with the mean diameter of the widest conduits. Maximum leaf conductance to water vapour recorded for the different species was negatively correlated with the corresponding R-venation. Sun-adapted species coping with the high water demand typical of sunny habitats appeared to have developed a highly efficient conducting system to supply living mesophyll cells with water. In contrast, species adapted to shady habitats showed higher R-venation values according to their lower need for investment of carbon into producing wide conduits in the leaf

Circadian regulation of leaf hydraulic conductance in sunflower (Helianthus annuus L. cv Margot)

The circadian regulation of leaf hydraulic conductance (K-leaf) was investigated in Helianthus annuus L. (sunflower). K-leaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. K-leaf was 4.0 e-4 kg s(-1) m(-2) MPa-1 during the light period (LL) and 30-40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for K-leaf at their subjective light or dark periods, K-leaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed K-leaf oscillating from light to dark values in phase with their subjective photoperiod indicating that K-leaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (R-leaf) of both LL and DL to the same value (1.0 e + 3 MPa m(2) s kg(-1)) that equalled the vascular resistance (R-v). The contribution of the non-vascular leaf resistance (R-nv) to R-leaf was of 71.9% in DL and of 58.4% in LL. The dominant R-nv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of K-leaf

Vulnerability to freeze stress of seedlings of Quercus ilex L.: an ecological interpretation

The vulnerability to freeze stress of seedlings of Quercus ilex L. was studied with the aim of defining the limits of the potential distribution area of this species in its northernmost habitat. In December 1996 seedlings were freeze stressed up to -8 °C for 3 d. Frost caused extensive functional damage to seedlings in terms of: a) leaf water status; b) root (Kr) and stem (Ks) hydraulic conductance; c) tissue disorder in the root (only nine seedlings out of 50 survived). In comparison with unstressed seedlings, Kr and Ks of freeze-stressed seedlings were reduced by 90 %. Root anatomy of freeze-stressed seedlings revealed that: a) cortex cells were dehydrated and had become separated from one another; b) the endodermis was oversuberized, thus isolating the stele from the cortex. Our conclusion was that Q. ilex is extremely vulnerable to freeze stress so that the distribution area of the species is restricted to zones with no frost events. (© Inra/Elsevier, Paris)La vulnérabilité au stress par congélation des semis de Quercus ilex L. : une interprétation écologique. La vulnérabilité au stress par congélation des semis de Quercus ilex L. a été étudiée avec l'objectif de définir les limites de l'extension géographique potentielle de cette espéce dans son habitat le plus septentrional. En décembre 1996 des semis ont subi le stress par congélation jusqu'à-8°C pour 3 j (figure 1). La gelée a provoqué des dommages remarquables aux plantes pour ce qui concerne : a) la condition hydrique des feuilles (figure 3) ; b) la conductivité hydraulique de la racine (K r) et du fuste (Ks) (figure 6) ; c) un désordre du tissu dans le racine (figure 2) (seulement 9 plantes sur 50 ont survécu). En comparaison avec des plantes non stressées le Kr et le Ks de plantes stressées par congélation avaient été reduits du 90 % (figure 7). L'anatomie de la racine des plantes stressées a révélé que : a) les cellules du « cortex » avaient été déshydratées et écartées les unes des autres ; b) l'endoderme avait été excessivement liègifié isolant le stèle du « cortex ». Notre conclusion était donc que Q. ilex est tellement vulnérable au stress par congélation que l'aire de distribution de l'espèce est limitée à des zones qui ne sont pas touchées par les gelées. (© Inra/Elsevier, Paris

Resistance to water flow through leaves of Coffea arabica L. is dominated by extra-vascular tissues.

The leaf hydraulic conductance (K-leaf) of Coffea arabica L. was measured for shoots exposed to nonlethal temperature stress or to a freeze - thaw cycle, and compared with K-leaf of non-stressed samples ( controls). Exposure to temperatures below 6degreesC for 1 h caused measurable damage to the functional integrity of cell membranes as shown by increased membrane leakiness to electrolytes. A 1 : 1 relationship was found to exist between relative electrolyte leakage and relative K-leaf suggesting that membrane damage caused K-leaf to increase. Low temperatures did not cause membrane disruption as shown by the comparison of chilled samples with frozen - thawed ones. In frozen leaves, membranes were extensively disrupted and both electrolyte leakiness and K-leaf increased 5-fold. Low temperatures did not induce alterations of the hydraulic properties of the leaf vascular system, as revealed by measurements of K-leaf after up to 500 cuttings of minor veins were made in the leaf blade of control and chilled leaves. Calculations showed that 62 - 75% of leaf hydraulic resistance resided in the extra-vascular water pathway. Results are discussed within the framework of our current understanding of leaf hydraulic architecture as well as in terms of plant adaptation to extremes in temperature

Impact of the leaf miner Cameraria ohridella on whole-plant photosynthetic productivity of Aesculus hippocastanum: insights from a model

The leaf miner Cameraria ohridella causes premature defoliation of Aesculus hippocastanum trees. In order to assess the whole-plant loss of productivity caused by the parasite, we monitored seasonal changes of leaf gas exchange and leaf area losses in horse chestnut trees freely infested or chemically treated to prevent moth infestation (controls). Data were integrated in a model and the annual loss of net primary productivity (NPP) was calculated for infested trees with respect to controls. Measurements showed marked vertical stratification of C. ohridella attacks, with lower crown strata being more infested than higher ones. Leaf gas exchange was maximum between May and early June, but it strongly decreased starting from mid-June even in controls. Model calculations showed that NPP loss of infested trees was about 30% on an annual basis (when the first moth attack is recorded at the end of April). Model simulations showed that postponing the start day of attack would have important positive effects on plant's NPP. For example, if the start day of attack was postponed to 20 May, the annual loss of NPP would be about 15%. Our study suggests that A. hippocastanum trees attacked by C. ohridella are not facing serious risks of decline, especially if methods are adopted to postpone the start day of attack (e.g. removal of fallen leaves in autumn). Our data do not support the view that plants need to be totally protected from the parasite by application of insecticides

Changes of pH of solutions during perfusion through stem segments: further evidence for hydrogel regulation of xylem hydraulic properties?

Changes in hydraulic conductivity (Kh) and pH were measured in stem segments of laurel (Laurus nobilis L.) during perfusion with iso-osmotic solutions of KCl, NaCl and sucrose. Sucrose had no effect on Kh while 100 mM NaCl or KCl induced up to 22 and 35 % increase of Kh with respect to deionized water, respectively. Increases in Kh were accompanied by a sharp drop in pH from 6.0 (inlet solution) to 5.0 (outlet solution). The same effect was observed with both KCl and NaCl solutions but not in the case of sucrose. Also, similar changes of Kh and pH were observed for stems killed after immersion in hot water. Our results might provide further evidence for ion-mediated regulation of xylem hydraulic conductivity based on the hydrogel properties of pectins at the pit membrane level