Andrea Nardini

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 (Ψ_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.

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 (Rvenation) is lower in sun- than in shade-adapted species. Rvenation 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. Rvenation 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 Rvenation. 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 Rvenation values according to their lower need for investment of carbon into producing wide conduits in the leaf.

We studied changes in the hydraulic conductance of leaves (K_leaf) between dawn and dusk during the growth period (July) and at midday at the beginning of autumn in four tree species. The main objectives of the study were to check the extent of diurnal and seasonal changes in K_leaf and the relationships between K_leaf, irradiance and leaf gas exchange. Two evergreen (Aleurites moluccana and Persea americana) and two deciduous trees (Platanus orientalis and Quercus rubra) were studied. Leaf hydraulic conductance was measured every 2 h between 0700 and 1900 h in July and compared with values measured between 0900 and 1300 h in October. Other variables measured were photosynthetically active radiation (PAR), leaf conductance to water vapor (g_L) and water potential (Ψ_L). In July, K_leaf varied by up to 75% in Pe. americana on a diurnal basis and by at least 44% in Q. rubra. The diurnal time course of K_leaf showed a distinct increase between dawn and late morning (1100 h) and a subsequent decrease in the evening in A. moluccana and Pl. orientalis, whereas in the other two species, K_leaf was highest just after dawn and lowest in the evening. In October, K_leaf of all the species studied was lower than in July, with differences of 20 to 28% for A. moluccana and Pl. orientalis and of 66 to over 70% in Pe. americana and Q. rubra, respectively. Significant correlations were found between PAR and K_leaf (in all species) as well as between g_L and K_leaf (in three out of four species). Leaf habit (evergreen or deciduous) did not influence absolute values of K_leaf or its diurnal variation.

The leaf miner Cameraria ohridella causes premature defoliation in Aesculus hippocastanum. The impact of the parasite on seasonal changes of water relations, hydraulics and growth of three-year-old A. hippocastanum seedlings was investigated. Leaf expansion was complete before the adults of the moth developed, so that growth of infested seedlings was similar to that of controls. Leaf conductance to water vapour of infested seedlings was reduced with respect to controls, both in mined and in still green portions of attacked leaves. In July and August, the hydraulic resistance of infested seedlings was higher than that of controls, due to anticipated increase of hydraulic resistance at the leaf, petiole and current-year stem level. This was interpreted as evidence of anticipated leaf senescence in infested plants, leading to early leaf abscission. Nevertheless, our data suggest that, under our experimental conditions, C. ohridella is not a major mortality factor for A. hippocastanum seedlings. Cameraria ohridella / Aesculus hippocastanum / seedling / hydraulic resistance / leaf Résumé-Variations saisonnières des propriétés hydrauliques, des relations hydriques et de la croissance de jeunes plants d'Aesculus hippocastanum infestés par la mineuse foliaire Cameraria ohridella. La mineuse foliaire Cameraria ohridella provoque des défoliations prématurées chez le Marronnier (Aesculus hippocastanum). L'impact du parasite sur les variations saisonnières des relations hydriques, des paramètres hydrauliques et de la croissance de plants d'A. hippocastanum âgés de 3 ans a été analysé. L'expansion foliaire étant terminée avant que les adultes de l'insecte soient développés, la croissance des plants infestés fut similaire à celle des plants témoins. La conductance foliaire à la vapeur d'eau des plants infestés fut réduite par rapport aux témoins, que ce soit dans les parties encore vertes des feuilles ou dans les parties attaquées. En juillet et août, la résistance hydraulique des plants infestés fut plus élevée que celle des témoins, en raison d'une augmentation anticipée de la résistance hydraulique des feuilles, des pétioles et des tiges de l'année. Ceci fut interprété comme une preuve d'une anticipation de la sénescence foliaire pour les plants infestés, conduisant à une abscission prématurée des feuilles. Néanmoins, nos données suggèrent que, dans nos conditions expérimentales, C. ohridella n'est pas un facteur majeur de mortalité pour les jeunes plant d'A. hippocastanum.

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 m² 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.

The leaf hydraulic conductance (Kleaf) of Coffea arabica L. was measured for shoots exposed to non-lethal temperature stress or to a freeze–thaw cycle, and compared with Kleaf of non-stressed samples (controls). Exposure to temperatures below 6°C 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 Kleaf suggesting that membrane damage caused Kleaf 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 Kleaf increased 5-fold. Low temperatures did not induce alterations of the hydraulic properties of the leaf vascular system, as revealed by measurements of Kleaf 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 frame work of our current understanding of leaf hydraulic architecture as well as in terms of plant adaptation to extremes in temperature.

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K_L) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K_L might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K_L on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K_L increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g_L, was used as a proxy for stomatal aperture and the time-course of changes in K_L and g_L was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K_L changes were not paralleled by g_L changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g_L response to irradiance. These leaves showed the same K_L response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K_L is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.

The hydraulic architecture of Laurus nobilis L. and Juglans regia L. leaves was studied using three different approaches: (1) hydraulic measurements of both intact leaves and of leaves subjected to treatments aimed at removing the extra‐vascular resistance; (2) direct measurements of the vascular pressure with a pressure probe; and (3) modelling the hydraulic architecture of leaf venation system on the basis of measurements of vein densities and conductivities. The hydraulic resistance of leaves (R_leaf) either cut, boiled or frozen–thawed was reduced by about 60 and 85% with respect to control leaves for laurel and walnut, respectively. Direct pressure drop measurements suggested that 88% of the resistance resided outside the vascular system in walnut. Model simulations were in agreement with these results provided vein hydraulic conductance was 0.12–0.28 that of the conductance predicted by Poiseuille's law. The results suggest that R_leaf is dominated by substantial extra‐vascular resistances and therefore contrast with the conclusions of recent studies dealing with the hydraulic architecture of the leaf. The present study confirms the ‘classical’ view of the hydraulic architecture of leaves as composed by a low‐resistance component (the venation) and a high‐resistance component (the mesophyll).

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13-week treatment period, soil water potential was –0.05 MPa for well-watered control seedlings (W) and –0.4, –0.8 and –1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meter. When scaled by leaf surface area, shoot hydraulic conductance did not differ among the treated seedlings, whereas root hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.