Hydraulic architecture of plants of Helianthus annuus L. cv. Margot: evidence for plant segmentation in herbs.

The hydraulic architecture of sunflower (Helianthus annuus L. cv. Margot) was studied in terms of the partitioning of the hydraulic conductance (Kleaf) of leaves inserted at progressively more apical nodes both in growing plants (GP) and in plants at full anthesis (mature plants, MP). Leaf conductance to water vapour (gL), leaf water potential (PsiL), leaf water potential at zero turgor (Psi tlp), and leaf osmotic potential at full turgor (pi0) were also measured. Sunflower plants showed gL and Kleaf values significantly increasing in the acropetal direction, while PsiL of basal leaves was significantly more negative than that of distal leaves; Psi tlp markedly decreased in the acropetal direction in MP so that leaves of MP retained increasingly more turgor the more apical they were. This hydraulic pattern, already present in very young plants (GP), strongly favours apical leaves. These data suggest that the progressive leaf dieback starting from the stem base, as observed when the inflorescence of sunflower reached maturity, might be due to time-dependent loss of hydraulic conductance. In fact, Kleaf loss was correlated with PsiL drop and stomatal closure. Leaf dehydration was aggravated by solute exportation from the basal towards the apical leaves, as revealed by the acropetal decrease of pi0. Kleaf was shown to be linearly and positively related to the prevailing ambient irradiance during plant growth, thus suggesting that leaf hydraulics is very sensitive to environmental conditions. It was concluded that the pronounced apical dominance of some sunflower cultivars is determined, among other factors, by plant hydraulic architecture.

Drought resistance of Ailanthus altissima: root hydraulics and water relations.

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.

Xylem cavitation in the leaf of Prunus laurocerasus and its impact on leaf hydraulics.

This paper reports how water stress correlates with changes in hydraulic conductivity of stems, leaf midrib, and whole leaves of Prunus laurocerasus. Water stress caused cavitation-induced dysfunction in vessels of P. laurocerasus. Cavitation was detected acoustically by counts of ultrasonic acoustic emissions and by the loss of hydraulic conductivity measured by a vacuum chamber method. Stems and midribs were approximately equally vulnerable to cavitations. Although midribs suffered a 70% loss of hydraulic conductance at leaf water potentials of -1.5 MPa, there was less than a 10% loss of hydraulic conductance in whole leaves. Cutting and sealing the midrib 20 mm from the leaf base caused only a 30% loss of conduction of the whole leaf. A high-pressure flow meter was used to measure conductance of whole leaves and as the leaf was progressively cut back from tip to base. These data were fitted to a model of hydraulic conductance of leaves that explained the above results, i.e. redundancy in hydraulic pathways whereby water can flow around embolized regions in the leaf, makes whole leaves relatively insensitive to significant changes in conductance of the midrib. The onset of cavitation events in P. laurocerasus leaves correlated with the onset of stomatal closure as found recently in studies of other species in our laboratory.

Refilling of embolized vessels in young stems of laurel. Do We need a new paradigm?

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than -1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N2 to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than -1 MPa. We have concluded that none of the existing paradigms explains the results.

Different strategies of drought resistance in three Mediterranean sclerophyllous trees growing in the same environmental conditions.

Measurements have been made of leaf conductance to water vapour, relative water content and water potential in Olea oleaster Hoffmgg et Link, Ceratonia siliqua L. and Laurus nobilis L., three evergreen sclerophyllous trees growing in Sicily at sea level. Measurements were made hourly in May and September 1986. Although all the three species are regarded as components of a homogeneous group (sclerophylls) and they all showed a high degree of sclerophylly, the strategies they adopted to withstand drought were completely different. Olea oleaster behaved as a 'drought-tolerating' species. Drought was 'avoided' by Ceratonia siliqua by a 'water-spending' strategy and by Laurus nobilis by a 'water-saving' strategy combined with the capability of recovering even minimal water losses by dropping leaf water potential drastically. Such differences in drought resistance are discussed in terms of differences in the bulk elastic modulus of leaf cells and wood anatomy.