Sequential Leaf Senescence and Correlatively Controlled Increases in Xylem Flow Resistance

Abstract

In bean, Phaseolus vulgaris L. (Contender), the directly measured hydraulic resistance of the xylem pathway between roots and primary leaf pulvinal junctions increased rapidly and progressively from 21 to 28 days after planting. These increases in xylem resistance (+390%) were specifically located in the pulvinal junction of the primary leaf. Moreover, they occurred just prior to the onset of primary leaf yellowing. Developmental increases in xylem hydraulic flow resistance and stomatal resistance, as well as subsequent primary leaf yellowing, were completely prevented by detopping the shoots above the primary leaves at 21 days. Thus, the onset of these senescence-associated symptoms was correla- (3,8,10,11). Primary leaf senescence is not simply a consequence of progressive shading by new shoot growth since it is not prevented by increased illumination (3, 10); nor is primary leaf senescence a programmed function of leaf age per se since removing the apical leaves above the primary node effectively inhibits the subsequent onset ofyellowing in the primary leaves (1,8,11). Some evidence suggests that programmed changes in the hydraulic architecture of the shoot may be involved (9, 10). Hydraulic resistances to flow into expanded primary leaves, calculated indirectly from water potential differences and transpiration rate, using an Ohm's Law analogy, are higher than for apical leaves (10). Similarly, the transpirational flow of solutes into the primary leaves of excised shoots declines progressively with age (10). Increasing diversion of essential xylem supplies of cytokinins, amino acids, mineral ions, and water away from basal towards more apical leaves, together with ongoing export of solutes from mature to young leaves via the phloem, would favor the prior senescence of the basal leaves (10).This paper describes the use of a direct, whole plant pressureflow assay to locate and directly quantify changes in hydraulic flow resistance of the xylem pathway. Relationships between such changes and the progressive stomatal closure characteristic of leaf senescence (3, 10, 13) are reported. Preliminary results are presented concerning a possible molecular mechanism for inducing changes in the resistance of xylem vessels to hydraulic flow and associated changes in leaf stomatal resistance. MATERIALS AND METHODSPlant Material. Bean plants (Phaseolus vulgaris, L. cv Contender) were germinated and grown on moistened vermiculite in a growth chamber under previously described conditions (10). In place of pots, each plant was grown in an individual black plastic bag (about 1 L capacity) with two small holes about 5 cm from the base. Two bags were placed in reservoirs consisting of 3 L pots filled to 12 cm from the base with water or, after seed germination, with 0.5 strength Hoagland solution. Solution levels were maintained by fresh additions of nutrient solution every 1 to 2 d. In some cases, one of each matched pair of 3-week plants was detopped by removing the shoot above the primary leaf( 11).Plant Resistances. ...

Effects of salinity on xylem structure and water use in growing leaves of sorghum

“…Schultz & Matthews (1993) found that the conductivity per xylem cross-sectional area increases by an order of magnitude during the period of leaf expansion in grape. Neumann (1987) found pulvinar xylem conductivity decreases four-fold during senescence of bean leaves. In general, a possible explanation for the small effect of xylem diameter on hydraulic conductivity is that the conductivity in mature leaves is affected less by the diameter of xylem elements of the growth zone than by hydraulic bottlenecks -constriction zones in roots, stems or leaf blades.…”

Hydraulic Signals from the Roots and Rapid Cell-Wall Hardening in Growing Maize (Zea mays L.) Leaves Are Primary Responses to Polyethylene Glycol-Induced Water Deficits

“…However, it seems that the rapid wall responses described here can be initiated by hydraulic signals alone. This is not a unique example of such signaling; longdistance hydraulic signals have also been invoked in control of stomatal closure, sequential leaf senescence, stem elongation, and rapid systemic responses to wounding (Raschke, 1970;Neumann, 1987;Nonami and Boyer, 1990a;Malone, 1992). Moreover, even hormonal messages, e.g.…”

“…The signaling process by which PEG treatment of roots induces cell-wall hardening in relatively distant leaves was also investigated. Possible involvement of hydraulic (Neumann, 1987;Kramer, 1988;Boyer, 1990a), hormonal (Saab andSharp, 1989;Davies and Zhang, 1991;Tardieu et al, 1993) and electrical (Pickard, 1973;Malone, 1992;Stahlberg and Cosgrove, 1992) signaling was considered: The relative importance of the different signals was evaluated by detennining leaf responses to PEG applied to live or killed roots. Killed roots responded hydraulically but were presumed unable to generate electrical or hormonal signals.…”

The effects of microbial exopolysaccharides (EPS) in vase water on the water relations and the vase life of Rosa cv. Sonia

“…In most of the work reported by other authors, purified bacterial polysaccharides (see Corey & Starr 1957) or artificially synthesized polymers were applied to elucidate the mechanisms of phytopathogenicity. The effects of polysaccharides on ornamental cut flowers, however, have rarely been studied (Hodgson et al 1949;Van Alfen & Turner 1975;Van Alfen & Allard-Turner 1979;Van Alfen et al 1983;de Stigter & Broekhuysen 1986;Neumann 1987). In the present work, molecular filtrates and retentates of supernatant fluids of pure cultures of several micro-organisms grown in an agitated sucrose-containing medium were used as a vase medium for cut roses cv.…”

“…Reports of levan-forming Bacillus and Pseudomonas species are mentioned by Sutherland (1977Sutherland ( , 1979 and by Powell (1979). Most of the organisms used in our experiments are known commonly to form exopolysaccharides (possibly levans) (Wilkinson 1958(Wilkinson , 1977Gorin & Spencer 1968;Govan et al 1979;Powell 1979;Anderson 1984;Marques et al 1986;Neumann 1987).…”