Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

Abstract

The water relations parameters involved in assimilate flow into developing wheat (Triticum aestivum L.) grains were measured at several points from the flag leaf to the endosperm cavity in normally watered (⌿ Ϸ Ϫ0.3 MPa) and water-stressed plants (⌿ Ϸ Ϫ2 MPa). These included direct measurement of sieve tube turgor and several independent approaches to the measurement or calculation of water potentials in the peduncle, grain pericarp, and endosperm cavity. Sieve tube turgor measurements, osmotic concentrations, and ⌿ measurements using dextran microdrops showed good internal consistency (i.e. ⌿ ϭ ⌿ s ϩ ⌿ p ) from 0 to Ϫ4 MPa. In normally watered plants, crease pericarp ⌿ and sieve tube turgor were almost 1 MPa lower than in the peduncle. This suggests a high hydraulic resistance in the sieve tubes connecting the two. However, observations concerning exudation rates indicated a low resistance. In water-stressed plants, peduncle ⌿ and crease pericarp ⌿ were similar. In both treatments, there was a variable, approximately 1-MPa drop in turgor pressure between the grain sieve tubes and vascular parenchyma cells. There was little between-treatment difference in endosperm cavity sucrose or osmotic concentrations or in the crease pericarp sucrose pool size. Our results re-emphasize the importance of the sieve tube unloading step in the control of assimilate import.One of the most striking aspects of fruit and seed physiology is the independence of their water relations from other parts of the plant. For seeds, stable water relations are required for normal embryo development (Walbot, 1978;Kermode, 1990; Bradford, 1994), for example, in preventing precocious germination. Embryo growth also requires a steady supply of nutrients. Since imported assimilates make up a large proportion of the embryo's osmotic environment, it is likely that these two requirements have some controls in common.In contrast to the embryonic environment, water relations and assimilate concentrations in the vegetative portion of the plant may vary considerably. Water potential especially may vary diurnally and/or decline progressively with the onset of drought conditions. Thus, reproductive structures, or at least their embryos, must be insulated from the variations in water potential experienced by the rest of the plant. At the same time, they typically maintain a constant growth rate (Barlow et al., 1980;Lang and Thorpe, 1989;Westgate et al., 1989;Lang, 1990) despite such variation.In part, this independence of seed and fruit water relations is often achieved by a functional discontinuity in the xylem at or near their point of attachment to the plant. Quite large ⌿ differences have been reported between seeds or fruit and vegetative portions of the plant, and even between the embryo and adjacent seed or fruit tissues (Bradford, 1994). However, as Bradford points out in his critical review of these issues, such differences have been reported even in instances where there seems to be no effective barrier to water movement (e.g. seeds in fleshy frui...

Water Relations Link Carbon and Oxygen Isotope Discrimination to Phloem Sap Sugar Concentration inEucalyptus globulus

“…In those studies, xylem water potentials were Ϫ0.7, Ϫ 0.5, and Ϫ0.2 MPa, respectively, all somewhat less negative than the shoot water potentials recorded in the present study. However, in peduncles of Triticum aestivum, sieve tube turgor pressures of 2.4 and 1.4 MPa were observed at apoplastic water potentials of Ϫ0.4 and Ϫ2.1 MPa, respectively (Fisher and Cash-Clark, 2000). This decrease in phloem sap turgor with increasing drought stress, as also seen for E. globulus in Figure 5, is likely to be qualitatively meaningful.…”

A Diurnal Component to the Variation in Sieve Tube Amino Acid Content in Wheat

“…During the necessary measurement of the volume of exuding sap in air, any significant evaporation would reduce the observed diameter of the exuding sap droplet, resulting in an overestimation of in vivo concentrations, a problem previously recognized by Fisher and Cash-Clarke (2000). Evaporation would be greater for more slowly exuding samples, as these require a longer period for volume measurement and have a greater average surface area-to-volume ratio.…”

Review: Nutrient loading of developing seeds

“…Plasmodesmata, interconnecting sieve elements with adjacent vascular parenchyma, exhibit unusually large size exclusion limits in wheat grains (Fisher and Cash-Clark 2000a) and Arabidopsis seeds (Stadler et al 2005). Their diametres, possibly as large as 42 nm (Fisher and Cash-Clark 2000a), may confer hydraulic conductivities of sufficient magnitude to permit sieve element unloading by bulk flow driven down large hydrostatic pressure gradients operating across this interface (∼1 MPa, Fisher and Cash-Clark 2000b). Bulk flow is the simplest mechanism to ensure homeostasis of hydrostatic pressures in the importing sieve tubes by coupling water and nutrient unloading rates (cf.…”

“…In relation to nutrient delivery function, post-phloem domains abut (e.g. cereals, Vicia, Pisum, Fisher and Cash-Clark 2000a;Patrick and Offler 2001;van Dongen et al 2003), or are separated from (e.g. Arabidopsis, Stadler et al 2005;cotton, Ruan et al 2001;Phaseolus, Patrick et al 1995), the underlying filial tissues (Fig.…”