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 (Psi approximately -0.3 MPa) and water-stressed plants (Psi approximately -2 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 Psi measurements using dextran microdrops showed good internal consistency (i.e. Psi = Psi(s) + Psi(p)) from 0 to -4 MPa. In normally watered plants, crease pericarp Psi 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 Psi and crease pericarp Psi 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.

Figure 1

Trends in endosperm water potential and its components in wheat grains during the final 3 weeks of grain filling. Total water potential was assumed to equal endosperm cavity sap Ψ_s. Endosperm solute potential was determined on sap released from the endosperm by two freeze-thaw cycles. Turgor pressure was calculated as (Ψ − Ψ_s).

Figure 2

Trend in grain pericarp Ψ during the final 3 weeks of grain filling. Ψ was measured by the use of dextran microdrops sited on the crease pericarp of attached grains.

Figure 3

Trends in endosperm cavity pressure (A), and in cavity volumetric modulus (B) during the final 3 weeks of grain filling. Measurements were made immediately after detaching the grain, using a modified pressure probe.

Figure 4

The effect of detaching grains, or of rapidly wilting the shoot, on endosperm cavity sap concentration (A and B) and cavity volume (C). In grain detachment experiments, grains were removed from the ear and placed in a humid chamber for the times indicated. Each plot represents grains from the same ear (one grain per point). In wilting experiments, a tiller on an illuminated plant was excised at ground level and allowed to wilt in the light. Grains were removed at intervals after excision. Changes in the cavity volume (C) were followed by inserting a fine-tipped capillary into the distal end of the cavity and recording the volume sucked from the pipette during the next several seconds, after which movement virtually ceased.

Figure 5

Exudation from the broken pedicel surface of grains detached 40 min earlier and placed in mineral oil.

Figure 6

Comparison of Ψ measured with dextran microdrops with values calculated from manometric measurements on the same tiller of sieve tube Ψ_p and Ψ_s. After a turgor measurement, the manometer tip was clipped off to confirm that the stylet was still exuding. The manometer was then broken from the plant surface and its contents were ejected into mineral oil to allow measurement of exudate Ψ_s by freezing point depression. A Ψ of zero cannot be measured by microdrops, but was assumed for detached tillers in water.

Figure 7

A two-step procedure was used to seal a manometer to an exuding stylet. During the first step (A), a fine-tipped pipette was used to collect exudate and to support the stylet while cyanoacrylate was polymerized around the stylet base. Following this, the manometer was slipped over the stylet and sealed to the surface of the first cyanoacrylate layer (B).