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

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

本研究针对正常供水（水势Ψ≈−0.3 MPa）与水分胁迫（水势Ψ≈−2 MPa）条件下的普通小麦（Triticum aestivum L.）发育籽粒中同化物运输相关的水分关系参数展开测定，测定位点覆盖从旗叶至胚乳腔的多个关键位置。测定参数涵盖筛管膨压的直接测定，以及采用多种独立方法对穗颈、籽粒果皮及胚乳腔的水势进行测量或计算。通过葡聚糖微滴法获取的筛管膨压、渗透浓度及水势测定结果，在0~−4 MPa范围内均呈现出良好的内部一致性（即Ψ = Ψ(s) + Ψ(p)）。在正常供水植株中，腹沟果皮水势与筛管膨压均较穗颈低约1 MPa，这提示连接二者的筛管存在较高的水力阻力；但泌出率相关观测结果却表明该段通路的阻力较低。在水分胁迫植株中，穗颈水势与腹沟果皮水势基本持平。两种处理条件下，籽粒筛管与维管束薄壁细胞间的膨压均存在约1 MPa的波动下降。两种处理间，胚乳腔的蔗糖与渗透浓度、腹沟果皮的蔗糖库大小均无显著差异。本研究结果再次强调了筛管卸载步骤在同化物输入调控中的核心重要性。