Aims As soil dries, the loss of soil hydraulic conductivity limits water supply to the leaves, which is expected to generate a nonlinear relationship between leaf water potential ( ψ leaf ) and transpiration ( E ). The effect of soil drying and root properties on ψ leaf and E remains elusive. Methods We measured E and ψ leaf of pearl millet for varying E and soil moisture using a root pressure chamber. A model of water flow in soil and plant was used to fit the ψ leaf ( E ) relationship. Results The relation between ψ leaf and E was linear at all soil moistures. The slope of ψ leaf ( E ) increased with decreasing soil moisture due to the decreasing soil-root conductance. The fact that the relation remained linear also in dry soils and high E is surprising. Indeed, it indicates that the gradients in soil water potential ( ψ soil ) were small, probably because of the large root surface (13.5 cm cm −3 ) active in water uptake. ψ leaf at E = 0 was less negative than ψ soil , indicating a more negative osmotic potential in the xylem than in the soil. Conclusions We propose that the linearity between ψ leaf and E and the high ψ leaf ( E = 0) compared to ψ soil support transpiration in drying soils.

中文翻译：

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土壤干燥过程中珍珠粟叶片木质部水势与蒸腾作用的线性关系

目的 随着土壤干燥，土壤导水率的损失会限制叶片的水分供应，预计这会在叶片水势 (ψ 叶 ) 和蒸腾作用 (E) 之间产生非线性关系。土壤干燥和根特性对 ψ 叶和 E 的影响仍然难以捉摸。方法我们使用根压力室测量了珍珠粟的 E 和 ψ 叶片的 E 和土壤水分变化。土壤和植物中的水流模型用于拟合 ψ 叶 ( E ) 关系。结果 ψ 叶与 E 之间的关系在所有土壤湿度下均呈线性关系。由于土壤根电导率的降低，ψ 叶 ( E ) 的斜率随着土壤水分的减少而增加。这种关系在干燥土壤和高 E 中也保持线性的事实令人惊讶。事实上，这表明土壤水势（ ψ 土壤 ）的梯度很小，可能是因为大的根表面 (13.5 cm cm -3 ) 活跃于吸水。E = 0 时的 ψ 叶比 ψ 土壤的负值小，表明木质部中的负渗透势比土壤中的负渗透势更大。结论 我们建议 ψ 叶和 E 和高 ψ 叶 (E = 0) 与 ψ 土壤之间的线性关系支持干燥土壤中的蒸腾作用。