Abdalla Mohanned, Carminati Andrea, Cai Gaochao, Ahmed Mutez Ali
Root-Soil Interactions, School of Life Sciences, Technical University of Munich, Freising, Germany.
Department of Horticulture, Faculty of Agriculture, University of Khartoum, Khartoum North, Sudan.
Ann Bot. 2024 Oct 30. doi: 10.1093/aob/mcae193.
Mucilage has been hypothesized to soften the gradients in matric potential at the root-soil interface, hereby facilitating root water uptake in dry soils and maintaining transpiration with a moderate decline in leaf water potential. So far, this hypothesis has been tested only through simplified experiments and numerical simulations. However, the impact of mucilage on the relationship between transpiration rate (E) and leaf water potential (ψleaf) at the plant scale remains speculative.
We utilized an automated root pressure chamber to measure the E(ψleaf) relationship in two cowpea genotypes with contrasting mucilage production. We then leveraged a soil-plant hydraulic model to reproduce the experimental observations and inferred the matric potential at the root-soil interface for both genotypes.
In wet soil, the relationship between the leaf water potential and transpiration rate (E) was linear for both genotypes. However, as the soil progressively dried, the E(ψleaf) relationship exhibited nonlinearity. Genotype with low mucilage production exhibited nonlinearity earlier during soil drying, i.e. in wetter soil conditions, (soil water content < 0.36 cm3 cm-3) compared to Genotype with high mucilage production (soil water content < 0.30 cm3 cm-3). The incidence of nonlinearity was concomitant with the decline in matric potential across the rhizosphere. High mucilage production attenuated water potential diminution at the root-soil interface with increased E. This shows, for the first time at the plant scale, that root mucilage softened the gradients in matric potential and maintained transpiration in drying soils. The model simulations indicate that a plausible explanation for this effect is an enhanced hydraulic conductivity of the rhizosphere in genotype with higher mucilage production.
Mucilage exudation maintains the hydraulic continuity between soil and roots and decelerates the drop in matric potential near the root surface, hereby postponing the hydraulic limitations to transpiration during soil drying.
黏液被认为可以缓和根 - 土界面基质势的梯度变化，从而促进干旱土壤中根系对水分的吸收，并在叶片水势适度下降的情况下维持蒸腾作用。到目前为止，这一假设仅通过简化实验和数值模拟进行了验证。然而，黏液对植物尺度下蒸腾速率（E）与叶片水势（ψleaf）关系的影响仍具有推测性。
我们利用自动根压室测量了两种黏液分泌量不同的豇豆基因型的E(ψleaf)关系。然后，我们利用土壤 - 植物水力模型重现实验观测结果，并推断出两种基因型在根 - 土界面的基质势。
在湿润土壤中，两种基因型的叶片水势与蒸腾速率（E）之间的关系均为线性。然而，随着土壤逐渐变干，E(ψleaf)关系呈现出非线性。黏液分泌量低的基因型在土壤干燥过程中更早地表现出非线性，即在比黏液分泌量高的基因型更湿润的土壤条件下（土壤含水量<0.36 cm3 cm-3）就表现出非线性（黏液分泌量高的基因型在土壤含水量<0.30 cm3 cm-3时才表现出非线性）。非线性的出现与根际基质势的下降同时发生。黏液分泌量高可减轻根 - 土界面水势随E增加的减小程度。这首次在植物尺度上表明，根系黏液缓和了基质势梯度，并在干燥土壤中维持了蒸腾作用。模型模拟表明，这种效应的一个合理原因是黏液分泌量高的基因型根际水力传导率增强。
黏液分泌维持了土壤与根系之间的水力连续性，并减缓了根表面附近基质势的下降，从而推迟了土壤干燥过程中对蒸腾作用的水力限制。
