Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK.
Centro de Investigación Forestal (CIFOR), Instituto Nacional de Investigación Agraria y Alimentaria (INIA), Crta A Coruña s/n, Madrid E-28040, Spain.
Tree Physiol. 2020 May 30;40(6):762-773. doi: 10.1093/treephys/tpaa037.
Soil moisture heterogeneity in the root zone is common both during the establishment of tree seedlings and in experiments aiming to impose semi-constant soil moisture deficits, but its effects on regulating plant water use compared with homogenous soil drying are not well known in trees. Pronounced vertical soil moisture heterogeneity was imposed on black poplar (Populus nigra L.) grown in soil columns by altering irrigation frequency, to test whether plant water use, hydraulic responses, root phytohormone concentrations and root xylem sap chemical composition differed between wet (well-watered, WW), and homogeneously (infrequent deficit irrigation, IDI) and heterogeneously dry soil (frequent deficit irrigation, FDI). At the same bulk soil water content, FDI plants had greater water use than IDI plants, probably because root abscisic acid (ABA) concentration was low in the upper wetter layer of FDI plants, which maintained root xylem sap ABA concentration at basal levels in contrast with IDI. Soil drying did not increase root xylem concentration of any other hormone. Nevertheless, plant-to-plant variation in xylem jasmonic acid (JA) concentration was negatively related to leaf stomatal conductance within WW and FDI plants. However, feeding detached leaves with high (1200 nM) JA concentrations via the transpiration stream decreased transpiration only marginally. Xylem pH and sulphate concentration decreased in FDI plants compared with well-watered plants. Frequent deficit irrigation increased root accumulation of the cytokinin trans-zeatin (tZ), especially in the dry lower layer, and of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), in the wet upper soil layer. Root hormone accumulation might explain the maintenance of high root hydraulic conductance and water use in FDI plants (similar to well-watered plants) compared with IDI plants. In irrigated tree crops, growers could vary irrigation scheduling to control water use by altering the hormone balance.
根区土壤水分异质性在幼苗定植和实施半恒定量土壤水分亏缺实验中都很常见，但与均匀干燥土壤相比，其对树木水分利用的调节作用尚不清楚。通过改变灌溉频率，在土壤柱中种植的黑杨（Populus nigra L.）上产生明显的垂直土壤水分异质性，以测试植物水分利用、水力响应、根植物激素浓度和根木质部汁液化学成分在湿润（充分灌溉，WW）、均匀（低频亏缺灌溉，IDI）和异质干燥土壤（高频亏缺灌溉，FDI）之间是否存在差异。在相同的体积土壤含水量下，FDI 植株的耗水量大于 IDI 植株，可能是因为 FDI 植株上层较湿润区域的根脱落酸（ABA）浓度较低，与 IDI 相比，根木质部汁液 ABA 浓度保持在基础水平。土壤干燥并未增加任何其他激素的根木质部浓度。然而，木质部茉莉酸（JA）浓度的植株间变异与 WW 和 FDI 植株叶片气孔导度呈负相关。然而，通过蒸腾流将高浓度（1200 nM）JA 供给离体叶片，仅使蒸腾作用略有减少。与充分灌溉植物相比，FDI 植物的木质部 pH 值和硫酸盐浓度降低。与充分灌溉植物相比，高频亏缺灌溉增加了根细胞分裂素玉米素（tZ）的积累，特别是在干燥的下层，以及乙烯前体 1-氨基环丙烷-1-羧酸（ACC），在潮湿的上层土壤中。根激素积累可能解释了与 IDI 植株相比，FDI 植株（类似于充分灌溉植株）维持高根水力导度和水分利用的原因。在灌溉的树木作物中，种植者可以通过改变激素平衡来改变灌溉计划来控制水分利用。
