Leaf growth and turgor in growing cells of maize (Zea mays L.) respond to evaporative demand under moderate irrigation but not in water-saturated soil

To test whether the inhibition of leaf expansion by high evaporative demand is a result of hydraulic processes, we have followed both leaf elongation rate (LER) and cell turgor in leaves of maize plants either normally watered or in water-saturated soil in which hydraulic resistance at the soil-root interface was abolished. Cell turgor was measured in situ with a pressure probe in the elongating zone of the first and sixth leaves, and LERs of the same leaves were measured continuously with transducers or by following displacements of marks along the growing leaves. Both variables displayed spatial variations along the leaf and positively correlated within the elongating zone. Values peaked at mid-distance of this zone, where the response of turgor to evaporative demand was further dissected. High evaporative demand decreased both LER and turgor for at least 5 h, with dose-effect linear relations. This was observed in five genotypes with appreciable differences in turgor maintenance among genotypes. In contrast, the depressing effects of evaporative demand on both turgor and LER disappeared when the soil was saturated, thereby opposing a negligible resistance to water flow at the soil-root interface. These results suggest that the response of LER to evaporative demand has a hydraulic origin, enhanced by the resistance to water flux at the soil-root interface. They also suggest that turgor is not completely maintained under high evaporative demand, and may therefore contribute to the reductions in LER observed in non-saturated soils.