Non-hydraulic signals from maize roots in drying soil: inhibition of leaf elongation but not stomatal conductance

Abstract

Conditions of soil drying and plant growth that lead to non-hydraulic inhibition of leaf elongation and stomatal conductance in maize (Zea mays L.) were investigated using plants grown with their root systems divided between two containers. The soil in one container was allowed to dry while the other container was kept well-watered. Soil drying resulted in a maximum 35% inhibition of leaf elongation rate which occurred during the light hours, with no measurable decline in leaf water potential (ψw). Leaf area was 15% less than in control plants after 18 d of soil drying. The inhibition of elongation was observed only when the soil ψw declined to below that of the leaves and, thus, the drying soil no longer contributed to transpiration. However, midday root ψw in the dry container (-0.29 MPa) remained much higher than that of the surrounding soil (-1.0 MPa) after 15 d of drying, indicating that the roots in drying soil were rehydrated in the dark.To prove that the inhibition of leaf elongation was not caused by undetectable changes in leaf water status as a result of loss of half the watergathering capacity, one-half of the root system of control plants was excised. This treatment had no effect on leaf elongation or stomatal conductance. The inhibition of leaf elongation was also not explained by reductions in nutrient supply.Soil drying had no effect on stomatal conductance despite variations in the rate or extent of soild drying, light, humidity or nutrition. The results indicate that non-hydraulic inhibition of leaf elongation may act to conserve water as the soil dries before the occurrence of shoot water deficits.

Plant response to mechanical resistance and air-filled porosity of soils under conventional and no-tillage system

“…This is indeed a limitation of our data. However, our results may be supported by the findings of Saab & Sharp (1989) with corn, and Blum et al (1991) with wheat. In split-root experiments, they observed that the exposure of a relatively small part of a plant root system to a dry soil significantly reduced plant growth.…”

Interaction with ethylene: changing views on the role of abscisic acid in root and shoot growth responses to water stress

“…Michelena & Boyer 1982). Furthermore, shoot growth can be so sensitive to soil drying that substantial inhibition can occur before the development of decreased water potentials in the aerial plant parts (Saab & Sharp 1989;Gowing, Davies & Jones 1990), and such observations have led to much interest in the involvement of non-hydraulic regulatory signals from the roots (Davies & Zhang 1991;Davies et al . 2000).…”

Do we know how plants sense a drying soil?

“…Two approaches were used in the studies that changed the view on how plants sense a drying soil. One approach consisted of splitting roots into two containers and applying a low soil water treatment in one of the containers (BLACKMAN & DAVIES, 1985;TERMAAT et al, 1985;ZHANG et al, 1987;SAAD & SHARP, 1989). In this approach, part of the root system was kept in dry soil and part in wet soil, but plants were still able to take up sufficient water to maintain leaf water potential as high as in the control plants, which had their entire root system in wet soil.…”

“…BLACKMAN & DAVIES, 1985;TERMAAT et al, 1985;GOLLAN et al, 1986;SAAD & SHARP, 1989), and the experiments supporting a stomatal response to leaves water status were in woody and semi-woody species (e.g. SALIENDRA et al, 1995;FUCHS & LIVINGSTON, 1996;COMSTOCK & MENCUCCINI, 1998).…”