Does ABA in the Xylem Control the Rate of Leaf Growth in Soil-Dried Maize and Sunflower Plants?

Abstract

Maize (Zea mays L.) and sunflower (Helianthus annus L.) plants were grown in large valumes of soil and leaf growth rate was monitored on a daily basis. Half the plants were given a soil drying treatment and when they showed a significant restriction of growth rate (compared to both teir daily growth rate before drying and the average growth rate of well-watered plants on the same day), leaf water relations were measured and xylem sap was extracted using several techniques. There was a significant negative log-linear relationship between the rate of leaf growth and the concentration of ABA in the xylem for both species. There was no clear relationship between leaf growth rate and leaf water potential or turgor for either species. Assessment of different methods for sampling xylem sap suggest that exudates collected from stem stumps or samples collected by pressurizing the whole root system are suitable for estimating ABA concentration in xylem, at least with large plants of maize or sunflower, provided the first few hundred cubic millimetres of collected sap are used for the assay. Centrifugation of sections of stems resulted in dilution of ABA in the xylem sap with sap squeezed from parenchyma tissue. This is because, at least in plants subjected to mild soil drying, the concentration of the ABA in the xylem is far higher than that in the cell sap of stem tissue. Results support the proposal that ABA plays a major role as a chemical signal involved in the root-to-shoot communication of the effects of soil drying. The non-hydraulic restriction of leaf growth by a chemical signal can be explained by the extra root-sourced ABA in the xylem and may be an important component of the modification of growth and development which results from prolonged soil drought.