© 1987 Heron Publishing—Victoria, Canada
Adaptations to soil drying in woody seedlings of African locust bean, (Parkia biglobosa (Jacq.) Benth.)
O. Osonubi (1) and F. E. Fasehun (2)
1. Department of Botany and Microbiology, University of Ibadan, Nigeria / 2. Department of Forest Resources Management, University of Ibadan, Nigeria / Received August 22, 1986
Summary
Stomatal conductance, transpiration and xylem pressure potential of African locust bean (Parkia biglobosa (Jacq.) Benth.) seedlings subjected from the sixth week after emergence to four weeks of continuous soil drought did not
differ from those of well-watered, control plants until two-thirds of the available soil water had been used. In both well-watered
and drought-treated plants, stomatal conductance was highest early in the day when vapor pressure deficits were low, but decreased
sharply by midday when evaporative demand reached its highest value. There was no increase in stomatal conductance later in
the day as vapor pressure deficit declined. The relationship between transpiration rate and xylem pressure potential showed
non-linearity and hysteresis in both control and drought-treated plants, which seems to indicate that the plants had a substantial
capacity to store water. The rate of leaf extension in African locust bean seedlings subjected to six consecutive 2-week cycles
of soil drought declined relative to that of well-watered, control plants, whereas relative root extension increased.
It appears that African locust bean seedlings minimized the impact of drought by: (1) restricting transpiration to the early
part of the day when a high ratio of carbon gain to water loss can be achieved; (2) utilizing internally stored water during
periods of rapid transpiration; (3) reducing the rate of leaf expansion and final leaf size in response to soil drought without
reducing the rate of root extension, thereby reducing the ratio of transpiring leaf surface area to absorbing root surface
area.
