Stomatal Control of Water Loss in Siratro (Macroptilium atropurpureum (DC) Urb.), a Tropical Pasture Legume

Stomatal conductance of siratro declined linearly as leaf water potential fell until zero conductance was reached at −10 bar. In a grass/legume pasture stomata of siratro responded to humidity (saturation deficit), and to a lesser extent to leaf water potential, such that leaf water potential did not fall below −9 bar, whereas that of the grass continued to decline for most of the day. The dual response of siratro to both humidity and leaf water potential suggests that this species has an efficient two-stage stomatal control of water loss which provides an explanation of its higher leaf water potential and greater drought avoidance compared with sown grasses in semi-arid areas of north-eastern Australia.