Physiological Reactions to an Imposed Drought by Two Twining Pasture Legumes: Macroptilium atropurpureum (Desiccation Sensitive) and Galactia striata (Desiccation Insensitive)

Abstract

Macroptilium atropurpureum cv. Siratro and Galactia striata were grown in both well-watered and droughted swards to allow comparisons between plants with similar form but different physiological responses to desiccation. Siratro maintained leaf water potentials (¨Ψl) above c. -2.2 MPa, while in G. striata they fell to c. -6.7 MPa. G. striata exhibited osmotic adjustment of up to 2.3 MPa and considerable desiccation tolerance, with relative water contents in live leaves as low as c. 25%. In Siratro, osmotic adjustment was less than 0.26 MPa and its relative water content was always above c. 65%. Leaves of droughted G. striata accumulated pinitol and proline, while Siratro did not accumulate substantial amounts either. Despite having consistently higher positive (calculated) turgor pressure, leaf expansion was limited by drought more in Siratro than in G. striata.

Well-watered plants of both species had similar abaxial conductance (g) and gross photosynthesis (P). However, as the drought progressed ΨI, g, and P fell more in G. striata than in Siratro due to a less effective extraction of soil water, which we attribute to shallower rooting habit in G. striata. The active mechanism of paraheliotropic leaf movement in Siratro reduced light interception at higher ΨI than did the passive rolling and drooping of leaves in G. striata. Both mechanisms reduced the mean daily integral of intercepted radiation, but leaf temperatures of droughted G. striata were sometimes considerably higher than those of Siratro. Rooting depth and the hydraulic conductance of the plant-soil system appear to be as important in determining plant responses to drought as are the physiological responses of their leaves.