| An ecophysiological study on two desert winter annuals of the southwestern United States was conducted. Both species possessed the ability to orient their leaf lamina perpendicular to the sun's direct rays (diaheliotropism). Malvastrum rotundifolium (Gray), which has simple leaves, maintained diaheliotropic leaf movements up to the wilting point of the plant (-4.0 MPa). Lupinus arizonicus (Wats), which has palmately compound leaves, varied its leaf movements from diaheliotropic at high leaf water potentials, to increasing degrees of paraheliotropic (orientation of the leaf lamina parallel to the sun's direct rays) as its bulk leaf water potential decreased. Field water relations properties of the plants were quite divergent, with M. rotundifolium maintaining leaf water potentials twice as negative as L. arizonicus, while leaf conductances were often quite similar. Leaf conductance, in both species, responded to bulk leaf water potential and ambient humidity levels. Leaves of L. arizonicus did not maintain positive net photosynthetic carbon gain beyond leaf water potentials of -1.6 to -1.8 MPa. Leaves of M. rotundifolium maintained positive net photosynthetic carbon uptake to leaf water potentials below -3.8 MPa. L. arizonicus possessed a combination of physiological and morphological properties which enabled it to maintain high leaf water potentials under drought conditions. M. rotundifolium adjusted its osmotic potentials, which enabled it to maintain metabolic processes in its leaves despite depressed leaf water potentials due to drought. The M. rotundifolium morpho-physiological pattern led to higher transpirational water losses for its leaves than leaves of L. arizonicus under equivalent conditions. An integration of these physiological properties with energy budget analysis of leaf orientation predicted that in terms of carbon gain and water use efficiency, a drought tolerant physiology was disadvantageous for a leaf displaying paraheliotropic movements, and a drought intolerant physiology was disadvantageous for a leaf displaying diaheliotropic movements. These results were extremely sensitive to the level of soil water availability. The kinetics of osmotic adjustment and soil drying would be critical to carbon gain and water use patterns of diaheliotropic species. |
