BENEFITS ASSOCIATED WITH STEEP FOLIAGE ORIENTATION IN 2 TUSSOCK GRASSES OF THE AMERICAN INTERMOUNTAIN WEST - A LOOK AT WATER-USE-EFFICIENCYAND PHOTOINHIBITION

Although steeply oriented foliage is a structural characteristic of ma ny tussock grasses in steppe and desert environments, benefits associa ted with these steep leaf angles have not been clearly demonstrated. T wo possible benefits of this foliage orientation, increasing water-use -efficiency and reducing photoinhibition, were investigated, in part b y model simulations, for Agropyron desertorum and Pseudoroegneria spic ata, two tussock grasses of the semi-arid steppe regions of the centra l Rocky Mountains, USA. Simulations using a whole-tussock photosynthes is model were used to assess water-use-efficiency in tussocks of varyi ng foliage orientation in summer conditions with minimal wind. Althoug h for horizontal leaves the simulations gave higher transpiration rate s than for leaves more steeply oriented due to greater incident solar shortwave flux, the calculated net transpirational losses were similar for all tussocks because of differences in the portion of sunlit leav es. In contrast computed daily net photosynthesis did differ between t ussocks of different foliage orientation when sunlit leaves were heate d to temperatures above that optimal for photosynthesis. Higher water- use-efficiency was calculated for tussocks with steeply oriented leave s, and resulted from higher daily net photosynthesis, not from differe nces in whole-tussock transpiration. In situ measurements of chlorophy ll fluorescence yield (F-V/F-M) were conducted in mid-July on intact l eaves on tussocks of A. desertorum and P. spicata fixed either horizon tally or at 70 degrees. While reduced yield was measured for all leave s at midday, F-V/F-M was 16% lower for leaves held horizontally than l eaves oriented at 70 degrees for both species. The potential for photo inhibition in these tussock grasses appears to be reduced by steeply o riented leaves.