Benefits associated with steep foliage orientation in two tussock grasses of the American Intermountain West. A look at water-use-efficiency and photoinhibition.

Abstract

Although steeply oriented foliage is a structural characteristic of many tussock grasses in steppe and desert environments, benefits associated with these steep leaf angles have not been clearly demonstrated. Two possible benefits of this foliage orientation, increasing water-use-efficiency and reducing photoinhibition, were investigated, in part by model simulations, for Agropyron desertorum and Pseudoroegneria spicata , two tussock grasses of the semi-arid steppe regions of the central Rocky Mountains, USA. Simulations using a whole-tussock photosynthesis model were used to assess water-use-efficiency in tussocks of varying foliage orientation in summer conditions with minimal wind. Although for horizontal leaves the simulations gave higher transpiration rates than for leaves more steeply oriented due to greater incident solar shortwave flux, the calculated net transpirationallosses were similar for all tussocks because of differences in the portion of sunlit leaves. In contrast computed daily net photosynthesis did differ between tussocks of different foliage orientation when sunlit leaves were heated to temperatures above that optimal for photosynthesis. Higher water-use-efficiency was calculated for tussocks with steeply oriented leaves, and resulted from higher daily net photosynthesis, not from differences in whole-tussock transpiration. In situ measurements of chlorophyll fluorescence yield (F v /F M ) were conducted in mid-July on intact leaves on tussocks of A. desertorum and P. spicata fixed either horizontally or at 70°. While reduced yield was measured for all leaves at midday, F v /F M was 16% lower for leaves held horizontally than leaves oriented at 70° for both species. The potential for photoinhibition in these tussock grasses appears to be reduced by steeply oriented leaves.