CONTRASTING PHOTOSYNTHETIC BEHAVIOR IN LEAVES AND TWIGS OF HYMENOCLEA SALSOLA, A GREEN‐TWIGGED WARM DESERT SHRUB

Abstract

The photosynthetic behavior of leaves and twigs was compared in Hymenoclea salsola T. and G., a subshrub of the Mohave and Sonoran deserts, in which both leaves and green twigs make substantial contributions to whole‐plant carbon gain. Light saturated photosynthesis in twigs was 0.62 times that of leaves (36.9 μmol m‐2 s‐1) when plants were well watered. Similar ratios were consistently observed in contrasting the photosynthetic responses of the two organ types to light, temperature, and intercellular CO2, regardless of whether rates were compared under saturating or highly limiting conditions of light or intercellular CO2. These scalar differences in photosynthetic rate between leaves and green twigs under a wide range of conditions were correlated with contrasting anatomical features such as chlorenchyma volume per projected area. Under normal ambient CO2 concentrations (350 μl 1‐1), twigs on well watered plants operated at lower intercellular CO2 concentrations than the leaves. Possible causes of this difference are discussed with respect to performance under well‐watered conditions, organ lifespans, and contrasting anatomical constraints. Twigs require larger investments than do leaves of both carbon and nitrogen per projected area of the respective organs, yet they realize lower photosynthetic rates per intercepted light. Twigs, however, fulfill additional roles besides photosynthesis such as structural support and vascular transport which does not allow them to be as anatomically specialized as leaves for photosynthesis. Twigs also have a longer expected lifespan than leaves with a larger fraction of them surviving the summer drought period. This was correlated with a greater tolerance of twig than leaf photosynthesis to low plant water potentials.