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Several studies have explored the contribution of stem photosynthesis to the total carbon gain of desert plants, but they have been limited to narrow desert systems in extra tropical regions. To have a better understanding of the role of photosynthetic stems in carbon fixation for tropical deserts, it is important to extend our studies into areas with different ecological conditions. This study compared the photosynthetic capacity, and seasonal variation of gas exchange of leaves and stems of Justicia californica, in the southern Sonoran Desert. It also compared their relative contribution to carbon gain, and their photosynthetic response to seasonal changes in xylem tension and vapor pressure deficit (VPD).

Leaves and stems showed the same photosynthetic capacity and similar diurnal patterns of A and g, during the rainy season. VPD and water potential influenced the diurnal and seasonal patterns of A and g(s), and their influence was different for leaves and stems. Stems were less sensitive to water stress than leaves. The relative contribution of stems and leaves to whole canopy carbon gain, depended mainly on the variation of their photosynthetic areas. On a year-round basis, leaves contributed 77% and stems 23% of the carbon gain. However, stems were the only source of carbon gain for about 7 months. (c) 2007 Elsevier Ltd. All rights reserved.