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Photosynthesis by nonsucculent green stems contributes a major portion of annual carbon assimilation in numerous C3 and some C4 desert perennials. For example, in the Sonoran Desert, stem photosynthesis was responsible for 40% of net annual assimilation in Cercidium floridum (Adams and Strain 1969) and for 72% in C. microphyllum, mostly by twigs less than 6 mm in diameter (Szarek and Woodhouse 1978a, 1978b). Because precipitation in deserts is sporadic, in leafy species large seasonal and year-to-year fluctuations have been observed in the relative contribution of stems to net carbon assimilation. In shrubs with ephemeral microphylls, e.g., Thamnosma montana and Salazaria mexicana, annual stem assimilation accounted for 83% of net carbon assimilation (Comstock et al. 1988). Species of Ephedra, Castela emoryi, and Koeberlinia spinosa, in which leaf biomass is negligible, and aphyllous, fleshy-stemmed Chenopodiaceae, such as Hammada scoparia, can be expected to depend almost entirely on stem photosynthesis (Kappen et al. 1976; Schulze et al. 1976). Drought deciduous perennials, such as Gutierrezia sarothrae (DePuit and Caldwell 1975), Hymenoclea salsola (Comstock and Ehleringer 1988), and other common chamaephytes (Comstock et al. 1988; Schmitt et al. 1993) may also depend substantially upon stem photosynthes. |
