SEASONAL ACCLIMATION OF STEM PHOTOSYNTHESIS IN 2 INVASIVE, NATURALIZED LEGUME SPECIES FROM COASTAL HABITATS OF CALIFORNIA

Responses of stem net photosynthetic rate (P-N) to irradiance (I), tem perature, vapor pressure, CO2 concentration, nitrogen content, and wat er potential were studied during early spring (March) and mid summer ( late July) in field populations of two stem photosynthesizing species introduced into coastal California sites. Stem P-N in Spartium junceum was inhibited in the summer compared with the spring due to low water potential and low tissue nitrogen content. Quantum yield decreased fr om March to July, along with a decrease in the demand function of the C-i response curves. Stem conductance was lower at all vapor pressure deficits during the summer, but C-i at a C-a of 350 mu mol mol(-1) inc reased for S. junceum from spring to summer. The thermal optimum for s tem P-N did not change between the two seasons, but that of S. junceum was 5 degrees C higher than that for Cytisus scoparius. The demand fu nction for the C-i curves of C. scoparius also decreased from spring t o summer, but the C-i at a C-a of 350 mu mol mol(-1) remained unchange d between the seasons. Thus, the water use efficiency of S. junceum de creased in the summer compared with that of the spring while that of C . scoparius remained unchanged between seasons. There was little evide nce for compensatory acclimation of stem P-N to changes in climatic co nditions in these species.