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), temperature, vapor pressure, CO2 concentration, nitrogen content, and water 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 from 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) increased for S. junceum from spring to summer. The thermal optimum for stem 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 function for the C-i curves of C. scoparius also decreased from spring to summer, but the C-i at a C-a of 350 mu mol mol(-1) remained unchanged between the seasons. Thus, the water use efficiency of S. junceum decreased in the summer compared with that of the spring while that of C. scoparius remained unchanged between seasons. There was little evidence for compensatory acclimation of stem P-N to changes in climatic conditions in these species.