Measuring and predicting net photosynthesis of honey mesquite (Prosopis glandulosa Torr. var. glandulosa)

Abstract

A portable, open CO2 exchange system was developed to measure net assimilation rates (NAR) on in situ, branches of honey mesquite (Prosopis glandulosa Torr. var. glandulosa). Xylem pressure potentials and leaf diffusive resistances to water vapor were concomitantly measured with NAR. Seventeen diurnal patterns of NAR were followed on honey mesquite at two Texas locations during the 1977 and 1978 growing seasons. Average daily quantum yields based on incident light were subsequently determined for each diurnal pattern. Maximum net CO2 assimilation rate for mesquite was 34.2 mg dm^-2 h^-1. Drought conditions in west Texas during 1977 caused a progressive shift in maximum NAR; maximum rates decreased and occurred earlier in the day for each sequential sampling period. The light saturation point for mesquite photosynthesis was estimated to be in the range of 40 to 70 nEinsteins cm^-2 s^-1. Optimum temperature for NAR of in situ mesquite was between 25 and 32°C. Xylem pressure potential had little affect on NAR until water stress increased to -35 bars; xylem pressure potentials below -35 bars severely reduced NAR. Overall, dally mean assimilation rate and quantum yield for mesquite were 3.19 ± 1.16 μmol cm^-2 h^-1 and 0.800 ± 0.328 x 10^-2 mol Einstein^-11, respectively. High water stress was found to reduce photosynthetic efficiency in honey mesquite by as much as 80 percent. A model was developed to show a mathematical relationship between NAR and components of the environment, and to predict daily diurnal patterns of NAR in mesquite. The model was intrinsically non-linear and had 12 estimable parameters. The only variable inputs required by the model were photon flux density, temperature and leaf conductance to CO2. The model accounted for at least 70 percent of the variability in NAR for mesquite.