SIMULTANEOUS EFFECTS OF FOLIAR NITROGEN, TEMPERATURE, AND HUMIDITY ONGAS-EXCHANGE IN PINUS-RADIATA

Seedlings of Pinus radiata were grown in a glasshouse in large pots wi th sand as the potting mix. They were kept well-watered and frequently supplied with nutrient solutions which contained different amounts of nitrogen for different treatments. Carbon assimilation and diffusive conductance of the foliage were measured under steady-state conditions at saturating light in all treatments. Experimental variables were le af-air vapour pressure difference and leaf temperature at time of meas urement. Data were fitted to a non-linear regression equation to exami ne responses of carbon assimilation, diffusive conductance, transpirat ion, assimilatory nitrogen-use efficiency, and assimilatory transpirat ion efficiency to foliar nitrogen concentration expressed on a leaf ar ea basis ([N]), to leaf temperature, and to leaf-air vapour pressure ( D). Parameters from the regression have been used to plot three-dimens ional surfaces, so that simultaneous effects of experimental variables can be easily visualised. Carbon assimilation increased linearly with foliar [N], declined exponentially as D increased, and had a broad te mperature optimum between c. 14 and 38 degrees C. Diffusive conductanc e increased linearly with foliar [N], was related to the reciprocal of D, and declined as temperature increased. Using climatic vapour press ure deficit and air temperature data for Canberra, ACT, and for Mount Gambler SA, and with the functions that had been fitted to experimenta l data, it was found that these regional climatic differences have pot ential for appreciably affecting carbon gain and water loss in the reg ions, which have P. radiata plantations. Predicted differences in carb on gain are of the order of reported differences in stem growth in the regions. This shows the need to take into account regional variation in climatic variables that strongly affect gas exchange when investiga ting regional differences in productivity.