Photosynthesis by mature needles of field-grown Pinus radiata

Abstract

Gas exchange was measured on needles of 14-year-old Pinus radiata in the Australian Capital Territory. The trees were growing at very different rates as a result of different inputs of fertiliser and water. Measurements were taken from late spring (October) 1987 until mid-autumn (April) 1988, a period with little rain and moderate to high temperatures. Despite large differences in needle nitrogen concentration between trees, the major determinant of the light-saturated rate of photosynthesis was water availability. On irrigated trees, the needles had consistently higher assimilation rates, as well as higher stomatal conductances. Light-saturated photosynthesis and stomatal conductance were closely correlated, but the droughted trees had higher instantaneous water-use efficiency, as indicated by the steeper slope of the assimilation versus stomatal conductance relationship. This difference was also seen in intercellular CO 2 partial pressure. which was significantly lower for needles of water-stressed trees than for those on well-watered trees. Stomatal conductance was very sensitive to leaf-to-air vapour pressure deficit, with much higher conductances for needles on irrigated trees for a given vapour pressure deficit. The sensitivity of somatal conductance to vapour pressure deficit declined between spring and autumm. On the few occasions that stomatal conductance exceeded 75 mmol H 2O m −2 s −1, CO 2 uptake was positively correlated with needle nitrogen concentration. Although needle chlorophyll and nitrogen concentrations had high positive correlation, they were weakly associated with photosynthetic rate. These results are generally consistent with much of the literature on conifers, and their implications for plant growth analysis and modelling of Pinus radiata growth are discussed.