REGULATION OF PHOTOSYNTHESIS IN EUCALYPTUS NITENS (DEANE AND MAIDEN) MAIDEN FOLLOWING GREEN PRUNING

Green pruning was used to induce source limitation in 1-year-old Eucal yptus nitens (Deane and Maiden) Maiden plants in an investigation of r egulation of net CO2 assimilation (A). The pruning regimes involved re moval of 0, 50% or 70% of the length of green crown, equivalent to 0, 78% or 95% of leaf area respectively. Gas exchange measurements were m ade on fully-expanded leaves at 80% of crown height prior to pruning, and at weekly intervals over the 8 weeks following pruning. An increas e in A was observed in response to pruning a week after treatment. In 50%-pruned plants this increase in A was observed for 6 weeks, but in 70%-pruned plants it was still evident at the end of the experiment. W hile leaf conductance (g) increased considerably following pruning, st omatal limitation was unaffected, indicating that the most important c hanges in conductance of CO2 were in the mesophyll. Both carboxylation efficiency (C-e) and RuBP regeneration capacity (V-j) increased follo wing pruning. The magnitude and duration of changes increased with pru ning severity. There was evidence that A was primarily limited by V-j, although in 70%-pruned plants C-e may have been the main limitation e arly in the experiment. The response of A to low p(O-2) indicated that A was not limited by rates of triose phosphate utilisation (TPU), and that, at more severe pruning levels, rates of TPU may have increased in the short term. Apparent quantim yield was unaffected by pruning, s uggesting no change in the production or consumption of ATP or NADPH(2 ). There was a negative linear relationship between A and the ratio of leaf area to aboveground dry mass, and a positive linear relationship between A and specific leaf area. Both suggested a link between rates of assimilation of the level of source limitation. The capacity of E. nitens to regulate photosynthesis in response to pruning increases th e potential for maintaining a balance between assimilation and utilisa tion of carbon, thereby mimimising the impact of pruning on growth. Ho wever there will be a level of source limitation at which photosynthes is is limited by the rate of one or more biochemical reactions, and ab ove which complete compensation is not possible. At such levels growth will usually be affected. In this experiment diameter growth was slig htly reduced by 50%-pruning, but both height and diameter increment we re substantially reduced by 70%-pruning.