Regulation of photosynthesis in Eucalyptus nitens (Deane and Maiden) Maiden following green pruning

Abstract

 Green pruning was used to induce source limitation in 1-year-old Eucalyptus nitens (Deane and Maiden) Maiden plants in an investigation of regulation of net CO₂ assimilation (A). The pruning regimes involved removal 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 made on fully-expanded leaves at 80% of crown height prior to pruning, and at weekly intervals over the 8 weeks following pruning. An increase 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. While leaf conductance (g) increased considerably following pruning, stomatal limitation was unaffected, indicating that the most important changes in conductance of CO₂ were in the mesophyll. Both carboxylation efficiency (C _e ) and RuBP regeneration capacity (V _j ) increased following pruning. The magnitude and duration of changes increased with pruning 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 early in the experiment. The response of A to low p(O₂) 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, suggesting no change in the production or consumption of ATP or NADPH₂. There was a negative linear relationship between A and the ratio of leaf area to above-ground 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 the potential for maintaining a balance between assimilation and utilisation of carbon, thereby mimimising the impact of pruning on growth. However there will be a level of source limitation at which photosynthesis is limited by the rate of one or more biochemical reactions, and above which complete compensation is not possible. At such levels growth will usually be affected. In this experiment diameter growth was slightly reduced by 50%-pruning, but both height and diameter increment were substantially reduced by 70%-pruning.