DECREASED RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE IN TRANSGENIC TOBACCO TRANSFORMED WITH ANTISENSE RBCS .5. RELATIONSHIP BETWEEN PHOTOSYNTHETIC RATE, STORAGE STRATEGY, BIOMASS ALLOCATION AND VEGETATIVE PLANT-GROWTH AT 3 DIFFERENT NITROGEN SUPPLIES

Wild-type tobacco (Nicotiana tabacum L.) plants and transgenic tobacco transformed with antisense rbcS to decrease expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) were grown at 300 mol.m -1.s-1 irradiance and 20-degrees-C at either 0.1, 0.7 or 5 mM NH4NO3. In high nitrogen (N), growth was reduced in parallel with the inhibition of photosynthesis when Rubisco was decreased by genetic manipulation. In limiting N, photosynthesis was reduced strongly when Rubisco was decreased by genetic manipulation, but growth was hardly affected. At all N levels, decreased expression of Rubisco led to a decrease in the amount of starch accumulated in the leaves. There was a large increase of the specific leaf area (SLA; leaf area maintained per unit dry weight in the leaf) in plants with decreased Rubisco. Increased SLA was associated with an increased inorganic and a decreased carbon contribution to leaf structural dry weight. The increased SLA represents a more efficient investment of photosynthate with respect to maximisation of leaf area and light interception, and partly compensates for the decreased rate of photosynthesis in plants with decreased expression of Rubisco. The changes of starch content and SLA were particularly large in limiting N, when growth rate was effectively independent of the rate of photosynthesis. Increased N availability led to a large increase of the shoot/root ratio, but only a small increase in SLA. It is argued that N availability and the availability of photosynthate both regulate storage and allocation of biomass to optimize resource utilization, but achieve this via different mechanisms.