RUBISCO ACTIVITY IS ALTERED IN A STARCHLESS MUTANT OF NICOTIANA-SYLVESTRIS GROWN IN ELEVATED CARBON-DIOXIDE

Dry matter and net photosynthesis of a wild type and a starchless muta nt NS 458 of Nicotiana sylvestris (Speg. et Comes) were studied after 25 d of CO2 enrichment. Plants were grown from seed in controlled envi ronment chambers and treatments of either ambient (35 Pa) or twice amb ient (70 Pa) CO2 were initiated when plants were 3-4 weeks old. Photos ynthetic rates measured at 35 and 70 Pa CO2 and at 900 mu mole quanta m(-2) s(-1) were unaffected (P>0.05) by 25 d of CO2 enrichment. Howeve r, a CO2-by-genotype interaction was observed indicating that photosyn thetic rates of the wild type but not the mutant at 35 Pa CO2 differed in response to CO2 enrichment. Photosynthetic enhancement was greater (P < 0.001) in the wild type than in the mutant when the measurement CO2 was doubled. Total biomass and leaf areas of the mutant and wild t ype also were unaffected by CO2 enrichment, although specific leaf wei ght increased 27% and 13% (P<0.001) for the wild type and mutant lines , respectively. Neither chlorophyll nor soluble leaf protein were affe cted by CO, enrichment. Starch, sucrose, glucose and fructose in wild type and mutant leaf samples were also unaffected by CO2 enrichment. R ubisco protein levels of the wild type and mutant were about 20% lower in elevated compared to ambient CO2-grown plants. Initial and total R ubisco activities of wild type and mutant leaf samples were not signif icantly different (P>0.05) between CO2 environments. However, initial Rubisco activity was more than 30% lower in mutant than than in wild t ype samples when results from ambient and elevated CO2-grown plants we re combined. Ribulose 1,5-bisphosphate and 3-phosphoglycerate were 280 % and 28% greater in the mutant than in the wild type, respectively. T hese findings suggested that photosynthesis rates of the mutant were l imited by Rubisco activity at 35 Pa CO2 and that end product synthesis rates limited photosynthesis of the mutant at 70 Pa CO2.