Changes in aldolase activity in wild-type potato plants are important for acclimation to growth irradiance and carbon dioxide concentration, because plastid aldolase exerts control over the ambient rate of photosynthesis across a range of growth conditions

Even though plastid aldolase catalyses a reversible reaction, does not poss ess properties allowing it to contribute to 'fine' regulation, and would th erefore be considered unimportant for the control of metabolism and growth, antisense transformants with a 50-70% decrease in aldolase activity showed an inhibition of photosynthesis and growth. We now show that acclimation o f photosynthesis to growth conditions includes and requires changes in plas tid aldolase activity. Wild-type potato plants and transformants were grown at low irradiance (70 mu mol m(-2) sec(-1)), and at high irradiance (390 m u mol m(-2) sec(-1)) at 400 or 800 p.p.m, carbon dioxide. (i) Ambient photo synthesis was always inhibited by a 30-40% decrease of aldolase activity, t he strongest inhibition being observed when plants were growing in high irr adiance and elevated carbon dioxide. (ii) The inhibition was due to a low r ate of ribulose-1,5-bisphosphate regeneration in low light, exacerbated by an inadequate rate of starch synthesis in high light and elevated carbon di oxide. Decreased expression of aldolase in antisense transformants was also accompanied by a decrease of fructose-1,6-bisphosphatase protein and activ ity, and Rubisco activity. Transcript levels for the plastid fructose-1,6-b isphosphatase and the small subunit of Rubisco did not decrease. (iii) In w ild-type plants, increasing the growth irradiance from 70 to 390 mu mol m(- 2) sec(-1) led to a 15-95% increase of the activity of eight Calvin cycle e nzymes, and increasing the carbon dioxide concentration from 400 to 800 p.p .m, led to a 5-35% decrease of these enzyme activities. The largest changes occurred for aldolase, and for transketolase which also catalyses a revers ible reaction and is not subject to 'fine' regulation.