DOES RUBISCO CONTROL THE RATE OF PHOTOSYNTHESIS AND PLANT-GROWTH - ANEXERCISE IN MOLECULAR ECOPHYSIOLOGY

Experiments are described in which tobacco (Nicotiana tabacum L.) tran sformed with antisense rbcS to decrease expression of ribulose-1,5-bis phosphate carboxylase-oxygenase (Rubisco) was used to evaluate the con tribution of Rubisco to the control of photosynthetic rate, and the im pact of a changed rate of photosynthesis on whole plant composition, a llocation and growth. (1) The concept of Bur control coefficients is i ntroduced. It is discussed how, with adequate precautions, a set of wi ld-type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficien t. (2) The flux control coefficient of Rubisco for photosynthesis depe nds on the short-term conditions. It increases in high light, or low C O2. (3) When plants are grown under constant irradiance, the Bur contr ol coefficient in the growth conditions is low (<0.2) at irradiances o f up to 1000 mu mol quanta m(-2) s(-1). In a natural irradiance regime exceeding 1500 mu mol quanta m(-2) s(-1) over several hours the flux coefficient rose to 0.8-0.9. It is concluded that plants are able to a djust the balance between Rubisco and the remainder of the photosynthe tic machinery, and thereby avoid a one-sided limitation of photosynthe sis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisc o had a higher Bur control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewe d as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A ch ange in the rate of photosynthesis did not automatically translate int o a change in growth rate. Several factors are identified which contri bute to this buffering of growth against a changed photosynthetic rate . (6) There is an alteration in whole plant allocation, resulting in a n increase in the leaf area ratio. The increase is mainly due to a hig her leaf water content, and not to changes in shoot/root allocation. T his increased investment in whole plant leaf area partly counteracts t he decreased efficiency of photosynthesis at the biochemical level. (7 ) Plants with decreased Rubisco have a lower intrinsic water use effic iency and contain high levels of inorganic cations and anions. It is p roposed that these are a consequence of the increased rate of transpir ation, and that the resulting osmotic potential might be a contributor y factor to the increased water content and expansion of the leaves. ( 8) Starch accumulation in source leaves is decreased when unit leaf ph otosynthesis is reduced, allowing a more efficient use of the fixed ca rbon. (9) Decreased availability of carbohydrates leads to a down-regu lation of nitrate assimilation, acting via a decrease in nitrate reduc tase activity.