Light Response of Photosynthesis and of RuBP Carboxylase Oxygenase Activity in Young, Mature, and Senescing Leaves of Red Clover (Trifolium pratense L.)

Summary The light response of the CO 2 exchange rate (CER) and of the activity of ribulose bis-phosphate carboxylase/oxygenase (RuBPCO) after rapid extraction (activity in vivo ) and after activation with CO 2 and Mg ++ (activity in vitro ) was determined in leaves of red clover ( Trifolium pratense L.) 1, 2 and 4 weeks after the petioles were first visible. They were referred to as young, mature, and senescent leaves, respectively. The age of the leaf affected CER at high but not at low photosynthetic photon flux density (PPFD) and thus influenced the shape of the light response curve. At high PPFD, CER was highest in mature leaves and lowest in those which were senescing. Responses to PPFD and to the age of the leaf were similar for CER and for in vivo activity of RuBPCO suggesting that, even at low PPFD, CER was controlled by RuBPCO activity. At PPFD higher than 400 °mol quanta m −2 s -1 , RuBPCO seemed to be fully activated in vivo . At high PPFD, differences in CER and in activity of RuBPCO in vivo between leaves of different ages were related to varying RuBPCO contents as estimated from the activity of RuBPCO after activation with CO 2 and Mg ++ in vitro . At lower PPFD, CER and in vivo activity of RuBPCO were independent of the age of the leaf despite differences in RuBPCO content. This was due to an effect of age on the degree of activation of RuBPCO in vivo below 300 °mol quanta m -2 s -1 .