CONCURRENT MEASUREMENTS OF OXYGEN AND CARBON-DIOXIDE EXCHANGE DURING LIGHTFLECKS IN MAIZE (ZEA-MAYS L)

Leaves of maize (Zea mays L.) were enclosed in a temperature-controlle d cuvette under 35 Pa (350 mu bars) CO2 and 0.2 kPa (0.2%) O-2 and exp osed to short periods (1-30 s) of illumination (light-flecks). The rat e and total amount of CO2 assimilated and O-2 evolved were measured. T he O-2 evolution rate was taken as an indicator of the rate of photosy nthetic noncyclic electron transport (NCET). In this C-4 species, the response of electron transport during the lightfecks qualitatively mim icked that of C-3 species previously tested, whereas the response of C O2 assimilation differed. Under short-duration lightflecks at high pho ton flux density (PFD), the mean rate of O-2 evolution was greater tha n the steady-state rate of O-2 evolution under the same PFD due to a b urst of O-2 evolution at the beginning of the lightfleck. This O-2 bur st was taken as indicating a high level of NCET involved in the buildu p of assimilatory charge via ATP, NADPH, and reduced or phosphorylated metabolites. However, as lightfleck duration decreased, the amount of CO2 assimilated per unit time of the lightfleck (the mean rate of CO2 assimilation) decreased. There was also a burst of CO2 from the leaf at the beginning of low-PFD lightflecks that further reduced the assim ilation during these lightflecks. The results are discussed in terms o f the buildup of assimilatory charge through the synthesis of high-ene rgy metabolites specific to C-4 metabolism. It is speculated that the inefficiency of carbon uptake during brief light transients in the C-4 species, relative to C-3 species, is due to the futile synthesis of C -4 cycle intermediates.