The influence of a decrease in irradiance on photosynthetic carbon assimilation in leaves of Spinacia oleracea L.

When leaves of Spinacia oleracea L. were subjected to a decrease from a saturating to a limiting irradiance, photosynthetic carbon assimilation exhibited a pronounced lag. This comprised a postlower-illumination CO₂ burst (Vines et al. 1982, Plant Physiol. 70, 629-631) and a slow increase in the rate of carbon assimilation to the new lower steady-state rate. The latter phenomenon was distinguishable from the former because it was present in leaves when photorespiration was inhibited by high concentrations of CO₂ or by 2% O₂. A lag which followed a decrease in irradiance was also evident in leaves of Zea mays in air or in isolated spinach protoplasts photosynthesising in high CO₂. The lag was not stomatal in origin. The origin of the lag which followed the decrease in irradiance was investigated. Measurements of total ¹⁴CO₂ fixation and ¹⁴C incorporated into sucrose during the transition in irradiance showed that sucrose synthesis displayed an overshoot during the transient which accounted for all of the carbon fixed during the first 90 s of the transition period. The behaviour of hexose phosphates in the intact leaf and in the cytosol was inconsistent with their supporting sucrose synthesis during the transient. It is concluded that the overshoot in sucrose synthesis imposes a drain on chloroplast intermediates which contributes to the temporary lag in the rate of carbon assimilation.