Photoassimilation of CO2 by Isolated Bundle‐Sheath Strands of Zea mays I. Stimulation of CO2 Assimilation by Adding Various Intermediates of the Photosynthetic Cycle; Evidence for a Deficient Photosystem II Activity

Isolated bundle‐sheath (BS) strands from leaves of mature maize plants show enhanced rates of CO2 fixation in the presence of reduced intermediates of the photosynthetic cycle (R5P, DHAP, FruDP.) 3PGA is the major labelled product of 14CO2 fixation whatever the substrate added. CO2 fixation is much lower with PGA than with reduced intermediates, suggesting a limited capacity of the cells to regenerate RuDP (the CO2‐acceptor) from PGA. These two experimental facts, which are characteristic features of bundle‐sheath photosynthesis for maize (a species with agranal bundle‐sheath chloroplasts) indicate that phaotosystem II activity is a limiting factor for the evolution of the bundle‐sheath photosynthetic process. Nevertheless, a reducing capacity arises as proved by sensitivity of CO2 fixation to DCMU, particularly when PGA is added to the bundle‐sheath. PGA synthesis occurs, in the presence of non‐limiting amounts of CO2, according to the equation: RuDP + CO2→ 2 PGA; the oxygen effect on 14CO2 fixation, at lower CO2 concentration, is interpreted as a dilution effect of the internal pool of 14CO2 by unlabelled CO2 generated by photorespiration.