Biochemical regulation during the mid-day decline of photosynthesis in field-grown sunflower

Sunflower (Helianthus annuus L.) was raised according to common agricultural practices except that drought was imposed from the 20th day after planting in order to accentuate the natural daily cycling in leaf water potential (Ψ_leaf). The nature of the processes involved in regulating assimilation on a diurnal time scale was then investigated using techniques of gas exchange analysis. Although Ψ_leaf decreased from -0.5 to -1.1 MPa between predawn and mid-afternoon, autoradiographs of recent ¹⁴C-photoassimilation in leaves showed no evidence for inhomogeneities in stomatal apertures at any time off the day. Only by severing the tap root and allowing the shoot to dry rapidly could 'patchiness' be induced. A significant diurnal decline in assimilation (A) (from ∼ 31 to 25 μmolCO₂ m^-2 s^-1) was accompanied by declining leaf conductance but, since there was no significant change in intercellular [CO₂] (C_i), it is unlikely that A was being limited by the supply of CO₂ for carboxylation. Instead, a decrease in the initial slope of A/C_i curves taken over the course of the day showing a decreased apparent affinity of the CO₂ reductive processes for CO₂, indicating chloroplast-level control. By briefly treating the leaf with intercellular CO₂ levels near the CO₂ compensation point, the relationship between A and C_i could be significantly shifted. These experiments revealed a strong biochemical element contributing to the lowre photosynthetic rates that develop over the course of a day in field-grown sunflower. In addition, it was seen that rapid adjustments could be made in the level of the biochemical control and that they were probably initiated by changes in the intercellular CO₂ levels.