Partitioning of photosynthetic electron flow between CO2 assimilation and O2 reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P (N)) and stomatal conductance (g (s)) of attached leaves decreased as leaf water potential (Psi(w)) declined from -0.3 to -2.9 MPa. Although g (s) decreased over the whole range of Psi(w), nearly constant values in the intercellular CO2 concentrations (C (i)) were observed as Psi(w) decreased to -1.8 MPa, but C (i) increased as Psi(w) decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (q(NP)) increased progressively. A highly significant negative relationship between q(NP) and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 mu mol m(-2) s(-1), the allocation of electrons from photosystem (PS) 2 to O-2 reduction was increased by 51 %, while the allocation to CO2 assimilation was diminished by 32 %, as Psi(w) declined from -0.3 to -2.9 MPa. A significant linear relationship between mean P (N) and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas q(P) declined only at very severe stress and the excess photon energy was dissipated by increasing q(NP) indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of q(NP) against photoinhibition in sunflower.