Regulation of photosynthesis in interior spruce during water stress: changes in gas exchange and chlorophyll fluorescence.

Photosynthetic response to water stress was analyzed in 1-year-old interior spruce (Picea glauca (Moench) Voss x P. engelmanni Parry hybrid complex) seedlings and emblings produced from somatic embryogenesis. Carbon dioxide uptake, oxygen evolution and chlorophyll fluorescence at 20 degrees C were monitored as predawn shoot water potential (Psi) decreased. Concurrently with stomatal closure, carbon assimilation declined rapidly as Psi decreased to -1.0 MPa. Oxygen evolution at 10,000 micro l CO(2) l(-1) declined continuously as Psi decreased to -1.6 MPa. At photon flux densities (PFD) above 50 micro mol m(-2) s(-1), photochemical efficiency of photosystem (PS) II observed during actinic light exposure (Phi(II), calculated as DeltaF/F(m)') decreased as Psi decreased. At the same PFDs, photochemical quenching (q(P)) declined with decreasing Psi and nonphotochemical quenching (q(N)) increased steadily. At PFDs below 50 micro mol m(-2) s(-1), major decreases in q(N) were not observed until Psi decreased below -1.6 MPa. We identified three phases of photosynthetic response to progressive water stress in interior spruce: a pronounced decline in gas exchange, subsequent photoprotective changes in chlorophyll fluorescence as primary photochemistry was down-regulated, and a decline in photochemical efficiency of dark-adapted needles.