Photosynthesis during desiccation in an intertidal alga and a land plant

This study was undertaken to determine how photosynthesis tolerates desicca tion in an intertidal alga Fucus vesiculosus L. and a terrestrial sunflower Helianthus annuus L. Photosynthetic O-2 evolution generally was inhibited at low water potentials (Psi(W)) but more in sunflower leaves than in Fucus fronds at the same Psi(W). As Psi(W) decreased, less carbon accumulated in an organic carbon store in Fucus. The inhibition of photosynthesis appeare d to be mostly biochemical because it could not be prevented by supplying a dditional CO2 or by supplying CO2 from the internal organic carbon store. T he inhibition of photosynthesis and carbon storage occurred after turgor di sappeared and thus when solute concentrations were increasing in the cells. Solute concentrations were much higher in Fucus than in sunflower. After d esiccation to the air-dry state (Psi(W) below -10 MPa), photosynthesis coul d not recover in sunflower but it recovered rapidly when Fucus was exposed to seawater. The lack of recovery in sunflower was associated with inabilit y to recover turgor probably because of breaks in cell membranes. The abili ty to recover in Fucus was gradually lost during 1.5 d of desiccation at 45 % relative humidity. At lower humidities, recovery was lost sooner as small amounts of water were removed. We conclude that photosynthesis tolerated d esiccation more in Fucus than in sunflower because of differences in the mo lecular environment around the photosynthetic enzymes. Important aspects of this environment were features that prevented membrane breakage but promot ed the retention of small amounts of water that were critical for viability .