XVI International Botanical Congess

Sulfur deficiency symptoms are light dependent and this has not been studied with respect to developmental age. Phaseolus vulgaris was grown at 200 or 50 µmol photons m-2 s-1. Added sulfur was 1.0 mM (normal), 0.1mM (low), or zero. The Plastochron Index showed a longer time interval for the initiation of the third trifoliate leaf in zero S-high light but no difference in any other treatment. At this developmental age, the photosynthetic rate of high light plants was 85% and 60% of controls in low and minus S. Stomatal conductance in high light was 62% and 28% of control in low and minus S. No difference in photosynthetic rate or conductance was shown in low light treatments. We conclude that decreasing sulfur availability negatively affects photosynthetic efficiency and conductance before leaf development is affected, but only in high light.