SHADE SPECTRAL QUALITY AND THE PHOTOSYNTHETIC CAPACITY OF SOYBEAN LEAVES

Soybean [Glycine max (L.) Merr. cv. Kent] plants were grown outdoors u nder a canopy of cascading mums and a canopy of a spectrally neutral s hade cloth to determine if the spectral quality of shade from vegetati on affected the photosynthetic acclimation to photosynthetically activ e radiation (PAR) in soybean. The canopies were matched for transmissi on of PAR and the frequency of sunflecks. The plant canopy transmitted more far-red radiation, and had a quantum flux ratio of red (660 nm) to far-red (730 mn) radiation of 0.25 compared to 1.28 for the cloth c anopy. There was no difference between die shade treatments in leaf ph otosynthetic capacity, the PAR-saturated rate of net carbon dioxide fi xation expressed per unit of area, for any of three batches of soybean plants. There were differences between shade treatments in total dry mass accumulation and in hypocotyl length, indicating that the plants perceived and responded to the spectral differences. In soybeans, phot osynthetic capacity is less sensitive to the net change in spectrum ca used by vegetative shade than are other morphological responses.