The Control of the Light Acclimation of Photosynthesis inGlycine max: Dependence on Import as Modified by Intraplant Shading

Abstract

Ward, D. A. and Bunce, J. A. 1986. The control of the light acclimation of photosynthesis in Glycine max: Dependence on import as modified by intraplant shading.—J. exp. Bot. 37: 615-624. The dependence of photosynthetic capacity on imported and locally-assimilated supplies of carbon during leaf development under different irradiances was investigated in Glycine max. The potential export of carbon to the developing, mainstem trifoliate leaf (source-potential) was restricted non-destructively by shading all lower, source leaves (source-shading), while local photosynthesis was modified concurrently by exposing the young leaf to different light levels during development. When source-shading was applied below the 2nd mainstem trifoliate leaf at the bud stage of development, photosynthetic capacity was unaffected in leaves which had developed under moderate and low irradiances (500 and 250 /?mol PAR m~2 s1, respectively), but was reduced significantly in leaves developed under a high irradiance (900 /?mol PAR m~2 s~'). If source-shading was applied beneath the 2nd leaf at unfolding, the reduction of photosynthetic capacity under the high irradiance was relatively minor. The photosynthetic capacity attained by the 2nd leaf during development under different irradiances was influenced by the previous light environment of the whole plant. In contrast to the 2nd leaf, the photosynthetic capacities of the 1st and 4th mainstem leaves were relatively unaffected by source-shading, even under the highest light regime. While photosynthetic capacity showed a widespread insensitivity to the light level of the lower region of the canopy, source-shading reduced final leaf size irrespective of node position or local irradiance during leaf development. These effects were not related to differences in daily photosynthesis by the expanding leaf, and are discussed