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Response of Beans to Shading¹

Two dry bean (Phaseolus vulgaris L.) cultivars were grown in controlled-environment chambers under standard light [390 µ Einsteins m^-2 sec^-1 (400 to 700 nm) = approx. 22,000 lux] and shaded light [55 µ Einsteins m^-2 sec^-1 (400 to 700 nm) = approx. 3,200 lux] intensities. Experimental observations included measurement of gas exchange and photosynthetic and respiratory enzyme activity along with examination of plant morphology, leaf anatomy, and chloroplast ultrastructure.

Shading reduced leaf number, area, and thickness. Photosynthesis/unit area of shaded leaves was decreased by an average of 38%. Transpiration was not significantly affected. Increased intracellular resistance of the shaded leaves was more important in reducing CO₂ uptake than was the increase in stomatal resistance. The increase in mesophyll resistance was reflected both in the biochemical reactions of the leaf (activity of all measured enzymes being reduced by 70%) and in the ultrastructure of the leaf chloroplasts (reduced quantities of starch in spite of extensive grana).

Key Words: Phaseolus vulgaris L. • Photosynthesis • Transpiration • Diffusion resistance • Leaf anatomy • Chloroplast ultrastructure • RuDP carboxylase • Malate dehydrogenase • Glycolate oxidase

² Assistant professor, Dep. of Agron. and Plant Genet., U. of Minn., St. Paul, MN 55108; biochemist, Int. Inst, of Trop. Agric., Ibadan, Nigeria; research associate; and professor, Dep. of Veg. Crops, Cornell U., respectively.

Received for publication October 7, 1974.