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Effect of Increasing Foliage Reflectance on the CO₂ Uptake and Transpiration Resistance of a Grain Sorghum Crop¹

The investigation reported here was designed to provide a physiological explanation for the grain yield increases which resulted from whitening of dryland sorghum crop canopy (Sorghum bicolor L. Moench ‘Hazera 726’) by a kaolin suspension spray. Two years of field measurements of labelled CO₂ uptake rates, on kaolin-coated and uncoated sorghum leaves, showed that the kaolin coating caused a 23% decrease of net photosynthesis similar to the 26% reduction of solar radiation absorption measured with a spectrophotometer immediately after the spray. The reduction in net photosynthesis persisted for several weeks and was actually larger 6 weeks after the last kaolin application.

Measurements of the dynamics of light dependent CO₂ uptake made on leaves of different ages with a series of neutral filters and independent measurements of green leaf area confirmed enhanced senescence of the kaolin-coated leaves.

By contrast, field measurements with a diffusion porometer demonstrated only minor reductions in the leaf surface diffusion resistance of kaolin-coated leaves.

It is concluded that, for the sorghum crop examined, the primary effect of kaolin sprays was to reduce the rate of photosynthesis, both by decreased light absorption and by enhanced senescence. Finally, a possible explanation is offered to reconcile the reduction in photosynthesis with the observed increase in the grain yield of the treated crop.

Key Words: CO₂ uptake • Light response of CO₂ uptake • Solar radiation • Absorptance • Leaf resistance

² Division of Agricultural Meteorology, The Volcani Center, Bet Dagan, Israel.

Received for publication February 13, 1976.