Photosynthesis, Chlorophyll, and Transpiration Responses in Aluminum Stressed Wheat and Sorghum 1

Aluminum is a major deterrent to crop production in acid soils. The specific effects of Al toxicity on photosynthesis, chlorophyll, and transpiration in Al stressed plants have received little attention. The objective of this study was to determine the influence of Al stress on photosynthesis, chlorophyll, and transpiration in wheat ( Triticum aestivum L.) and sorghum [ Sorghum bicolor (L.) Moench] under greenhouse conditions. Plants were grown in modified Steinberg solution containing 10 Al rates (0, 1.9, 3.7, 9.3, 18.5, 37.1, 74.1, 148, 222, and 297 μmol L −1 ) in the greenhouse for 28 days after transplanting. The initial pHs were 4.0 and 3.3 for wheat and sorghum, respectively. Photosynthesis and transpiration measurements were made on intact recently matured leaves. Chlorophyll concentrations were determined on the same tissue. In wheat, Al toxicity decreased photosynthesis, chlorophyll concentration, and transpiration. The critical Al toxicity levels in wheat blade associated with decreases in photosynthesis, chlorophyll concentration, and transpiration were 0.15, 0.13, and 0.14 mmol kg −1 , respectively. In sorghum, photosynthesis and chlorophyll concentration were also decreased by Al stress, but transpiration was increased. The critical Al toxicity levels for these parameters in sorghum could not be determined due to low levels of Al concentration in the blade. Transpiration in nonstressed sorghum was approximately one‐third the rate of wheat. Thus, the growth reduction of wheat and sorghum under Al stress may be due in part to decreased photosynthesis.