EFFECT OF PLANT HEIGHT ON LIGHT INTERCEPTION, WATER USE AND YIELD OF GRAIN SORGHUM IN PURE AND MIXED CANOPIES

Advantages of short over tall sorghum genotypes include resistance to lodging and adaptation to mechanical harvesting. Grain yield comparison of sorghum height types are, however, inconsistent. Experiments were designed to relate sorghum height to growth and development in pure and mixed canopies in the summer of 1978 and 1979. The treatments were: the 1-dwarf, 2-dwarf, 3-dwarf, a 33% by weight mixed-intercrop and alternating rows of the height isolines. Water-use efficiency (WUE), photosynthetic light (PAR) interception, flag leaf temperatures, stomatal resistance, xylem-water potential, dry matter accumulation and grain yield of the height isolines in the pure, mixed and alternated rows were determined.;The water extraction profiles of the pure isolines were similar except the 3-dwarf plots allowed more surface evaporation because of incomplete ground cover. Tall plants had the lowest xylem-water potential and highest stomatal resistance, 3-dwarf plants had the reverse while 2-dwarf plants were intermediate. The tall canopy had the highest evapotranspiration in both seasons. The 2-dwarf plants had consistently the warmest canopy temperature of the isolines during peak insolation periods.;Differences in dry matter of the isolines were mainly in the stem fraction rather than leaves or heads. The tall plants produced about the same total dry matter as the mixed and alternated plots. Relationship of dry matter production to transpiration normalized with water vapor pressure gave slope factor (k) of 0.173, 0.222 and 0.296 mb for the 3-, 2- and 1-dwarf canopies. Tall plants had the highest WUE for dry matter but the 2-dwarf plants were more efficient with respect to grain water-use efficiency. The 2-dwarf produced the most grain in both seasons. Tall plants yielded more grain in the mixed than in either pure or alternated canopies. The short 3-dwarf plants in the mixed plots were shaded and, therefore, yielded the lowest in dry matter and grain.;The plant population of approximately 175,000 plants/ha, gave leaf area indices greater than 3.7 from half-bloom through grain filling. Only the 3-dwarf plots had incomplete ground cover which allowed significantly higher PAR transmission than either the 1- or 2-dwarf canopies. Fewer tillers and open arrangement of leaves on the tall 1-dwarf plants allowed higher PAR transmission and, therefore, lower interception compared to the 2-dwarf canopy though these differences were not statistically significant. Intercepted PAR in the mixed and alternated rows were similar to that in 1- or 2-dwarf plots.