The yield of maize hybrids has increased significantly during the past 30 yr. Changes in climate, management techniques and genetics have contributed to yield gains. The genetic contribution has ranged from 33 to 79%. A field study was developed to identify physiological components of this genetic improvement in eight widely grown hybrids in Ontario from 1959 to 1988 ((1) Dekalb 29, (2) Pride 5, (3) Warwick 263, (4) United-H-106, (5) PAG SX111, (6) Pioneer 3978, (7) Pioneer 3851, (8) Pioneer 3902). A part of this study investigated differences in photosynthetic response to irradiance (PRI) and in crop growth rates and machine-harvestable grain yields of the eight hybrids at three plant densities. The occurrence of an unseasonally cool night during the measurement period also indicated hybrid differences in the effect of cool temperatures on photosynthetic response. Hybrid differences in PRI resulted from stress, either in the form of low night temperatures or high plant densities. When stressed, PRI was reduced more in the oldest hybrids than in the most recent. Large increases in machine-harvestable yield and rate of dry matter accumulation between the oldest and most recent hybrids were paralleled by slight increases in PRI among hybrids when unstressed, i.e. low population, seasonal temperatures, but the hybrid differences in response were generally not significant (P > 0.10). The importance of increased PRI, particularly when associated with stress, is discussed in terms of Canadian production.Key words: Stress, low temperature, plant density, crop growth rate
Optimizing nitrogen rate and planting density for sunflower under irrigated conditions of Punjab