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The effect of radiation on yield formation in irrigated and fertilized spring wheat was quantified, focusing on the crop growth and partitioning during the spike growth period, critical for the determination of number of grains. Most of the data were obtained from five experiments at Balcarce, Argentina, with cv. PROINTA Oasis. Additional data were obtained from an experiment in Sonora, Mexico. A wide range of incident radiation during spike growth was achieved, both by natural variation among experiments and by applying shading nets. Spike growth period was defined as the interval during which spikes achieved from 5% to 100% of the dry weight they accumulated by day 7 after anthesis excluding grain weight. Radiation level during this period affected yield mainly through variation in number of grains m(-2). Differences in grains m(-2) were related to differences in dry weight of spikes m(-2) measured 7 days after anthesis excluding grain weight. Above 106 g m(-2) of spikes, the relationship between grains m(-2) and dry weight of spikes m(-2) was approximately linear with positive slope and intercept. The slope obtained at Balcarce (58 grains g(-1) spike) was not different from those obtained in Sonora (58-62). For low weight of spikes (< 106 g m(-2)), the slope increased and the intercept decreased. Thus, the relationship was not linear and the number of grains g(-1) spike was not constant, reaching its maximum value at 106 g m(-2) of spikes. The cause of the variation in number of grains g(-1) spike due to radiation does not seem to operate through a change in spike rachis proportion. Nevertheless, variation in radiation level caused greater variation in dry weight of spikes m(-2) than in number of grains g(-1) spike. For Oasis at Balcarce, the duration of the spike growth period (27 days) was steady between shading levels and experiments. Thus, the smaller dry weight of spikes m(-2) induced by low radiation was due mainly to the lower spike growth rate. Crop and spike growth rate were positively related although low radiation increased mean and maximum partition to spikes. Crop growth rates, during the spike growth period, was linearly related to intercepted PAR (photosynthetically active radiation), and radiation-use efficiency was little affected by radiation level. Shading increased green area ratio (i.e., the ratio of green area index to crop dry weight) but it slightly affected green area index and extinction coefficient. Therefore, the percentage of intercepted PAR was not affected by radiation level. Intercepted radiation was the main factor determining both crop and spikes growth during spike growth period, and grain number m(-2) was linearly related to accumulated intercepted PAR during this period. (C) 1997 Elsevier Science B.V.