GRAIN-YIELD IN WHEAT - EFFECTS OF RADIATION DURING SPIKE GROWTH PERIOD

The effect of radiation on yield formation in irrigated and fertilized spring wheat was quantified, focusing on the crop growth and partitio ning during the spike growth period, critical for the determination of number of grains. Most of the data were obtained from five experiment s at Balcarce, Argentina, with cv. PROINTA Oasis. Additional data were obtained from an experiment in Sonora, Mexico. A wide range of incide nt radiation during spike growth was achieved, both by natural variati on among experiments and by applying shading nets. Spike growth period was defined as the interval during which spikes achieved from 5% to 1 00% of the dry weight they accumulated by day 7 after anthesis excludi ng grain weight. Radiation level during this period affected yield mai nly through variation in number of grains m(-2). Differences in grains m(-2) were related to differences in dry weight of spikes m(-2) measu red 7 days after anthesis excluding grain weight. Above 106 g m(-2) of spikes, the relationship between grains m(-2) and dry weight of spike s m(-2) was approximately linear with positive slope and intercept. Th e slope obtained at Balcarce (58 grains g(-1) spike) was not different from those obtained in Sonora (58-62). For low weight of spikes (< 10 6 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 radiati on does not seem to operate through a change in spike rachis proportio n. Nevertheless, variation in radiation level caused greater variation in dry weight of spikes m(-2) than in number of grains g(-1) spike. F or 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 positive ly related although low radiation increased mean and maximum partition to spikes. Crop growth rates, during the spike growth period, was lin early 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 ind ex to crop dry weight) but it slightly affected green area index and e xtinction coefficient. Therefore, the percentage of intercepted PAR wa s not affected by radiation level. Intercepted radiation was the main factor determining both crop and spikes growth during spike growth per iod, and grain number m(-2) was linearly related to accumulated interc epted PAR during this period. (C) 1997 Elsevier Science B.V.