[Is it possible to increase kernel number per unit area?] | Es posible incrementar el número de granos por unidad de superficie?

The formation of the number of grains per m2 in 15 semi-dwarf primarily spring bread wheat varieties, obtained in Argentina since 1984, is analyzed using different strategies. The data came from experiments conducted without limitations of water or nutrients at Escobar (34º and 40' S, 58º 39'W, altitude 22 m) and Balcarce (37º 45' S, 58º 18' W, altitude 130 m), in the Buenos Aires province. The number of grains per m2 is seen as a result of: (a) the duration of the spike growth period (b) the rate of the crop growth during this period (c) the partitioning of the dry weight of the spikes during this period and (d) the number of grains produced per gram of spike (i.e. the factor of the spike fertility). The spike growth period was defined as the interval in which the spikes accumulated from 5 to 100% of the dry weight that was achieved seven days after the anthesis (excluding the weight of the grains). For this set of data, the yield (range: 5.3 to 9.1 t/ha, at 14% humidity) related best with the number of grains per m2 than with the grain weight. The duration of the spike growth period varied between 24 and 30 days (average 27 days) between cultivars and was weakly related with the length of the crop cycle. All varieties were able to intercept a major proprtion (85%) of the PAR (photosynthetically active radiation) available during the spike growth period. The coefficient of extinction of the PAR, measured in two experiments, did not show any clear differences among the cultivars. There were only some significant differences among the cultivars with respect to their efficiency in the use of radiation (mean 2.6MJ m2 d-1) and in the rate of crop growth (mean: 26 g m2 d-1) during the spike growth period. The differences in the partitioning of the spikes (range: 26 to 39%) were more frequent. The dry weight of the spikes m2, a week after the anthesis, associated more with the partitioning of the spikes than with the duration of their growth period or with the rate of the crop growth. These results coincide with the majority of research findings that have analyzed the effect of the dwarfing genes. However, as the majority of the cultivars studied here were semi-dwarf, neither the dry weight of the spikes or any other of its components can explain the variations in the number of grains per m2. On the contrary, clear differences in the fertility factor of the spikes among the cultivars (range 52 to 114 grains g-1 of spike) were found and the number of grains per m2 was strongly associated with this factor. The introduction of dwarfing genes has allowed increasing the number of grains per unit area via better partitioning of the spike. As a result, the number of grains is expected to increase with the increase in the fertility factor of the spike or eventually the duration of the spike growth period or the rate of crop growth during this stage.