THE EFFECTS OF RADIATION AND NITROGEN ON NUMBER OF GRAINS IN WHEAT

The possible interacting effects of shading and N supply on number of grains of Triticum aestivum L. (cv. Buck Nandu) were investigated at B alcarce, Argentina, during the 1988/89 and 1989/90 growing seasons. Sh ading was imposed from c. 13 days before anthesis to 6 days after, and four rates of N fertilization were supplied within each shading treat ment around the date of terminal spikelet formation. Water and other n utrients were not limiting. Total grain yield was strongly correlated with grain number/m(2), regardless of shading or N supply. At the high est N rates, grain number and dry weight of spikes at anthesis were li nearly related to a photothermal quotient, i.e. the ratio of intercept ed photosynthetically active radiation (PAR) to mean temperature minus 4.5 degrees C, during the period from 20 days before anthesis to 10 d ays after. The response of grain number to the photothermal quotient w as interpreted in terms of the supply of assimilates to the spike at a nthesis, which determined flower survival. The response of dry weight of spikes to photothermal quotient was interpreted in terms of crop gr owth rate since there was a linear relationship between crop growth ra te and intercepted radiation. The lowest N rates reduced the number of grains/m(2), at any given photothermal quotient. Since the reduction in grain number also occurred at any given dry weight of spikes, it ca nnot be explained by a reduced supply of assimilates to the spikes. Gr ain number responded directly to the supply of N to the spike, probabl y through the survival of differentiated flowers. The relationship bet ween spike growth rate and crop growth rate was not affected by N supp ly. Crop growth rate was reduced by reduced N supply, because less rad iation was intercepted and because radiation-use efficiency was lowere d. These results indicate that current models for determining yield an d number of grains/m(2), based on crop growth, are not adequate when N is deficient.