A study of grain yield and nitrogen content in different shoots of wheat

Three field experiments were conducted on different high fertility soils during the years 1979-80 and 1980-81 to investigate factors controlling grain yield and nitrogen content in wheat (Triticum aestivum L.). Autumn-sown Kopara, a standard New Zealand tall cultivar, and Rongotea and Oroua, newly released semidwarf cultivars, were used in the first experiment, and Kopara and Rongotea in the second and third experiments. In the first experiment, nitrogen was applied at the tillering stage (0, 80 kg N per ha), after the double ridge stage (0, 40 kg N per ha), and around ear emergence (0, 20 kg N per ha) and in the second and third experiments, application of nitrogen (0, 50, 100 kg N per ha) was restricted to the tillering stage only. In addition, two plant populations, 250 and 380 plants per m², were established. Irrigation was introduced, as a treatment, only in the second experiment. The grain yield of Rongotea was greater than that of Kopara only in one of the three experiments. This was associated with the production of more ears per plant and development of heavier grains. In the other experiments, however, grain yield of Rongotea was similar to Kopara, with failure to produce either more ears per plant or heavier grains. The yield of Rongotea was also higher than that of Oroua, due mainly to the production of heavier grains. Cultivars varied in tiller production and in final ear number per plant. The differences in tiller production among these cultivars did not appear to be associated with the levels of nitrogen (N %) or DM accumulation per plant at the tillering stage. It is suggested that other mechanisms affecting the distribution of assimilate are involved. The data on individual shoots indicated that grain set in Rongotea was consistently lower than in the other two cultivars and that this was caused mainly by fewer grains per spikelet in the central region of the ear. This characteristic was not related to any detectable difference in the ear weight at anthesis among these cultivars. Application of nitrogen, generally, had little effect on overall grain yield and its effect on yield components was small, variable and often compensating. Irrigation increased grain yield per plant by 51%. This yield improvement resulted from increased survival of tiller and also from high grain set. The effect of irrigation, however, varied with plant population, being relatively greater at low plant population than at high. High plant population increased the ear number per unit area but reduced the grain number per ear so that there was no significant difference in grain yield between the two populations studied. The reduction in grain number per ear at high plant population was mainly associated with a reduction in the DM accumulation per shoot. The partitioning of DM between grain and non-grain, represented by the Harvest Index (HI), was not affected by a change in plant population. Investigation of individual shoots showed that the mainstem (MS) had highest grain weight per ear largely because of the greater number of grains per ear. The difference between first tillers (T1) and the rest of the tillers (TR) was negligible. The MS also accumulated greater amounts of DM than tillers at all stages of development. The differences between cultivars in nitrogen accumulation in the shoots were related to variable DM accumulation whereas the differences due to nitrogen additions were related to variable nitrogen concentration (r = 0.99*). The grain-nitrogen percentages in the various shoot categories (MS, TI, TR) were similar. There was no consistency in the differences between cultivars from experiment to experiment. Kopara had a higher grain-nitrogen percentage than Rongotea and Oroua in the first experiment. In the second experiment, however, Rongotea was higher than Kopara whilst in the third experiment, they were similar. These differences were largely associated with the variation in Harvest Index (HI) or in the levels of nitrogen per unit of biomass accumulated. Application of nitrogen, irrespective of timing, failed to produce any variation in grain-nitrogen percentage in the first experiment, but in the second and third experiments, the response was quadratic and linear respectively. Grain nitrogen percentage was linearly related to nitrogen-percentage of the shoot at maturity in both the experiments (r = 0.99*). Seeding rate did not affect the grain-nitrogen percentage but irrigation depressed it in all the shoots. The depressing effect of irrigation on grain-nitrogen percentage was linked with a greater DM production relative to nitrogen accumulation in the shoot and either an increase in the Harvest Index (HI) or a decrease in the Nitrogen Harvest Index (NHI) caused by irrigation. These results are discussed and interpreted in relation to DM and N accumulation and partitioning in different parts of the plant.