Nitrogen supply at various stages of development in wheat : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science in the University of Canterbury [Lincoln College]

The importance of nitrogen in plant nutrition has long been recognised. It is an indispensable constituent of amino acids, proteins, and nucleic acids, all of which are essential in plant growth and development. In wheat production, nitrogen has two specific roles to play: its effect on grain yield, and its influence on grain protein content, upon whose quantity and quality the baking properties of flour principally depend. Thus for its use to be economically feasible, nitrogen fertiliser must provide increased return, either in grain yield per acre or in grain protein content (in countries where premiums are payable for it). Most investigations carried out so far have indicated that it may do both, which justifies the interest in nitrogen fertilization among wheat growers (NeNeal and Davis, 1954). In New Zealand, owing to the very high cost of nitrogen fertilizers and the small amount of arable cropping in relation to the areas under improved grass-clover pastures, little if any nitrogen is supplied to wheat (Walker, 1956). Thus it is not surprising that very little work has been done here on the problem of nitrogen top-dressing of wheat. Although it is generally accepted that nitrogen fertilization has a definite place in wheat production, the question of when to apply nitrogen to the crop is still very controversial. Most of the investigations during the last 50 years or so have been conducted in the field where the responses to nitrogen have been vitiated by an interplay of other environmental factors, both climatic and edaphic. It is commonly realise that field experiments are not likely to determine an optimum time for nitrogen application until we are able first to define much more accurately than at present the environmental factors prevailing during crop growth. Another factor contributing towards the inconsistency of results of field experiments is that the components of grain yield viz., ear number per unit area, grain number per ear and single grain weight, are highly interdependent and compensating. Furthermore, it has become increasingly obvious that they are not the determinants of yield and tell us little about the underlying physiological factors governing the performance of the crop. Thus research during the last two or three decades has gradually turned towards a more dynamic and physiological approach, looking at nitrogen fertilization and other environmental variables as factors influencing growth, flowering and grain development of cereal plant. As a result of this intensive and searching study, photosynthesis has rightly emerged as the central process determining yield, and thus increasing emphasis is being placed on the size, longevity and efficiency of the photosynthetic system. However, grain yield consists of only a particular fraction of the total or biological yield and so it does not depend on the whole photosynthetic performance of the crop. The question of the partition of assimilates between grain yield and the rest of the biological yield is very complex and to date it is still far from being fully resolved. Possibly due mainly to the time-sequence of their initiation, the compensating nature of the yield components is such that ear number tends to dominate the picture and mask the expression of the components within the ear. Thus relatively little attention has been paid specifically to the effect of nitrogen supply on the development of the ear. Thus relatively little attention has been paid specifically to the effect of nitrogen supply on the development of the ear. The results of Single (1964) have demonstrated convincingly that further work on this aspect is highly desirable. The present study was designed as a result of these considerations and with the aim of gaining some information regarding the effect of nitrogen supply on (1) the development of the wheat inflorescence in relation to its contribution to grain yield; (2) grain protein content; and (3) the underlying physiological factors that govern grain yield. The investigation was conducted in glasshouses where the environmental conditions were partially controlled. Observations were confined to the main shoot and all tillers were removed soon after their emergence. Two New Zealand varieties, Arawa and Hilgendorf ’61, were chosen for this study partly because they are grown widely in the South Island, but mainly because of the contrasting nature in size and structural organisation of their ears (Langer, 1965). It was surmised that this difference would be reflected in their reaction to nitrogen treatments.