Analysis and modelling of the effects of nitrogen on the growth, partitioning and quality of malting barley

Abstract

In this project we have studied the interaction of nitrogen uptake and carbon assimilation in relation to its effects on partitioning, development and variations in yield and characteristics of grain quality. Experiments were carried out using controlled nutrient environments. These experiments have been compared with specially grown field trials to ensure general validity. If a crop is grown without water stress or diseases it is possible to measure the parameters of growth and manipulate growth without complicating factors. Soil is a very complex system which is still not well understood. It is possible to detect, particularly in breeders' small plot trials - but also in farmers' fields where precise measurements are made, large variation in crop growth and yield over very short distances.

The initial phase of work in the project consisted of growing field trials and experimenting with possible controlled nutrient systems. After a year of experimentation, a perlite bed system, protected by a polythene tunnel and with trickle irrigation, was adopted (PTX). A spring barley variety (cultivar) by nitrogen trial (VNT) indicated that while the lowest level of top dressing resulted in lower yield it also resulted in the harvest of more nitrogen than applied to the crop. The impact of restricted nitrogen uptake on nitrogen productivity was explored in the PTX. Sieving fractions of the grain from the VNT led to an hypothesis that nitrogen content of grain was dependent on grain position of the ear and ear position in the developmental hierarchy. This hypothesis was tested and elaborated with novel effects found on germination rate in the PTX. At the whole plant level differences between cultivars were found in the magnitude of changes of nitrogen concentration of the grain (or ear) in response to reductions in nitrogen supply. Within the plant, nitrogen concentration of grains was found to change with position on the ear, highest concentrations being found near the top of the ear. This effect was most pronounced in the main-stem, present in tiller T1 and not detectable in T3. Grain size, as in previous studies, varied with stem type, grain position and nitrogen uptake. Greatest variation was experienced in later tillers that experience most competition for resources. In contrast, grain borne on main-stem ears showed little variation in response to nitrogen uptake. Grain size is known to affect germination rate. However, an equally important and previously unknown effect is the variation in germination rate with grain position on the ear. This was present in all stem types and nitrogen treatments. A systematic increase in the time to germinate as one moves from the base of the ear to the top was found in all ears. The scale of this change differed between the two cultivars tested.

The development and growth of crops in the PTX was carefully followed. The major effect of nitrogen was on total dry matter production which was manifest in the total amount of dry weight present in the tillers. Once inside either tiller or main-stem the proportions partitioned to tissue types (leaves, stems and ears) was unaffected by nitrogen. Tiller development lagged that on the main-stem (partitioning to the ears was later and nitrogen concentration in leaf and stem tissues on tillers greater). Effects on partitioning of nitrogen were similar. The main contrast being the much greater proportions of nitrogen residing in the leaves. Hence, during grain growth the leaves were the main tissue source for nitrogen and stems the main source for carbon. These data allowed the development of a mechanistic model for nitrogen limited growth in barley. Attempts were made to base the interpretation of nitrogen uptake and carbon assimilation interactions on the use of existing simulation models of cereal growth and one under development specifically for barley. The CERES model for wheat, produced in the USA under the IBSNAT programme, was first explored in a parallel project. It was found to be unsatisfactory for our purposes. We then developed a simulation model based on two other models. Sensitivities to empirical functions of partitioning and critical nitrogen concentrations at the whole plant level convinced us that an entirely new approach was required based on mechanisms operating at the cellular level. The mechanism chosen was that nitrogen concentration of the photosynthetic system is proportional to the light intensity incident on the tissue. The nitrogen content of the non-reproductive biomass (leaf plus stem tissue) was shown to be the main driving force for carbon assimilation. Early uptake of nitrogen is critical for attaining high growth rates. The history of nitrogen uptake determines the nitrogen productivity of a crop.