Nitrogen and the leaf growth of temperate cereals

Under agricultural conditions where soil moisture is adequate, low nitrogen (N) availability is usually the main soil factor limiting the growth and yield of temperate cereals. A major part of the positive response of plant growth to additional N is a result of greater leaf area, an important determinant of plant photosynthetic capacity. This thesis investigated various aspects of the influence of N on the leaf growth of temperate cereals. Data were presented in Chapter 2 which investigated the influence of additional N as nitrate (NO₃⁻) or ammonium (NH₄⁺) on reserve mobilisation and seedling growth prior to emergence from the substrate. The amount of N assimilated was similar with either form of N, but as a result of enhanced endosperm mobilization, seedling dry weight (d.wt) was greater with NO₃⁻. When seedlings were supplied chloride, reserve mobilisation and seedling growth were as great as with NO₃⁻. It was concluded that the increased rate of mobilisation of seed reserves and subsequently greater seedling growth with additional NO₃⁻ were due to greater seedling water uptake, probably acting via increased seed water contend A similar mechanism, but acting directly via the seed, was suggested for enhanced reserve mobilisation with increased levels of endogenous seed N. Chapter 3 investigated the influence of N form and availability on the growth of individual main stem and tiller leaves. With increasing external N concentrations over the range 0 to 2.5 - 5 mol m⁻³ leaf growth characteristics and maximum leaf area attained were similar with N supplied as NO₃⁻, NH₄⁺ or glutamine. Leaf area increased further with increasing external concentrations of NO₃⁻ or glutamine to 20 mol m⁻³ but with NH₄⁺ it usually declined substantially. As leaf growth was similar with NO₃⁻ or glutamine over a wide range or external N concentration, it was suggested that the site of N assimilation is probably not a major factor in determining the extent of individual leaf area development. However, it is possible that factors associated with NH₄⁺toxicity influence the growth of leaves. It was demonstrated in Chapter 4 that greater individual leaf area with additional NO₃⁻ was associated with an increase in both cell number and size. Increased cell division was thought to be due to increased availability of both photosynthate and N. It was proposed that greater cell size with additional NO₃⁻ was due to an increase in the availability of osmoticum, primarily sucrose. Also, it was suggested that at higher external NO₃⁻ concentrations, additional types of osmotica, such as NO₃⁻, counter ions and organic acids, are also available as a result of assimilation and storage of NO₃⁻ in the leaves. The influence of N availability and form on shoot to root d.wt ratio (S:R) and leaf d.wt as a fraction of total plant d.wt (LWR) were investigated in Chapter 5. It was shown that regardless of whether N was supplied as NO₃⁻, NH₄⁺ or glutamine, increasing external N concentration resulted in an increase in plant reduced-N content and S:R or LWR, though at any given total plant d.wt, all three parameters were greater for plants supplied NH₄⁺ or glutamine. Hence, at any given plant N content, S:R or LWR were similar, regardless of N form supplied. These results were discussed in terms of a proposed mechanism for the control of S:R by N. It was also shown that leaf area produced per unit leaf N was greater for plants supplied NO₃⁻ compared to NH₄⁺ or glutamine; this does not appear to have been reported previously. In Chapter 6 it was demonstrated that despite relatively high initial levels of soil N, fertilizer N applied at sowing had positive effects on the grain yield of all the temperate cereals investigated. The reason for the increase was similar for all species: additional N increased the fraction of available photosynthetically active radiation (PAR) intercepted by increasing the rate of canopy development. As a result, crop dry matter (DM) production increased and as harvest index (HI) was not affected, grain yield was greater with additional N. Differences between species in the amount of grain produced were not associated with the amounts of PAR intercepted or DM produced, but were related to differences in HI.