Leaf emergence of spring wheat receiving varying nitrogen supply at different stages of development.

We examined effects of nitrogen (N) supply on leaf emergence of spring wheat (Triticum aestivum L.) grown in sand with nutrient solution containing different N concentrations (9NO3: 1NH4). In expt 1, the cultivar 'Gamenya' received nutrient solution twice weekly containing a constant N supply ranging from 50 to 2400 microM N. In expts 2 and 3, cultivars 'Aroona' and 'Gamenya' were irrigated hourly with nutrient solution containing either low (L = 50 micrroM N) or high (H = 2000 microM N) N supply. In expt 2, the N supply to half of the plants receiving L and H was changed at the double ridge stage of apical development, producing plants receiving LL, LH, HL and HH. In expt 3, N supply was changed firstly when the main stem apex was vegetative (one to two leaves) and secondly when the main stem apex was at double ridge stage (four to five leaves), producing plants receiving LLL, LHL, HLH and HHH. Leaves on the main stem and primary tillers were counted. Rate of leaf emergence was estimated from regression of number of leaves against thermal time; the phyllochron was calculated as 1/ rate of emergence. Severely N-deficient plants (which had at least a 60 % reduction in shoot dry weight) had slower rates of leaf emergence (expt 1). Fluctuating N supply sometimes, but not always, changed the rate of leaf emergence (expts 2 and 3). The N supply before double ridge stage had bigger effects on the phyllochron than that afterwards (expt 3). The phyllocrons of the main stems were generally lower than those of tillers, with a greater difference between stems in N-deficient plants. Low N supply at the vegetative apex stage decreased the total number of leaves on the main stem, while low N supply after double ridge did not.