'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser III. The influence of water deficit and heat shock

Post-anthesis drought and heat shock have been implicated in previous studies as factors contributing to ‘haying-off’ in wheat, but their relative importance has not been investigated. To separate the effects, wheat plants were grown at 2 levels of nitrogen (N) and then exposed to different levels of post-anthesis water deficit in factorial combination with the presence or absence of heat shock. The growth, yield, leaf carbon exchange, water use, and the contents of protein and soluble carbohydrate were measured and compared with the field results reported in Papers I and II of this series. The experiment consisted of wheat plants (cv. Janz) grown in 1·2-m-long tubes outdoors through winter and spring in Canberra, with either nil or 240 kg N/ha applied. The tubes were supported in a refrigerated box to maintain temperatures representative of those of soil in the field, and arranged to form mini-canopies with a density of 29 plants/m2. After anthesis, half of the plants at both levels of N were watered according to their transpiration demand and the other half at 75% of demand to reduce gradually the store of soil water so that water deficit could be initiated at the same time as heat shock. Fifteen days after anthesis, different temperatures were imposed by moving half of the plants into an adjacent glasshouse where heat shock was imposed by raising the air temperature to maxima of ~35ºC for 3 days, to simulate the pattern of temperatures experienced in the field during a heat wave. During this time, the control plants experienced daily maxima of ~25ºC. Following the heat shock, all plants were placed outside and rewatered to enable the assessment of treatment effects on potential leaf function. Both water deficit and high temperature reduced assimilation. After these measurements were taken, well-watered control plants were irrigated according to transpiration demand and the plants with imposed water deficit were watered at 50% of this amount. Yields increased in response to N at both levels of water status and both levels of temperature. That is, there was no evidence of the haying-off reported in Papers I and II of this series. Two factors are proposed to account for the difference between the field crops and the plants grown in the mini-canopy here. Firstly, the pattern of soil-water use differed from the field studies reported in Paper I, with the high-N plants using more soil water than low-N plants during grain filling. Secondly, the level of water-soluble carbohydrates (WSC) in the tube-grown plants of high-N status was greater than that for plants of low-N status, which was opposite to the pattern for field-grown plants reported in Paper II. In addition, the concentrations of WSC in the tube-grown plants were higher than those in the field-grown plants, apparently because lower spike density allowed better penetration of light into the mini-canopies and led to greater assimilate storage than by the denser field crops. The results confirm the conclusion of Paper I that high temperature is not necessary for haying-off, although it is likely that it would worsen the haying-off caused by post-anthesis drought and low WSC reserves in the field. The absence of the haying-off response in this experiment was mostly because the supply of WSC from the sparse canopy was adequate to ofiset the reduction of assimilation due to water deficit and heat shock. A contributing factor to haying-off in the field may therefore be dense canopies resulting in low levels of WSC