'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser III. The influence of water deficit and heat shock
Authors: van Herwaarden A. F. , Richards R. A. , Farquhar G. D. , Angus J. F. 
Published: August 2004

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