The responses of sweet corn biomass and yield to timing and severity of water deficit were determined in an experiment using a mobile rainshelter. Six irrigation treatments were applied such that plots experienced: (1) no water deficit; (2) full water deficit; (3) moderate pre-silking deficit; (4) severe pre-silking deficit; (5) moderate post-silking deficit; or (6) severe post-silking deficit. Drought was quantified using the concept of potential soil moisture deficit, which was calculated from climatic data. Potential soil moisture deficit can be related simply to a wide range of plant performance variables, making it possible to compare the relative importance of variables in determining the overall response of the crop to drought. For all treatments, yield was related strongly to biomass, especially that accumulated after silking. Biomass, in turn, was reduced by water deficit, mainly because of reduced radiation use efficiency, but also because of reduced total radiation interception, particularly in early deficit treatments. Both water use efficiency and transpiration efficiency increased with water deficit, even though soil evaporation as a proportion of total water use also increased with deficit. There was no stage of crop development at which yield was particularly sensitive to water deficit, although yield components changed with timing of deficit. Importantly, measures of potential soil moisture deficit integrated the effects of timing and severity of drought, making it possible to simply and mechanistically account for the effects of water deficit on biomass and particularly yield.
The potential for nitrate (NO3–) to leach is enhanced following cultivation of pastoral land, due to rapid mineralization of labile soil organic matter (SOM). In a 7‐yr field trial in New Zealand, we examined the impacts of tillage intensity [intensive (plowing to 20 cm), minimum, or no‐tillage] and winter cover crops (forage rape) on NO3– leaching following cultivation of permanent (sheep [Ovis aries]‐grazed) pasture to grow arable crops (the rotation included barley [Hordeum vulgare L.], wheat [Triticum aestivum L.], and pea [Pisum sativum L.]). Permanent pasture and permanent fallow (maintained plant‐free using herbicides, i.e., not cultivated or fertilized) treatments were included as controls. Losses of NO3––N were calculated from soil solution NO3––N concentrations (measured in ceramic suction cups installed at 600 mm) and drainage volumes. Cumulative NO3––N leached over 7 yr ranged from 20 to 428 kg N ha– 1, with least N lost under pasture. Residual soil mineral N in autumn accounted for ∼30% of the variability in leaching. Nitrate leaching under arable crops generally increased rapidly as winter rainfall (range 78–352 mm yr– 1) increased. Winter cover crops were effective in reducing NO3––N leaching losses, particularly in drier winters when about 50% less N leached where cover crops were grown. On average, annual leaching was only 10 to 18 kg N ha– 1 in the presence of cover crops. Tillage had relatively little influence on leaching, though use of minimum tillage for autumn cultivation resulted in significantly (P < 0.001) less NO3––N leaching than either intensive or no tillage. Largest leaching losses were recorded in the unfertilized, permanent fallow where there was no plant sink for NO3–N derived from SOM mineralization. Growing a crop during the winter period, combined with good N management practices to minimize pre‐winter soil mineral N, provides the best option to keep NO3– leaching within the acceptable range for arable cropping in New Zealand.
The development of mucilage in the epidermal cells of canola seeds (Brassica campestris L. cv. Candle) was studied with light and electron microscopy from 5 days after pollination to maturity. During the first 17 days starch was deposited in amyloplasts. At or near the 17th day mucilage appeared between the plasmalemma and the outer tangential wall of the epidermal cells. As the volume of mucilage increased, starch grains disappeared and were totally absent by 25 days. Membrane-bound structures and Golgi bodies were visible within the cytoplasm adjacent to the site of mucilage deposition. At maturity the seed epidermal cells were totally devoid of cytoplasm and engorged with mucilage.
The response of sweet corn canopy development to timing and severity of water deficit was determined in an experiment using a mobile rainshelter. Six irrigation treatments were applied such that plots experienced: (1) no water deficit; (2) full water deficit; (3) moderate pre-silking deficit; (4) severe pre-silking deficit; (5) moderate post-silking deficit; or (6) severe post-silking deficit. Soil moisture content profiles were measured using neutron moisture probes and water deficit was quantified using the concept of ‘potential soil moisture deficit’ (Dp), which was calculated from climatic data. Water deficit reduced leaf area, and the effect varied depending on its timing and severity. Early water deficit reduced the rate of leaf expansion (but not duration of expansion) and, consequently, the maximum area of individual leaves; it therefore reduced the maximum leaf area index (LAI). Moderate post-silking water deficit did not significantly reduce maximum LAI, but it hastened leaf senescence. Severe water deficit late in crop growth reduced canopy development by decreasing maximum LAI and advancing leaf senescence. A model was developed which accounted for these effects. Two measures of Dp were defined which related to both short-term transient responses and those integrated over longer times. The model made it possible to simply and mechanistically describe the effects of water deficit on canopy development.
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