Many abnormal embryosacs (43% of the total observed) were recorded in ears from corn plants that had been drought stressed twice over a period of 10 days during embryosac formation (treatment T3). Also, hermaphrodite spikelets were commonly observed. Yery few grains were produced by these plants. In ears of plants drought stressed once over a period of five days during embryosac formation (treatment T2), 15% of the observed embryosacs were abnorraal compared with only 2.5% in ears of the unstressed control plants. The grain yield of T2 plants was not significantly reduced. Both T2 and T3 treatments greatly delayed silking; in the former there was a lag of 13 days between anthesis and silk emergence, and 16 days in T3 as compared to 3 days in the controls. Plants were hand‐pollinated to prevent sterility caused by the lack of pollen during silking, so that the large reduction in grain yield by treatment T3 was due mostly to embryosac abortion. The surmise that femaleness of corn plants was reduced by drought stress because of a reduction in auxin levels was discussed.
Water use by maize at three densities was measured by a complete water balance, including soil moisture, rain, applied irrigation and deep drainage in crops irrigated so that relative turgidity at noon remained above 90 per cent (i.e. no-stress conditions). Total evapotranspiration between planting and harvest, 56 cm., was not significantly affected by density, but evapotranspiration was slightly higher at higher densities when ground cover was less than 50 per cent. Actual evapotranspiration rose to 1 cm. per day during anthesis and grain filling. A fifth of the water applied moved beyond the root zone, indicating that work on more permeable soils would be inaccurate if the deep drainage component was ignored.
An experiment was designed to study the effect of gypsum and drought stress on growth, water status and yield with maize (Zea mays L.). Water status was assessed under field conditions by measurements of relative turgidity (RT) at noon. No stress was defined as RT greater than 90% — drought stress as RT less than 90%. A nostress crop was compared with a crop drought stressed for a period of 20 days during male meiosis and with a crop drought stressed during grain filling (after pollination to maturity). These treatments were set out in a randomised block design with a further comparison between the untreated sodic clay soil (control) and the same soil treated with gypsum. Gypsum and no‐stress conditions stimulated tiller production so that the final effective plant densities were above optimum. The no‐stress treatments produced large amounts of dry matter but this was channelled into sterile tillers rather than into grain. Drought stress during male meiosis stunted growth, stunted tassel development, reduced chlorophyll content and light absorption (during the period of stress) but, overall, was not very detrimental to grain yield. Drought stress during grain filling reduced grain yield by 50%; grain weight was reduced and a very high proportion of photosynthate went into sterile tillers.
Summary Two rapid methods of assessing relative turgidity in maize are described. The first involved measuring the water uptake by discs of leaf tissue but omits the oven dry weight from the calculation. Where the position on the leaf was standardized, the relationship was linear (r = 0.99***) over a wider range than normally encountered in field conditions. The regression relationship held for plants grown under a wide variety of environmental conditions and at various stages of development. The second method utilized a series of photographic standards of plants at different relative turgidities. Relative turgidity could then be estimated visually by comparing the plant and the photographic standards. Accuracy with either method was similar to the experimental error of relative turgidity measured by normal means. Both offer a considerable saving in time, especially the photographic standard method which allows instantaneous readings to be made. This should permit field workers to assess rapidly the water status of their crop without the labour involved in conventional measurements.
Results from an experiment with maize (variety NEH 1151) at three plant densities indicated that normally favourable environmental conditions promoted tillering and it was difficult to maintain the desired tiller densities. The higher densities produced more vegetative dry matter but less grain than the lower densities. In a second experiment, where tillers were removed from the plants, there was no advantage because the remaining tillers grew larger to compensate for the removal. It is concluded that it is desirable to breed or select for non-tillering varieties where other management parameters can be controlled.
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