Abstract

Media Summary

Key words

Maize, high temperature stress, water deficit, anthesis and grain set

Introduction

In dry land regions of the north-eastern Australia, water stress and high temperature are regarded as severe constraints to maize production even under conditions where the soil profile is fully recharged at the beginning of the growing season. The floral structure of maize, notably the separation of male and female floral organs and the near-synchronous development of florets on a single ear, borne on a single stem, is extremely sensitive to moisture and temperature stress during anthesis (Johnson and Herrero, 1981; Thompson, 1975). However, Albrecht and Carberry (1993) reported that non-lethal water deficit prior to anthesis did not significantly affect the grain yield for north Australian conditions. At temperatures above 38^oC, poor seed set in maize has been attributed to both a direct effect of high temperature (Johnson and Herrero, 1981, Carberry et. al., 1989) and pollen desiccation (Schoper et al. 1986; Lonnquist and Jugenheimer 1943). Muchow (1990) found that during five growing seasons in north Australia, yield was unaffected by temperature, which ranged from 25.4 to 31.6 ^oC during the period from pollination to 80% maximum grain size.

The expansion of maize production in Australia is challenged with the issue of how extreme environmental conditions will impact on maize growth and yield. Stabilization of maize yield in marginal dry land regions is an exercise in risk management. Consequently, a field study was undertaken in south-eastern Queensland with the objective of exploring the response of maize to water deficit under stored water conditions. Different sowing dates were used to obtain different levels of water deficit and incident temperatures during crop growth.

Materials and Methods

The experiment was conducted at the University of Queensland, CSIRO field facility at Gatton (Lawes) (27° 33’S, 152° 20’E, 89m elevation), on a deep Vertisol of moderate fertility. A split-split-plot experimental design with three replications was used. The treatments were maize (Pioneer- C87) and the sowing dates (24^th October and 20^th December 2001) and row spacing (solid and double skip configuration where alternatively two rows are planted and two rows are not planted). The experimental plots were irrigated before sowing with a total of 150 mm applied on September 11^th, 17^th and 24^th and rainfall of 36 mm received before planting. A row spacing of 1.0 m was used in the solid planted (SP) configuration and double skip configuration with the population density of 6 and 12 plants m^-2. The plot size was 16m x 20m. The maize was harvested regularly for partitioning into grain and dry matter from 4 m^-2 and 8 m^-2 areas for solid and double skip treatments respectively. Observations of tasselling, silking and physiological maturity were made on experimental treatments. The grain was oven-dried then weighed. Yield components, kernel number, 100-grain weight, and harvest index (HI) were measured at maturity.

Meteorological data were collected adjacent to the experimental site using an automatic weather station. Soil water content was measured using gravimetric water content of soil cores to 0.10 m and a neutron probe (CPN, Model 503 DR) to 1.9 m for greater depths - one tube was sited at the mid-interrow position in solid row configuration and three access tubes were installed in double skip row configuration plots with one tube on the midrow and two tubes at 50 cm intervals to the centre of the skip configuration. The readings were made using 16-second count at 20 cm depth intervals for layers 10-30, 30-50, 50-70, 70-90, 90-120,120-150,150-170,170-190 cm every 10 days during the season. Plant available water content (PAWC) at periods throughout crop growth was calculated as the percentage of water remaining above the crop lower limit. PAWC was averaged across the three sampling points for the double skip configuration.

Results and Discussion

Temperature

Air temperatures recorded during the crop growth period for the two sowing dates are presented in Figures 1a and 2a. The summer was very warm, with the October sown maize experiencing high temperatures during anthesis (maximum daily temperature of 41.0^o C). The hourly cumulative degrees above 38^o C during this period was 16.7º.C hours (Fig. 1b). The December sown maize crop experienced a 3.0^o C.hours period of temperature above 38^o C during grain filling, although after the completion of pollination and anther development.

Plant available water percentage

Plant available water percentage for the October sowing is presented in Figure 1c. The initial PAWC at sowing was 60% of a full profile and increased during early vegetative growth to more than 80% PAWC. Around anthesis, the solid-planted crop had extracted soil water down to 25% PAWC while maize planted to double skip configuration had 37% PAWC remaining. Soil water depletion during the grain filling period was severe with the crop experiencing terminal drought. Similarly, the December sown crop had an initial plant available water content of 170 mm, which was 80% PAWC and extraction reduced PAWC at anthesis to 36 and 35% for the solid and skip configurations respectively (Figure 2c). Total soil water extraction occurred early in grain filling and late rain appeared to have not been utilised by the crop.

Total biomass and grain yield

The growth data presented in Figures 1d and 2d showed linear growth rates up until anthesis but limited growth thereafter due to severe water deficit in both sowing treatments. The two sowing dates experienced similar levels of water deficit and consequently produced close to the same total biomass at maturity for the two row configurations (Table 1). In both sowings, the solid configuration produced more biomass than the double skip configuration.

The key result from this study was the significantly lower grain yield and associated grain numbers and HI achieved in the October sowing compared to the later December sowing date (Table 1). The maize crop sown in October experienced extremely high air temperatures over several days at the time of anthesis (silking) whereas the maize sown in December escaped such high temperatures at anthesis. While soil water in both sowings and configurations was relatively low at anthesis (25-35% PAWC), both soil water and growth rates prior to anthesis (the period where grain number is determined in maize) suggested that severe water deficit occurred during grain filling and after grain number had been determined. These data indicate a contribution of high air temperatures to reduced grain numbers and grain yields of maize planted in the October treatment.

Table 1. Grain yield, total biomass, kernel number, seed weight, and harvest index for the treatments during summer 2001

Figure1. (a) Temperature and rainfall (b) days above 38^oC (c) total biomass and grain weight g/m² for October sown maize at Gatton during 2001 summer.

Figure 2. (a) Temperature and rainfall (b) days above 38^oC (c) total biomass and grain weight g/m² for December sown maize at Gatton during 2001 summer.

Conclusion

This paper shows that two sowings of maize experienced similar growth conditions, commencing with good starting soil moistures and ending in terminal drought around anthesis. Lower grain yields, grain numbers and HI were recorded in the sowing treatment where extreme air temperatures (>38^oC) coincided with anthesis. High temperatures probably affect pollen viability directly, since the crop showed no visible symptoms of water stress (Herrero and Johnson 1980; Schoper et al 1987). This result supports the findings of Johnson and Herrero, (1981), Schoper et al. (1987a,b), Carberry et al. (1989) and Jones et al (2000), that high air temperatures can affect grain set in maize. These findings highlight the importance of the kernel set process in determining variability in grain yield in areas where spikes of high temperature can occur with unpredictable timing during the growing season. Clearly, the high air temperature effects can also be compounded by water stress occurring at the same time to further decrease kernel set under dry land environments.

Acknowledgements

This study was conducted by the senior author as part of his Ph.D., research under the sponsorship of ACIAR’s John Allwright Fellowship. Special thanks must go to Dr. R.A. Fischer, Country Manager of the ACIAR project and Dr.Brian Keating of the CSIRO Sustainable Ecosystems. We also acknowledge the advice of Allan Lisle, Statistical Advisor and Neil Huth of the CSIRO Sustainable Ecosystems.

References

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