Effects of soil water supply and temperature on the photosynthesis of white clover and paspalum in irrigated pastures

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Abstract
Field canopy chambers were used to measured the effect of a range of soil water contents from surface ponding to very dry soil and the effect of high summer temperatures on the photosynthesis of irrigated white clover and paspalum pastures. Water was ponded for 5-12 days on swards of white clover and paspalum at full cover and on others that had been defoliated to a height of 50 mm. Photosynthetic rate was monitored each day and compared with a non-ponded control. In all cases there was little response in photosynthetic rate to ponding either with or without supplemental nitrogen fertiliser. Photosynthesis of full swards of white clover and paspalum was monitored during a drying cycle following irrigation and compared with the photosynthesis of a well-watered control. Soil water deficit was expressed in terms of cumulative evaporation minus rainfall (mm E-R) after irrigation. Water deficit stress reduced the maximum photosynthetic rate of white clover by 50% (from 0.8 mg CO2/m2 .cntdot. s at 25 mm E-R to 0.4 mg CO2/m2 .cntdot. s at 75 mm E-R), but the photosynthetic rate of paspalum did not decline until 70 mm E-R. At high irradiance, temperatures between 24 and 33.degree. C had little effect on the photosynthetic rate of well-watered white clover, whereas the rate in paspalum was higher at temperatures between 29 and 38.degree. C than at temperatures of 24-29.degree. C. The slow surface drainage and subsequent rapid drying of the root-zone of flood irrigated soils, combined with the high temperatures experienced in northern Victoria during summer, favour paspalum and severely limit the productivity of white clover.
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