Effects of stage of growth,irrigation frequency and gypsum treatment on CO2 exchange of lucerne (Medicago sativa L.) grown on a heavy clay soil |
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| Authors: | D M Whitfield G C Wright O A Gyles A J Taylor |
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| Institution: | (1) Irrigation Research Institute, 3616 Tatura, VIC, Australia |
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| Abstract: | Summary Effects of weekly (W) and fortnightly (F) irrigation schedules on CO2 assimilation by lucerne grown on untreated (C) and gypsum-ameliorated (G) heavy clay soil were investigated. Leaf area limited rates of assimilation during the initial stages of regrowth, but rates of up to 9 g CO2 m–2 h–1 were measured once full ground cover was achieved after approximately two weeks. High rates were maintained until the fifth week of regrowth (one week after full flower), after which there was a marked decline.During the fourth week of regrowth, afternoon rates of canopy photosynthesis under less frequent irrigation were less than those measured at similar irradiance during the morning. This was evidenced as hysteresis in the light response curves and was apparent in all treatment during the final stages of the experiment.For the first five weeks of regrowth, daytime integrals of photosynthesis were directly related to the amount of light intercepted by the crops. The mean efficiency of utilisation of light in CO2 assimilation was 6.2 g CO2 MJ–1 in all treatments apart from that on untreated soil under the fortnightly irrigation regime (treatment FC). Its mean efficiency was 5.1 g CO2 MJ–1. The amounts of CO2 assimilated exceeded the growth and respiratory requirements of the above-ground components of the crops, and it was estimated that 25% and 40% of the assimilated carbon was partitioned to and retained in the roots and crowns of the weekly and fortnightly irrigated crops, respectively.Results are appraised in terms of the response of lucerne to moisture deficits. Implications for above-ground dry matter production are also discussed. |
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