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Effects of irrigation on CO2 exchange of wheat

D.M. Whitfield, C.J. Smith and O.A. Gyles

Institute for Irrigation and Salinity Research, Tatura, Vic. 3616.

Effects of irrigation regime on CO2 exchange of wheat were measured during heading and grain filling to quantify effects of treatment and environment. Relationships between CO2 exchange, green leaf area index (LAI) and yield were investigated.


Daily rates of CO2 exchange (PN) of a rainfed crop (treatment RF) and of crops irrigated at fortnightly (treatment IF) and weekly (treatment Iw) intervals were measured between heading and maturity using the field chambers described by Connor et al (1). Each treatment was sown to cultivar 'Condor' and received 150 kg N ha-1, 30 kg P ha-1 and 5 tonne gypsum ha-1 prior to sowing on June 14, 1984. Irrigation treatments were imposed 120 days after sowing (DAS 120), 20 days before the crops reached anthesis. 22 mm of a total of 210 mm of rainfall fell between DAS 120 and 180.

Results and discussion

A maximum green leaf area index of approximately 7 was achieved by DAS 108. In treatment RF, LAI decreased almost linearly after DAS 120 until none remained at DAS 160. In treatments I and I LAI was maintained at the maximum for approximately 140 days after which it also decreased to reach zero at approximately DAS 180 and 187, respectively. Grain filling, which commenced at DAS 1148, finished by DAS 167 in treatment RF, and by DAS 180 in treatments I and I In treatment I P increased from 140 g CO2 m-2 day-1 at DAS 108 to a maximum of 70 g CO2 m-2 day at DAS 150 (see Fig. 1). It then decreased in WI N parallel with LAI to zero at approximately DAS 180. At the other extreme, treatment RF reached a maximum of 55g CO2 m-2 day-1 before a rapid decrease to approximately 10 g CO2 day-1 at anthesis. This was again associated with changes in LAI. Rainfall and soil reserves maintained this rate until approximately DAS 160. Before anthesis, the response of treatment IF was the same as that of treatment I. In subsequent drying cycles, rates equal to those of treatment I were maintained for only approximately one week after irrigation, but were generally restored at the next irrigation.

The data suggest that PN is strongly related to LAI and hence leaf area duration and, further, that grain filling is maintained while PN is positive. The association with LAI is circumstantial, as the contribution of the ears was not measured. The data also demonstrate the sensitivity of PN to irrigation frequency. However, factors other than PN were important in yield determination because yields of treatments IF and Iw were not significantly different.

1. Connors,D.J., Palta,J.A., and Jones,T.R. 1985. Field Crops Res. 12:281.

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