Seasonal variation in radiation use efficiency of irrigated rice

Radiation use efficiency (RUE), the amount of carbon assimilated per unit intercepted photosynthetic photon flux density (Qi), is important in understanding and modeling the relationship between plant growth and the physical environment. Field experiments were conducted in 1998 and 1999 to evaluate RUE as a function of growth and development in a commercial rice field near El Campo, TX. Half-hourly averages of carbon assimilation were obtained from tower-based conditional sampling (CS), and Qi was measured with quantum sensors. We found a high correlation between half-hourly averages of CO2 exchange rate (CER) and Qi. RUE steadily declined during growth of the primary crop, from 3.18 to 2.65 μg μmol−1 quanta, in 1998 and 1999, respectively, for leaf area index (LAI) between 1 and 2, to 1.57 and 1.55 μg μmol−1 quanta, respectively, for LAIs exceeding 5. Post-anthesis values of RUE were substantially lower than pre-anthesis values at comparable LAIs. Post-anthesis RUE in the ratoon crop was also lower than pre-anthesis RUE. Daily totals of CER and Qi values were also strongly correlated. We concluded that when RUE is used as a model parameter, it must be changed for differing LAI and for pre- and post-anthesis periods.