Seasonal variation in radiation used efficiency of irrigated rice

Radiation use efficiency (RUE), the amount of carbon assimilated per unit i ntercepted photosynthetic photon flux density (Qi), is important in underst anding 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 fr om tower-based conditional sampling (CS), and Qi was measured with quantum sensors. We found a high correlation between half-hourly averages of CO2 ex change rate (CER) and Qi. RUE steadily declined during growth of the primar y crop, from 3.18 to 2.65 mug mu mol(-1) quanta, in 1998 and 1999, respecti vely, for leaf area index (LAI) between 1 and 2, to 1.57 and 1.55 mug mu mo l(-1) quanta, respectively, for LAIs exceeding 5. Post-anthesis values of R UE were substantially lower than pre-anthesis values at comparable LAIs. Po st-anthesis RUE in the ratoon crop was also lower than pre-anthesis RUE. Da ily 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 differi ng LAI and for pre- and post-anthesis periods. (C) 2001 Elsevier Science B. V. All rights reserved.