Maize Radiation Use Efficiency under Optimal Growth Conditions

and Amthor (1999) suggested that the RUE era in crop modeling should be closed. Accurate measurement of crop growth and radiation use efficiency A number of factors contribute to the variation in (RUE) under optimal growth conditions is required to predict plant reported estimates of RUE (Sinclair and Muchow, 1999). dry matter accumulation and grain yield near the genetic growth potential. Research was conducted to quantify the biomass and leaf Estimates of RUE depend on whether radiation is meaarea index (LAI) accumulation, extinction coefficient, and RUE of sured as total solar radiation or as PAR. While some maize (Zea mays L.) under conditions of optimal growth. Maize was authors suggest that conversion of RUE based on solar grown in two environments over five growing seasons (1998–2002). radiation to that based on PAR is achieved simply by Total aboveground biomass at maturity ranged from 2257 g m 2 in multiplying by the fraction of total solar radiation that 1998 to 2916 g m 2 in 2001; values that are considerably greater than is photosynthetically active (usually 0.5, Sinclair and the biomass achieved in most previous studies on RUE in maize. Muchow, 1999), it has been pointed out that the approPeak LAI ranged from 4.8 to 7.8. Maize extinction coefficients during priate multiplication factor depends on canopy LAI vegetative growth (k ) were within the range of recently published (Bonhomme, 2000). The radiation intercepted by a crop values (0.49 0.03), with no clear pattern of differences in k among years. Seasonal changes in interception of photosynthetically active is different from that absorbed by it and, therefore, radiation (PAR) were similar across all but one year. Estimates of introduces variation in RUE calculations. In agreement RUE were obtained using the short-interval crop growth rate method with Sinclair and Muchow (1999), Bonhomme (2000) and the cumulative biomass and absorbed PAR (APAR) method. suggests that assuming 85% of intercepted PAR (IPAR) Values of RUE obtained using the two methods were 3.74 ( 0.20) g is absorbed by the leaf canopy is accurate when canopy MJ 1 APAR and 3.84 ( 0.08) g MJ 1 APAR, respectively, and did LAI is large, but the value is smaller when canopies not vary among years. This compares to a published mean RUE are less dense. Variation in estimates of RUE can be for maize of 3.3 g MJ 1 of intercepted PAR (Mitchell et al., 1998). substantially reduced by measuring both intercepted Moreover, RUE did not decline during grain filling. Differences in and absorbed radiation continuously during a sampling biomass accumulation among years were attributed in part to differences in observed radiation interception, which varied primarily due period. to differences in LAI. Maize simulation models that rely on RUE for Maize grain yield is determined, in part, by kernel biomass accumulation should use an RUE of 3.8 g MJ 1 APAR for number at harvest (Tollenaar et al., 2000), which is sensipredicting optimum yields without growth limitations. tive to environmental conditions (Lizaso et al., 2003) and not completely dependent on total biomass production (Rajcan and Tollenaar, 1999). The most effective P dry matter accumulation depends on the total approach to predicting kernel number per plant depends C fixed by photosynthesis and the fraction of that on the average daily IPAR around silking (Lizaso et C converted to dry matter (Norman and Arkebauer, 1991). al., 2003). Therefore, accurate prediction of daily IPAR In the absence of biotic and abiotic stresses, plant dry is often a critical component of maize simulation models matter accumulation depends on the quantity of radiabecause it determines daily biomass increase as well tion absorbed by the canopy (e.g., Kiniry et al., 1989; as kernel number. Radiation interception is primarily Monteith, 1977; Sinclair and Muchow, 1999). The relationdetermined by the LAI (Bonhomme, 2000; Lindquist ship between plant dry matter and radiation intercepted and Mortensen, 1999; Muchow, 1988) and an index of has been termed the radiation use efficiency (RUE, g the efficiency of radiation interception, the extinction MJ 1; Monteith, 1977). A number of crop growth simucoefficient (k; Lizaso et al., 2003). lation models have been developed using the RUE conThe definition of plant growth also determines the cept to forecast crop growth and yield in different enviestimated value of RUE. Growth can be determined ronments (Brisson et al., 2003; Jones and Kiniry, 1986; using net CO2 uptake, total aboveground dry matter, or Muchow et al., 1990). These models generally calculate total dry matter (including roots, Arkebauer et al., 1994). daily biomass production as the product of the quantity Growth is most commonly reported based on net aboveof radiation intercepted and RUE (Lecoeur and Ney, ground biomass production because destructive sam2003). However, the empirical nature of RUE and the pling of biomass is easier than long-term measurement low precision with which it can be estimated (Mitchell of canopy CO2 uptake, and obtaining estimates of root et al., 1998) may cause significant uncertainty about the biomass is difficult (Sinclair and Muchow, 1999). Accuaccuracy of model simulations. Considering this, Loomis racy of measured crop biomass can contribute greatly to variation in estimated RUE, and care must be taken Dep. of Agron. and Hortic., Univ. of Nebraska–Lincoln, P. O. Box 83095, Lincoln NE 68583-0915. Received 15 Mar. 2004. *CorrespondAbbreviations: APAR, absorbed photosynthetically active radiation; ing author ([email protected]). CGR, crop growth rate; DOY, day of year; DVS, development stage; IPAR, intercepted photosynthetically active radiation; k, extinction Published in Agron. J. 97:72–78 (2005). © American Society of Agronomy coefficient; LAI, leaf area index; PAR, photosynthetically active radiation; RUE, radiation use efficiency. 677 S. Segoe Rd., Madison, WI 53711 USA