RADIATION-USE EFFICIENCY RESPONSE TO VAPOR-PRESSURE DEFICIT FOR MAIZEAND SORGHUM

Variability within a crop species in the amount of dry mass produced p er unit intercepted solar radiation, or radiation-use efficiency (RUE) , is important for the quantification of plant productivity. RUE has b een used to integrate (1) leaf area, (2) solar radiation interception, and (3) productivity per unit leaf area into crop productivity. Respo nsiveness of RUE to vapor pressure deficit (VPD) should relate closely to responsiveness of CO2 exchange rate (CER) to VPD. The objective of this study was to compare independent RUE measurements to published r esponse functions relating VPD with RUE of maize (Zea mays L.) and gra in sorghum [Sorghum bicolor L. (Moench)]. Data sets from five location s covering a wide range of mean VPD values were compared to published response functions, Predicted RUE values were nearly always within the 95% confidence intervals of measurements. Measured RUE of maize decre ased as VPD increased from 0.9 to 1.7 kPa. For sorghum, measured value s of RUE agreed closely with predictions. RUE of sorghum decreased as VPD increased from 1.1 to 2.2 kPa. The relative RUE:VPD responses for these two species were similar to CER:VPD responses reported in the li terature, Thus, these RUE:VPD responses may be general and appear to b e related to carbon exchange rates. We calculated the expected impacts of VPD on RUE at three USA locations during maize and sorghum growing seasons. The RUE:VPD equations offer hope in describing location effe cts and time-of-year effects on RUE. (C) 1998 Elsevier Science B.V.