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Drought stress reduces yield of maize ( Zea mays L.) and other grain crops by (i) reducing canopy absorption of incident photosynthetically active radiation (PAR), (ii) reducing radiation use efficiency (RUE), and (iii) reducing harvest index (HI). The primary objective of this work was to quantify yield losses attributable to each of these components for maize exposed to drought stress in a 2‐yr field study. A second objective was to examine the relationship between RUE at the single leaf level (estimated using chlorophyll fluorescence techniques) and RUE at the whole crop level. Two levels of soil water deficit and a control treatment were established using drip tape irrigation, and dry matter harvests were taken at midseason and at physiological maturity. Mild and severe water stress treatments reduced final grain yield by 63 and 85%, respectively, in 2000, and by 13 and 26%, respectively, in 2001. Reduction of intercepted PAR (IPAR) was generally a very minor yield loss component. Yield losses attributable to reduced RUE and reduced HI were of similar magnitude. Weekly chlorophyll fluorescence measurements were used to estimate the average quantum efficiency of photosystem II at a photosynthetic photon flux density of 1200 μmol m −2 s −1 (Φ II1200 ) for each plot. Crop dry matter accumulation was not linearly related to IPAR, due to decreased RUE in the water stress treatments. However, the linear relationship was restored when daily IPAR was multiplied by the current estimate of Φ II1200 , suggesting that Φ II1200 can be used as an indicator of whole‐crop RUE.
Earl et al. (Thu,) studied this question.