(A.R.) While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO 2 concentration ([CO 2 ]) is predicted to enhance yield and offset these detrimental factors. However, C 4 photosynthesis is usually saturated at current [CO 2 Global climate change, in the form of rising temperature and altered soil moisture, is projected to decrease the yield of food crops over the next 50 years (Thomson et al., 2005). Meanwhile, the simultaneous increase in CO 2 concentration ([CO 2 ]) is predicted to stimulate crop production and offset these detrimental components of climate change (Thomson et al., 2005). This encouraging projection results from speciesspecific ''CO 2 fertilization'' factors in yield models (Phillips et al., 1996;Brown and Rosenberg, 1999;Parry et al., 2004;Thomson et al., 2005). These simulate the enhancements of net CO 2 assimilation rate (A) and yield observed, for both C 3 (17%-29%) and C 4 crops (6%-10%), under elevated [CO 2 ] in controlled environment studies (Kimball, 1983;Allen et al., 1987).While early projections of ''[CO 2 ] fertilization'' were based on studies in glasshouses and other protected environments, Free-Air Concentration Enrichment (FACE) experiments are fully open-air trials of crop performance. They provide realistic simulations of future growing conditions and provide perhaps the best opportunity to requantify CO 2 fertilization effects and elucidate the mechanism of crop response. FACE experiments on the C 3 crops rice (Oryza sativa), wheat (Triticum aestivum), and soybean (Glycine max) have observed smaller increases in yield than were predicted from the early chamber studies Long et al., 2005;Morgan et al., 2005). Yet the primary response mechanisms of C 3 crops have not been controversial . First, elevated [CO 2 ] directly stimulates A, growth, and yield by decreasing photorespiration and accelerating carboxylation by Rubisco. Second, it decreases stomatal aperture, which can reduce plant water use and indirectly enhance performance by ameliorating water stress. In contrast, the response of C 4 crops to future elevated [CO 2 ] is uncertain. In C 4 plants, Rubisco is

Heat stress around flowering has negative effects on maize (Zea mays L.) grain yield. Most research on this topic focused on the response of pollen viability and pollination constraints, and little is known about the relative response to heat of plant grain yield (PGY) components [kernel number per plant (KNP) and individual kernel weight (KW)] and the physiological determinants of grain yield [light interception efficiency (ei), radiation use efficiency (RUE), and harvest index (HI)]. Field experiments were performed to study the response of physiological traits to contrasting air temperature regimes at ear level [nonheated control (TC) and heated (TH; with air temperature >35°C around noon)]. Heating was performed during periods of approximately 15 d at two growth stages [presilking (GS1) and postsilking (GS2)]. All silked ears received fresh pollen. Heating during GS1 caused (i) a larger delay in silking date than in anthesis date, (ii) an increase in male and female sterility, and (iii) a reduction in plant height and leaf area index, but not in ei Heating always caused a reduction in (i) plant and ear growth rates (EGR) around silking, (ii) RUE around silking, and (iii) HI and KNP. Final PGY was related to KNP (r2 = 0.89, p < 0.001) but not to KW. Variations in KNP were explained (r2 = 0.71, p < 0.0001) by variations in EGR postsilking (EGRPOST) and not presilking, evidence of long‐term effects of heat stress during GS1 Variations in EGRPOST depended on variations in RUE postsilking (RUEPOST) and not on biomass partitioning to the ear.

In maize (Zea mays L.), the later‐fertilized ovaries often abort, thereby reducing kernel set. We examined whether altering the time interval between pollination of florets within an ear or between ears could affect final kernel number per plant. Synchrony of pollination was varied by natural‐ and hand‐pollination of four hybrids, contrasting in prolificacy (ears plant−1). Plants were grown in the field at low (2.5 and 3 plants m−2) and high (7.5 and 9 plants m−2) plant populations, without water or nutrient stress. Increasing plant population generally delayed silk appearance, but most silks were exposed within 5 d after silking (DAS). Synchronous pollination of all exposed silks on apical and sub‐apical ears 5 DAS improved kernel number (KN) per plant and the floret fertility index (FFI = number of kernels/number of pollinated silks), relative to open‐pollinated plants. At low plant populations, the KN plant−1 increase resulted primarily from a large increase (39–535%, depending upon the hybrid) in kernels on sub‐apical ears. At high plant populations, only apical ears set kernels. Synchronous pollination increased KN in these ears 8 to 31%, depending on the hybrid. Thus, timing of pollination had a large impact on kernel set, and the disadvantage associated with an ontogenetic delay in silk emergence could be partially overcome by synchronous pollination. Because delayed pollination of early‐silking ovaries allowed a greater number of the late‐silking ones to set kernels, factors other than assimilate availability per fertile floret likely are involved in controlling kernel set.