High Temperatures around Flowering in Maize: Effects on Photosynthesis and Grain Yield in Three Genotypes

To aid breeding for heat-tolerant germplasm we analyzed the effects of high temperatures on the CO2 exchange rate (CER), crop growth rate (CGR), kernel number (KN), and grain yield (GY) in a 30-d period bracketing flowering. Field experiments, including three maize (Zea mays L.) hybrids with temperate (Te), tropical (Tr) and temperate x tropical (Tx) adaptation were performed in two experiments (Exp. 1 and 2). Hybrids were subjected to high temperatures induced by shelters during a 15-d period before (H1; pre-flowering) or after silking (H2; postflowering). Crop growth rate was measured during the 30-d period bracketing silking (CGR(CP)), H1 (CGR(PRE)), and H2 (CGR(POST)). Relative to nonstressed conditions, CER was reduced by 17 and 16% in H1 and H2. Moreover, CER was associated with CGR(CP) (r = 0.78; p <= 0.001), CGR(PRE) (r = 0.39; p <= 0.05), CGR(POST) (r = 0.51; p <= 0.01), KN (Exp. 1, r = 0.53; p <= 0.01; Exp. 2, r = 0.49; p <= 0.01), and GY (Exp. 1, r = 0.59; p <= 0.01; Exp. 2, r = 0.46; p <= 0.05). As a result of heat stress, CGR(CP) (H1, -17%; H2, -29%), KN (H1, -7%; H2, -45%), and GY (H1, -10%; H2, -45%) were reduced relative to the control treatment. Stronger reductions for all traits in H2 relative to H1 emphasize the importance of sufficient CER during this period. The effect of high temperature on CER differed among hybrids (Tx > Te = Tr) and is promising for future germplasm screening.