Recurrent Phenotypic Selection for High and Low Photosynthesis in Two Maize Populations1

Photosynthesis is a primary trait influencing yield potential of maize (Zea maysL.), but limited breeding work has been done to increase photosynthetic rate per unit leaf area. We conducted five cycles of recurrent phenotypic selection for high CO2‐exchange rate (CER) in two maize populations, BSUL1(RPH) and BSUL2(RPH), and three cycles of selection for low CER in BSUL2(RPL). Selection advance for high CER was significant in both populations. Actual genetic advances for high CER for BSUL1(RPH) were 1.6 and 1.3%/cycle at vegetative (CER 1) and grain filling (CER 2) growth stages, respectively. BSUL2(RPH) showed a per cycle increase of 1.7% at CER 1 and 1.3% at CER 2. Estimates of Δpα clearly showed that selection for high CER at CER 1 caused significant weighted‐average changes in allelic frequencies at CER 1 (0.28 ± 0.05) and CER 2 (0.25 ± 0.09) in BSUL1(RPH) and in BSUL2(RPH) at CER 1 (0.29 ± 0.05) and CER 2 (0.26 ± 0.09). Actual advance for low CER in BSUL2(RPL) was statistically significant at CER 1, but it was only 0.7%/cycle. Correlated responses in CER at CER 2 for BSULl(RPH) and BSUL2(RPH) provided additional evidence that similar genetic mechanisms control CER of maize at both developmental stages. Our data demonstrated that leaf photosynthesis of maize can be improved by recurrent selection.