Experiments were conducted during the growing seasons of 1975 and 1976 to determine changes in morphological and physiological traits associated with recurrent selection for grain yield in maize (Zea mays L.). Four variety hybrids, BSSS(R)CO x BSCBI(R)CO, BSSS(R)C7 x BSCBI(R)C7 [from a reciprocal recurrent selection program involving Iowa Stiff Stalk Synthetic (BSSS) and Iowa Corn Borer Synthetic # 1 (BSCB1)], BS12CO x B14A, and BS12C6 x B14A [from a half-sib selection program involving the open-pollinated variety Alph (BS12) and the inbred tester B14A] were grown at 59 300 and 98 800 plants/ha near Ames, Iowa We obtained data on CO2-exchange rate (an estimate of photosynthetic rate), grain yield, grain-yield components, flowering dates, maturity traits, light interception and use, shelling percentage, harvest index, and various other plant traits.
COz-exchange rate did not change appreciably with recurrent selection for yield. Grain yield per hectare and per plant were larger for the improved than unimproved hybrids. Grain-yield components did not change significantly with recurrent selection. Kernel weight of BSSS(R)C7 x BSCB1 (R)CT, however, was larger than that of BSSS(R)C0 x BSCB1 (R)C0. Pollen-shed-to-silking interval was shorter for the improved than the unimproved hybrids, and grain-filling duration was longer in C7 x C7 than in Co x C O of the reciprocal recurrent selection program. Furthermore, improved hybrids were characterized by smaller tassels and more upright canopies. Usually, plant traits and leaf-area-related traits were similar for all hybiids.
Although dry-matter productivity was similar for all hybrids, those that were improved by recurrent selection produced more grain per unit leaf area and per unit light interception. Also, BS 12C6 x B14A was characterized b~ยข a higher harvest index than BS12CO x B14A.
We concluded that the source (i.e., photosynthetic capacity) was not limiting grain yield in BSSS(R) x BSCBI(R) and BS12. Increased grain yields that resulted from recurrent selection were consequences of longer grain-filling duration for BSSS(R) x BSCBI(R) and increased translocation of photosynthate from source to sink for both BSSS(R) x BSCBI(R) and BSl2.