Search

Plant breeders and plant physiologists would like to know which plant traits have changed to permit new varieties to produce higher yields than older varieties. We studied crosses of improved and unimproved cycles from three recurrent selection programs conducted to increase grain yield in maize (Zea maysL.). Our research was conducted to study changes in several physiological traits that could be important to increased grain production. Results suggested that an extended grain filling period and later plant senescence rather than an increased rate of grain filling was associated with grain yield increases in BS10(FR) x BS11(FR). Testcrosses of the improved Lancaster population produced more dry matter and translocated dry matter to developing kernels at a faster rate and for a longer period of time than did testcrosses of the unimproved population. Crosses of improved cycles of BSSS(R) and BSCBl(R) generally displayed more rapid filling periods and later plant senescence than did crosses of their unimproved cycles. Crosses of improved cycles generally produced more dry matter and translocated a larger percentage of dry matter to the grain.

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.

We measured CO2‐exchange rates (CER), morphological traits, and yield traits of 64 maize (Zea maysL.) inbred lines randomly derived from Iowa Stiff Stalk Synthetic (BSSS). Our experiments were grown for 2 years at one location. Genotypic correlations among these traits were calculated to determine if selection for high CER would cause correlated responses in other traits. We also studied possible physiological limitations to grain and dry matter yield of these inbreds. Genotypic correlations between CER and plant traits were low, suggesting that one could develop high CER genotypes displaying various morphological traits. Nonsignificant correlations between CER and yield traits indicated CER was not the primary factor limiting yield of these inbreds. Correlations showed that dry matter yield was limited by amount of leaf area and by traits associated with efficient use of light energy. We concluded grain yield of these inbred lines was limited by sink size and by lack of a receptive sink because: 1) grain yield was highly correlated with ear length, ear diameter, 300‐kernel weight, and number of ears per plant and 2) potential dry‐matter production during grain filling was 208 g plant ‐1compared with 49.6 g plant‐1actually stored in the grain.

Barrenness, the failure of a plant to produce a normal ear, is a major limitation to efficient conversion of light energy into grain in maize (Zea maysL.) grown at high plant densities. Our research was conducted to determine magnitudes of genetic variability and heritabilities for various traits of the maize population Iowa Upright Leaf Synthetic #1 (BSUL1) and to determine which traits were associated with barrenness and grain yield at high plant densities. We grew 144 S1lines from BSUL1 for 2 years at one location and at two plant densities. Results indicated that BSUL1 contained large amounts of genetic variability for the 14 traits studied at 42,300 and 96,800 plants/ha. Significant heritabilities ranged from 0.31 for grain/leaf area at the high density to 0.91 for tassel branch number (low density). Phenotypic and genotypic correlations indicated that silking traits, pullen‐shed‐to‐silking interval, and grain/plant were highly correlated with grain yield and barrenness at both densities. Canopy orientation traits generally showed weak associations with grain yield and barrenness. Correlations and genotype × year interactions suggested that selection at a low density (42,300 plants/ha) in one location for traits correlated with barrenness should improve BSUL1 for tolerance to high plant densities.

Maize (Zea maysL.) hybrids were grown at a high plant density (98,800 plants/ha) to determine the influence of several morphological and physiological traits on barrenness, the major factor limiting grain yields at high stand densities. Correlation and multiple regression analyses indicated density‐tolerant maize genotypes would be characterized by rapid completion of silk extrusion, coincidence of pollen‐shed and silk extrusion, rapid growth of the fust ear and first‐ear silk, prolificacy, reduced tassel size, and efficient production of grain per unit leaf area. Breeding populations composed of these “plant types” should allow isolation and development of high‐yielding and density‐tolerant maize genotypes.

We studied the inheritance of tassel branch number in maize (Zea maysL.) by analyzing data collected from five generations (P1, P2, F1, F2, and F3) of a cross involving two inbred lines that differed significantly for tassel branch number. Estimates of genetic effects and genetic variance components were obtained from generation mean analyses and expectations for generation variances, respectively. Our data indicated that: 1) tassel branch number was quantitatively inherited (minimum of eight genetic factors involved), 2) high tassel branch number was dominant to low tassel branch number (data from crosses involving other parental inbreds showed the expression of dominance was genotype dependent), 3) no reciprocal effects were detectable, 4) additive gene action was most important, but dominant gene action should be evaluated carefully in future studies, and 5) heritability estimates were high, suggesting selection for tassel branch number should be possible.

Photosynthesis is a primary trait influencing yield potential, and it should be improved in a yield‐potential selection program. Information concerning the genotypic variability and inheritance of photosynthetic efficiency in maize (Zea maysL.) must be obtained before a yield‐potential selection program is initiated. We used CO2‐exchange rate (CER) of sections from excised leaves to estimate photosynthetic efficiency of 64 random inbred fines of maize derived from Iowa Stiff Stalk Synthetic (BSSS). Our study was conducted for years at one location. Measurements of CER taken during vegetative growth (CER 1) and during grain filling (CER 2) were used to estimate genotypic variances and heritabilities and to calculate selection advances for these two stages of plant development. We also evaluated the possibility of using specific leaf weight and leaf thickness as index traits for CER selection. Results showed that estimates of additive variance were 1.7 and 4.9 times larger than estimates of genotype ✕ environment variance at CER 1 and CER 2, respectively. Narrow‐sense heritabilities were 0.58 for CER l and 0.80 for CER 2, and realized heritability values were 0.72 (CER 1) and 0.66 (CER 2). A high genotypic correlation between CER at CER 1 and CER 2 indicated selection at either developmental stage would produce nearly identical results. Actual selection advances (15% selection intensity) represented increases of 14 and 22% above population means for CER 1 and CER 2, respectively. Our data suggested that measurement of two replications of four plants/plot grown in two environments would provide most efficient selection advance for inbreds. We concluded that selection for improved CER in BSSS would be possible and that selection must be for CER per se.

The advantage of vertically oriented leaves in crop canopies has been studied with variable results. In many maize (Zea maysL.) populations there is a wide variation of leaf orientations. Nine maize inbreds visually selected for upright, intermediate, and horizontal leaf orientation were grown in 51‐cm rows at three plant densities at Iowa State University. Each inbred was evaluated for 2 years for leaf area index, light extinction coefficient (k), dry‐matter yield, and grain yield, and 1 year for leaf orientation and leaf display. We developed a new leaf orientation parameter, leaf orientation value (LOV), to account for the ability of leaves to maintain the same orientation for their entire length. This parameter was easier to measure than k, is a good indicator of leaf orientation, and could be used as a selection index for leaf orientation. Differences in dry matter yields and grain yields across leaf orientation groups agreed with expectations based on theories involving leaf orientation and leaf area index; i.e., yield advantages for genotypes with upright leaves were observed only at high leaf area indices.

We evaluated cold‐tolerance responses [measured by percentage emergence (30 days after planting), emergence index (an estimate of emergence rate), and seedling dry weight (sampled 42 days after planting)] of 34 maize (Zea mayL.) inbred lines adapted to various latitudes in North America. Evaluations were performed in field experiments planted in early April at Ames and Algona, Iowa, in 1974 and 1976. Objectives of our study were: i) to assess genetic variability and breeding potential for improvement of cold tolerance within maize germplasm adapted to North America, ii) to study associations of cold‐tolerance traits with other plant traits (including grain yield), and iii) to examine relationships between geographical locations of origin and cold‐tolerance responses of inbred lines adapted to North America. We observed large amounts of variability for each of the three cold‐tolerance traits. Means ranged from 27.5 to 82.9% for percentage emergence, 20.0 to 24.0 days for emergence index, and 0.33 to 1.16 dg for seedling dry weight; respective heritability estimates were 0.85 ± 0.06, 0.72± 0.06, and 0.80 ± 0.06. Genotypic correlations among the three traits were high, suggesting selection for improved cold tolerance (as an aggregate of the three traits) should be possible. Environments and genotype ✗ environments mean squares were highly significant; therefore, evaluations of cold tolerance of maize inbred lines should be conducted in more than one environment. Correlations of all three traits with juvenile plant height and leaf number (measured in early July), 50% silk emergence, mature plant height, and grain yield usually were low. Thus, seedling cold tolerance was not associated with vegetative vigor of juvenile plants, flowering date, or mature plant height. Seedling dry weight, however, was significantly correlated with grain yield (r = 0.48**), indicating good seedling vigor was related to high grain yield. Cold‐tolerance response generally was not related to geographical location of adaptation; however, 9 of the 10 best inbred lines were adapted to the central latitudes of the U.S. Corn Belt. Inbreds from the northern and southern regions ranked average or lower for cold tolerance. The three best cold‐tolerant inbreds were B73, (V3 x B14)‐2‐1 (both from Iowa), and Mo 17 (from Missouri). Breeding populations developed from the best inbreds in our study should contain gene complexes for good cold tolerance, plus variable flowering dates, plant heights, and grain yields.

We grew four maize (Zea maysL.) genotypes, representing a wide range of leaf area/plant, at two plant densities in 1973 to develop a simple, rapid, and nondestructive method for estimating average leaf area/ plant. Immediately after pollen‐shed, we measured lengths and maximum widths of all leaves on all plants in each plot and subsequently calculated individual leaf areas. Individual leaf areas were summed to give total leaf area/plant. Correlations of individual leaf areas with total plant leaf area revealed that leaf #8 (from the top of the plant) was the most highly correlated (r = 0.95). Therefore, the regression of total plant leaf area on area of leaf #8 was computed and the resulting b‐value (9.39 ± 0.04) used as the leaf area factor for estimating average leaf area/plant. Results indicated average leaf area/plant for normal maize plants can be estimated by measuring length and maximum width of leaf #8 on several competitive plants/ plot, calculating area of leaf jS, and multiplying that leaf area by 9.39. Genotype means can often be obtained across replications by measuring only one plant/plot. This method is rapid, nondestructive, and should allow estimation of leaf area of more genotypes and/or experimental treatments/experiment than is currently feasible with conventional techniques.

Expected genetic responses were used to compare the simultaneous improvement of several agronomic traits for three selection index methods in recurrent selection programs. Observations for various agronomic traits in maize (Zea maysL.) were obtained from 144 S1lines of the BSSS2 maize population grown in several locations during the 1970‐72 growing seasons. These traits were European corn borer (Ostrinia nubilalis(Hubner)) resistance, cold tolerance (percentage of emergence, emergence index, and seedling dry weight), grain yield, grain moisture, root and stalk lodgings, and corn root worm (Diabroticaspp.) resistance (root damage, root size, and secondary root growth). Seven of them, grain yield, corn borer resistance, percentage emergence, emergence index, seedling dry weight, root size, and secondary root growth, were used to construct three types of index selection for S1selection. The conventional indices constructed from two sets of arbitrarily assigned relative economic weights did not give satisfactory predicted improvement for all traits included in individual indices. Most of the predicted improvement went to grain yield and percentage of emergence. Base indices, in which relative economic weights were used as index coefficients, were 95 and 97% as efficient as conventional selection indices. Hence, the application of the base index would be preferable with S1selection when precise relative economic values of the traits included in the indices are known. When modified selection indices (based on desired gains) were used, final goals from recurrent selection of the traits considered (S1testing and 10% selection intensity) were predicted to be attained after 14 cycles or 28 years of selection. In another scheme of recurrent selection, corn borer resistance and cold tolerance traits would be evaluated in the S1generation, and yield, root size, and secondary root growth would be tested hi the S2generation. With 44 and 8% selection intensities in the S1and S2generations, respectively, improvement of all traits was expected to reach final goals after 10 cycles or 30 years of selection. Assuming that the two sets of the arbitrarily assigned relative economic weights were realistic, modified selection indices (based on their contributions to aggregate genetic advance) were shown to be only 46 and 61% as efficient as conventional indices for the specified desired gains and relative economic weights considered. Modified selection index with the S1testing scheme was expected to give 53% of improvement for yield, compared with single trait selection based on yield alone. Use of the modified index selection is recommended for the simultaneous improvement of several agronomic traits in recurrent selection programs when the relative economic values of the traits are difficult to determine. Results from comparisons of predicted gain from recurrent selection with S1testing and a combination of S1and S2testings suggested that S1testing would be preferable, because fewer resources would be required and the construction of selection indices would be less complicated.

We conducted experiments in 1973 and 1974 (planted each year as early as possible in the spring) to evaluate effectiveness of recurrent selection for cold tolerance in two Iowa maize (Zea maysL.) breeding populations, BSSS2(SCT) and BSSS13(SCT). Progress from selection for cold tolerance was measured by changes in percentage emergence, emergence index (based on emergence rate), and seedling dry weight. Changes in other agronomic traits were studied also. Relatively more progress for improved cold tolerance was realized from two cycles of recurrent selection in BSSS13(SCT) than from three cycles of recurrent selection in BSSS2(SCT). Percentage emergence and seedling dry weight of BSSS13(SCT) were improved 8.4%/cycle and 0.6 dg/cycle, respectively. These traits did not respond consistently to selection within BSSS2(SCT). Emergence index was not changed by selection in either population. Also, one cycle of selection based on data collected from growth‐chamber plantings of both populations did not choose significantly different genotypes from those chosen by selection based on data collected from field plantings. The C2cycle of BSSS13(SCT) was characterized by more vigorous seedling and juvenile plant growth, an earlier flowering date, and a lower harvest moisture than the C0cycle. The C3cycle of BSSS2(SCT) showed an earlier flowering date, shorter mature plant height, and lower harvest moisture than the C0cycle. Grain yields of advanced selection cycles of BSSS2(SCT) were lower than, and those of BSSS13(SCT) were equal to, that of the highest‐yielding hybrid check.

Index selection was developed to help breeders practice simultaneous selection for several traits. Our objective was to compare selection differentials, expected gains, and relative index efficiencies of several indexes constructed to improve cold tolerance of two maize (Zea maysL.) populations. Cold tolerance traits were percentage emergence, emergence index (i.e., rate of emergence), and seedling dry weight. Best predicted results for all traits were given by a rank summation index, a multiplicative, weight‐free index, and a base index (index weights were reciprocals of phenotypic standard deviations). These indexes were not seriously affected by unequal variances among traits and combined 1) simplicity of use, 2) freedom from need to estimate genetic parameters, and 3) good selection differentials and predicted gains in each trait and in the aggregate genotype. Our results also showed that selection for dry weight/plot identified lines with excellent percentage emergence and seedling dry weight.

We evaluated two U.S. Corn Belt maize (Zea maysL.) populations (BSSS2 and BSSS13) for percentage emergence, emergence index (emergence rate), and seedling dry weight in two different “cold” environments: early field planting and growth chamber at constant 1O.OC. Genotypic variances of the traits within environments were large, but genotype ✕ environment interaction was large and significant so that parameter estimates (δ2gand h2) from data combined across environments were low. Genotypic correlations suggested that within a population the same, or very similar, genetic systems conditioned cold tolerance in both environments. These genes were independent of genes controlling stand and maturity under “normal” planting conditions. Predicted selection responses indicated field selection for cold tolerance would be more efficient.

Photosynthesis is a primary trait to be improved in a selection program designed to improve yield potential of maize (Zea maysL.). Previous studies have suggested that selection for high CO2exchange rate (CER) among inbred parents would be effective. We conducted experiments to evaluate the effectiveness of selection for high CER among inbred lines of maize. Actual advance for one cycle of selection among inbred lines was 2.5 (4.7%) and 2.3 (5.7%) mg CO2dm−2hour−1during vegetative growth and during mid‐grain filling, respectively. Realized heritabilities for CER were 0.33 and 0.26 for vegetative and grain‐filling growth stages.