Genetics of Photoperiod Sensitivity and Seasonal Effect in Com (Zea Mays L.)
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1978
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Forty-three corn inbreds were screened for photoperiod insensitivity in controlled growth chambers maintained at 12 and 16 hour daylengths. Tassel initiation determined quantitatively (0.4 mm) was the criterion used in the screening. Photoperiod insensitive and sensitive lines were identified by the above criterion. Tassel elongation of two photoperiod insensitive inbreds (Va35 and 0h43) and two sensitive inbreds (Hi30 and Tx601) under 10, 12, 14 and 16 hour day lengths was logarithmic with time after tassel initiation. No critical day length was differentiated in this study for corn. Long day lengths delayed tassel initiation resulting in a higher leaf number at the time of tassel initiation for photoperiod sensitive genotypes.
In photoinduction studies, it was found that increasing the number of short days decreased the number of days to tassel initiation and anthesis for photoperiod sensitive genotypes. Conversely, increasing the number of long days increased the number of days to tassel initiation and anthesis for these genotypes. In the hybrid CM104 x Tx601, plants were found to be non-responsive to day length changes during the first twenty days from planting. Tassel initiation time as well as the period of tassel development were affected by photoperiod, with stronger influence on the former than the latter.
Ten inbreds of differing sensitivity to photoperiod were used to generate a diallel cross. The inbreds were Va35, B37 and 0h43 (insensitive); A619, Hi25 and Hi26 (intermediate); and Hi30, CM104 and Tx601 (sensitive). The inbreds and their 45 F-^g were evaluated under normal day length (average 12.5 hours) and extended day length of an additional 4 hours of incandescent light in Hawaii. Days to anthesis and silking were delayed tinder extended day length for all genotypes, with sensitive genotypes having the greatest delays and insensitive genotypes having relatively smaller delays. Plant height and ear height were significantly increased under the extended day length in sensitive genotypes and their crosses with insensitive or intermediate genotypes.
Lower grain yield was observed under extended day length for genotypes insensitive or intermediate to photoperiod. Cob length was increased although no difference in filled ear length was observed among the insensitive genotypes but was increased among the intermediate genotypes. Both kernels per row and 100 kernel weight were decreased slightly in both group of genotypes. With insensitive x sensitive or intermediate x sensitive crosses, extended day length increased grain yield, cob length and filled ear length and led to slight increases in kernels per row. Sensitive genotypes showed drastic yield reductions under extended day length with decreases in cob length, filled ear length, kernels per row, 100 kernel weight and kernel depth. These reductions in yield and yield components were attributed to diseases and poor pollinations, but not day length effects. Row number on the whole was relatively unaffected by photoperiod. Counts of leaf number and kernel initials per row were increased by long days and with increasing sensitivity to photoperiod of the genotypes.
Days to tassel initiation was found to be highly correlated with days for tassel development, leaf number, days to anthesis and silking. Using only the photoperiod sensitivity, days to tassel initiation was found to have a high correlation with leaf number, days to anthesis and silking.
Photoperiod sensitivity under normal and extended day length were expressed in days to tassel initiation, days to anthesis, days to silking and leaf number use in the diallel analysis. General combining ability (GCA) and specific combining ability (SCA) mean squares were significant with sensitivity expressed in days to tassel initiation, anthesis and silking. This indicates that both additive and non-additive genes contributed to the genetic variation of photoperiod sensitivity. When leaf number was used, only GCA mean squares were significant. Nevertheless, the GCA/SCA ratio was extremely high in all cases indicating that GCA was more important than SCA. Narrow sense heritability estimates were very high and ranged from 73.9% to 94.7%. In the graphical analysis using photoperiod sensitivity expressed in leaf number, it was found that low sensitivity was partially dominant to high sensitivity, and that photoperiod sensitivity was controlled by a minimum of 2 genes showing some degree of dominance.
Generation mean analysis based on days to anthesis under long days for crosses involving a series of insensitive x sensitive inbreds were carried out. Photoperiod sensitivity and maturity were confounded in this evaluation. Depending on the crosses, additive and/or dominance gene effects were important in controlling days to anthesis with epistasis detected in some crosses. Narrow sense heritability estimates for days to anthesis from the summarized data was 57.43%. The minimum gene number controlling days to anthesis (confounded with photoperiod sensitivity) was on the average 4 (Castle-Wright) or 5 (Sewall Wright) genes with earliness being dominant.
The same set of diallel crosses was evaluated for grain yield and yield components under winter and summer conditions. Planting in winter reduced grain yield by an average of 54.570 from summer values. Cob length, kernels per row and 100 kernel weight were also reduced in winter as compared to summer. GCA and SCA mean squares for all characters were highly significant in both seasons, with GCA/SCA ratio greater than unity. In the combined analysis, GCA. x season and SCA x season mean squares were also highly significant for all the characters studied. GCA/SCA ratios were higher in the winter than in the summer plantings with the exception of kernels per row. The tropical inbreds CM104 and Tx601 showed high combining ability for grain yield under both seasons as compared to the temperate inbreds.
A monthly planting using 6 hybrids was carried out for a period of 20 months. Significant hybrid x months interaction was detected for all characters. Days to mid-silk was earlier in the summer months than in the winter. Plant and ear height, yield and yield components, were generally greater in the summer than in the winter months. The performance somewhat progressed according to the cyclical climatic variations. Plantings in April through August in 1976 were identified as the best months for high grain yield. The best overall hybrid was H763, a temperate by tropical cross.
Multiple regression analyses were also conducted using solar radiation, maximum and minimum temperatures and light duration values as independent variables. Generally, it was found that all the climatic factors studied were important in affecting flowering. Day length was found to have an important effect on cob length and grain yield. In a separate analysis without using the day length variable, solar radiation was also found to be important in affecting grain yield. Of the three months, solar radiation on the third month of the plant's growth, corresponding to grain filling period, was more important than the effects of solar radiation on the earlier two months in contributing to grain yield.
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