One of the factors that has hindered introgression of tropical germplasm from its area of adaptation into the central corn belt has been sensitivity to the long-day conditions characteristic of the corn belt growing season. Most tropical germplasm will not flower until late September or October, if at all, under long-day conditions. Genes that condition this photoperiod sensitivity have been reported by various researchers to be recessive. There may be, however, many other factors involved in flowering that are dominant. If this hypothesis is correct, then one method that may be useful to alter flowering initiation is mutagenesis. In 1985, we began a project to move one of the tropical populations into the corn belt without first introgressing corn belt germplasm to facilitate flowering. We based this project on pollen mutagenesis. Our approach was to mutagenize a photoperiod-sensitive population grown in a short-day environment, self the resulting M1 plants in a similar short-day environment, and grow the M2's (S1's) in a long-day environment to select for those plants that could flower and set seed.
Mutagenesis was conducted according to Neuffer's standard pollen mutagenesis protocol (M.G. Neuffer, Maize for Biological Research, p. 61, 1982). In addition to the standard 0.063% EMS in paraffin oil (Treatment 1), we also used 0.10%, EMS (Treatment 2). Pure paraffin oil was used as the control treatment. Pollen from a tropical population was bulked, treated with EMS in paraffin oil, and used to pollinate 226 plants at Homestead, Florida. One hundred and six of these were pollinated by Treatment 1, while 120 ears were pollinated by Treatment 2. In addition, 100 ears were pollinated with pollen that had been exposed to paraffin oil only. At Homestead, 74 ears from Treatment 1 were harvested with M1 seed set ranging from 3-121 kernels per ear. Sixty-one ears from Treatment 2 were harvested with seed set ranging from 3-107 kernels per ear. The control treatment yielded 77 well-filled ears.
During summer 1985, M1 seed of the three treatments was planted at Homestead.
One hundred fifty M2 ears were obtained for Treatment 1, 197 for Treatment
2, and 94 for the control. M2 progenies were grown at Johnston, Iowa. Self-pollinations
were made on those plants that silked and shed pollen during the long-day
conditions at Johnston during summer 1986. Following is the flowering distribution
of the M2 families grown at Johnston in Growing Degree Units (GDU) = (degree
F max + degree F min) divided by 2; 86F is upper max and 50F is lowest
|Less than 1599 GDU||3||1||0|
|more than 1900 GDU||10||23||8|
Treatment 1 resulted in 66 families that flowered between 1550 and 1900 GDU's. Treatment 2 resulted in 63 families that flowered between 1550 and 1900 GDU's. The control treatment had 20 families that flowered less than 1900 GDU's. The earliest control family that flowered was 130 GDU's later than the earliest family from either Treatment 1 or Treatment 2. M3 seed was harvested from these plants and planted at Johnston in summer 1987. One hundred seven M3 (S2) families were selected based on agronomic appearance and early flowering. These are being topcrossed with 2 elite proprietary inbreds this winter at Homestead. Progenies will be yield tested during summer 1988 at 3 Pioneer research stations. A total of 3 locations for each station will be grown. Each location will have 2 replications of each topcross combination, resulting in 18 reps of data for each topcross/S2 combination. The best performing S2's will be advanced either as inbreds per se or will be used as a source of germplasm for introgression into elite inbred lines.
M.C. Albertsen and W Salhuana
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