In the past, most studies dealing with the effects of gibberellic acid (GA) on genetic dwarfs of corn have dealt primarily with the vegetative characteristics of
the plant and suppression of tassel development. The present investigations sought not only to repeat some of this work both in the field and under controlled environmental conditions, but also to re-examine some of the general observations with particular attention being paid to the cob, a structure which has been all but overlooked in this particular mutant.
Plants were treated with serial concentrations (10-6 M to 10-3 M) of GA on the average of every four days. Treatments began at the time of emergence of the first leaf through the coleoptile, and ceased at the time of emergence of the first silk emergence. Each plant received a total of 100 ul of GA per visible leaf per treatment. The application was divided between the crown and at the ligule-leaf sheath junction of the expanded leaves.
Analysis of the data to be reported resulted from experiments initiated at the laboratories of Drs. Richard Greyson and Dave Walden at the University of Western Ontario in 1978 and continued at the University of Louisville in 1980-81.
In both controlled and field conditions, only the highest concentration, 10-3 M, of GA severely reduced the development of the tassels. Unlike some previous reports, the inhibition was not 100%. Inhibition ranged from completely sterile, poorly developed tassels to tassels that were 70-80% inhibited but still able to produce and shed pollen. Field grown plants exhibited a greater degree of inhibition than plants grown under controlled conditions. The inhibitory effects of GA were first seen as a reduction in the number of laterals on the tassel and a reduction in the number of florets reaching maturity on each lateral. Tassels produced on plants treated with 10-4 and 10-5 M GA had twice the number of laterals as the control plants. The main axis of the tassel was invariably the last portion of the tassel to show any reduction in development. The lower concentrations of GA produced no measurable reduction in tassel development, in fact, there was an increase in the number of tassel laterals at the lower GA concentrations.
Anthers produced on the 10-3 M treated plants were almost 20% smaller than those produced on control plants. When pollen grains from 10-3 M GA-treated plants were plated onto pollen germination medium, fewer grains germinated and a large number of these pollen grains spontaneously ruptured at a much higher frequency than grains from the control anthers. Studies are presently underway to examine the structure of these anthers and pollen grains to determine if structural variations can account for the apparent weakness of this pollen.
Previously reported experiments with GA reversible mutants rarely described effects on cob development. Anther ear is a mutant in which the cob contains, in addition to the female flowers, fully-formed, post-meiotic, non-dehiscent anthers. Apparently the normal genetic timing mechanism that causes the male parts of the flower to spontaneously abort in early development fails to act correctly. Coe and Neuffer have reported that GA-treated plants produce cobs that are "reverted," that is, they have gynoecious flowers rather than the expected perfect (hermaphroditic) flowers peculiar, but not unique, to this mutant. Present experiments have confirmed this observation, but have made a more detailed analysis of the cobs.
Close observation of developing cobs reveals that the expression of sex within the flower appears as a gradient within the cob. The terminal portion of the cob is a sizable extension that contains only male flowers at maturity. These flowers initiate ovules in the early stages of development, and as in a tassel, abort early in development. These male flowers contained three or six mature anthers, but were never observed to shed pollen even though they were not enclosed in the cob and were exposed to the air. The major portion of the cob contains the normally expected, for an1, perfect flowers.
For the purpose of analysis, cobs were divided into three regions: Region 1, (R-1) terminal portion containing only male flowers (ovular stump may be present); Region 2, (R-2) containing male and female floral parts (hermaphroditic); and Region 3, (R-3) containing only female flowers (no evidence of stamens). The R-1 region of the cob normally dries up and is usually lost or discarded at harvest, and thus may be overlooked by someone examining only field-dried material.
As can be seen in Table 1, the application of gibberellin to plants of this mutant has a marked effect upon the sexual expression in the cobs.
Cobs from plants treated with 10-3 M GA were markedly smaller than those on the control. Much of this reduction was due to reversion of these cobs and the concomitant reduction of the R-1 region of the cob. Ninety-five percent of the 10-3 M GA-treated cobs were totally reverted. Dissection of these cobs under the microscope revealed no evidence of stamens. Five percent of these cobs were only partially reverted, and dissection revealed the presence of aborted stamens of almost normal size. The plants treated with 10-4 M produced cobs that were very much like the partially reverted cob produced at the higher concentration. A casual observation might indicate that these cobs were reverted; however, observation with a hand lens or microscope revealed numerous stamens that had aborted prior to meiosis and were now flaccid, semi-transparent structures similar to those reported in vitro (MNL 55:116). At 10-5 M GA most of the stamens appeared normal and were post-meiotic, but each cob still carried a large number of poorly developed or aborted stamens in the R-1/R-2 interface region. The cobs produced with 10-6 M GA were indistinguishable from the control. Thus the suppression of maleness as expressed in the cobs has the characteristics of a concentration related effect. As the concentration of GA increases, there is a concomitant decrease in the maleness and an increase in femaleness which is measurable in the sex expression gradient seen in these cobs.
Arnold J. Karpoff
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