Studies on perennial maize
--Y.C. Ting
In the spring of this year, I received a package of maize kernels of both diploid and tetraploid varieties from Dr. D.L. Shaver, Cornnuts, Inc., Salinas, CA. I was very excited about these acquisitions because I have tried several times without success to transform annual maize into perennial through teosinte introgression. The failure to achieve my goal was partly due to the severe cold in the Boston area. Now the gift from Dr. Shaver gave me another chance to do experiments with maize perennialism. Table 1 depicts the results.
Table 1. Characteristics of perennial maize.
| Characteristic | 2n | 4n |
| Earliness in germination | 10 days | 7 days |
| Tiller proliferation | 7.6(6)* | 2.4(10)* |
| Ovule fertility | 82%(381)** | 95%(535)** |
| Anthocyanin gene (b1) | No | Yes |
| Ga gene | No | Yes |
| Propagule regeneration | No | 20%(10)* |
The kernels of the two varieties of perennial maize were sown in pots in the greenhouse in early January. It took seven days for the tetraploids to germinate and 10 days for the diploids. When the plants were about 10 weeks old, with well-developed tillers, their main stalks (culms) were cut off at a point above the soil surface. After cutting, the tillers continued to grow vigorously. As soon as the plants had reached 20 weeks old, they were transplanted to the field; this was in late May. Ten tetraploids and six diploids were maintained for observations. However, no new tillers of these plants generated in the field.
On average, the number of tillers per plant varied in diploids vs. tetraploids when counts were made at the 25-week stage. There were eight tillers for diploids, and only two for tetraploids. Since tillering is a required characteristic for maize perennialism, attention was focused on it. In addition, ovule fertility of these plants was also examined. It was found that 95 percent of the ovules of tetraploids set well-developed kernels upon selfing, while only 82 percent of the ovules of diploids produced kernels.
Regarding synthesis of anthocyanin pigments, the tetraploids were heterozygous for the b1 gene located on chromosome 2. They segregated into purple and green plants in the selfed progeny, while the diploids did not. In order to have triploid perennial maize, the above two varieties were hybridized with the tetraploids as female parent. Nineteen ears of the tetraploids were pollinated with pollen from three different diploids. Only three ears bore kernels. Five presumptive triploid kernels were obtained. It seems that the tetraploids might possess a gametophyte factor in chromosome 4, but not the diploids.
Another required character for maize perennialism is the growth or regeneration of propagules into tillers. This was also investigated in the greenhouse. The procedure was to decapitate the seedlings at various stages of growth. Results showed that when decapitations were applied to the four-week-old plants, no growth of the propagules was observed, even though a drop of cytobeinin, 6 BAP (6-benzyl-aminopurine, 5 mg/l), was dripped on the surface of cuttings. However, when the plants were six weeks old, 20 percent of the decapitated plants of tetraploids grew into plantlets using the same treatment (details are not provided here). In contrast, no response was found for the diploid plants. On average, 10 plants were employed in this test for each of the two ploidy levels.
Before frost came, all the above healthy field-grown plants were moved to the greenhouse. If their growth, or regeneration of propagules, is continued next year, it would prove that the perennialism is present in these plants.
Perennial maize is of great importance in crop improvement and production. If its basic genetics, particularly at the molecular level, is fully explored, new directions in maize farming will be established in the near future. For instance, like sugar cane, once it is planted, continued harvests can be reaped for several years.
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