The increase in the capitula of zinnia plants (Zinnia violacea Cav.) was investigated by analyzing the production of shoots. The effects of removing the buds for capitula and application of BA on the production of shoots were also evaluated. It took ≈40 to 50 days from the emergence of axillary buds to the opening of the capitula at the apices of the shoots from these axillary buds. The application of BA shortened the number of days for the same process. The difference in the number of days from emergence of the axillary buds to that of the first descendant axillary buds was ≈25. The total number of capitula opened was greater in plants with the bud removal treatment than in intact plants. Chemical name used: (N-phenylmethyl)-1H-purine-6-amine (BA).
The causes of low production of seeds capable of producing double-flowered plants and of high production of seeds capable of producing single-flowered plants were investigated in zinnias (Zinnia violaceu Cav.). Poor pollination was a major cause of the low seed set. A tubular floret produced abundant pollen; however, the pollen flow to ray stigmas was limited due to the infrequent visitation by pollinators. Moreover, in the double-flowered capitula, newly opened ray petals overlapped on the pistils that unfolded the previous day. These phenomena were considered to cause low seed set in double-flowered plants. Actually, capitula with more tubular florets produced more seeds than those with fewer tubular florets. Pollen germination and plants near zinnias had additional possible influences on seed production of double-flowered zinnias.
The seed producing system in viola (Viola ×cornuta) was investigated to improve seed yield and to save labor. In a flower five anthers sequentially dehisced; pollen grains were continuously supplied to the anterior petal, which played a significant role in pollination, throughout the flowering period. Evidence from pollen and ovule number suggests that the species is facultative autogamy. Each flower opened more than 10 days was independent of the success in fertilization and kept seed producing ability during the flower longevity period. Pollen grains also maintained viability during the flower longevity period. Pollinators were indispensable for pollination of viola, but pollination in viola was done by a different mechanism from the typical insect-mediated pollination that sticky pollen grains adhere to the exposed stigmas. Pollen grains, accumulated around the entrance of the stigmatic cavity, entered into the cavity by the movement of pollinators. Although the visitation of pollinators was occasional, solitary bees primarily contributed to the pollination of viola. On the other hand, germination of pollen grains on the stigmatic surface was under 50%. Seed set was much lower than the germination percentage of pollen grains. A viola flower had the ability for additional pollinations and fertilization for some days after the fertilization success in some ovules in the flower. This characteristic suggested that repeated pollination is effective to increase the number of mature seeds in a capsule.
The seed-producing system of salvias (Salvia splendens Sello) was investigated. The number of florets opening per day per plant increased with the increase in days from anthesis. Pollinators that effectively worked were small-sized insects. High pollen viability was observed on the stigmatic surface, and pollen tubes reached the ovules within 3 hours after pollination. Fertilized ovules became mature seeds within 25 days after fertilization. Pollination within 1 day after opening of florets resulted in a high percentage of seed setting. The pollen-ovule ratios indicated that salvias were facultative xenogamous. Actually, the salvias had heterostyle florets and the ability to set seeds without pollinators.
In double-flowered (Dbf) zinnias (Zinnia violacea Cav.), seed lots inevitably contain seeds for single-flowered (Sgf) plants. A Sgf capitulum is composed of many disk florets and some ray florets with wide ovaries. A Dbf capitulum is composed of many ray florets with ovaries of various widths. Based on differences in morphology of florets that develop in Sgf and Dbf capitula and the fact that doubling is a genetically controlled trait, seeds of two cultivars of Dbf zinnias were classified into three groups by morphology of the pericarps: thin (RT) and wide (RW) seeds (achenes) produced by ray florets, and seeds (D) produced by disk florets. Plants were grown from these seeds and the ornamental value of their capitula was evaluated. Percentage of plants with Dbf capitula and the number of ray florets in a capitulum were highest for plants from RT seeds. Thus, the ornamental value of Dbf zinnias could be heightened by selecting RT seeds.
Seeds of `Kumamotonokagayaki'(Kk), `Goldenball' (Gb), and `Purple Gem' (PG) (Zinnia violacea Cav.) were harvested at various stages of maturity and subsequent seed and plant performance were evaluated. The largest increase in ovule or embryo length and width occurred from 0 to 10 days after pollination (DAP). The seed weight was unchanged after 23, 25, and 30 DAP for Kk, Gb, and PG, respectively. The pericarp color was completely green from seeds harvested 20 DAP, while the seeds harvested from 30 to 35 DAP contained a mixture of green and brown seeds. Pericarp color change from green to brown was not a reliable index to harvest seeds. The percentage germination increased from seeds harvested from 10 to 19 DAP for all cultivars. From 20 to 40 DAP, germination was unaffected and >90%, while the rate of germination as measured by days to visible germination decreased slightly. Seeds harvested 20 to 40 DAP produced plants with similar growth and quality characteristics. Considering a 20- to 30-day difference between the first and last floret to open in a capitulum, capitula should be harvested 50 days after anthesis for good seed quality and to prevent shattering.
Daiichiro Miyajima and Masaaki Nakayama
Composition of capitula and their making by florets of zinnia (Zinnia violacea Cav.) were analyzed to improve seed production. For each cultivar, mature capitula were classified into three types based on shape. A capitulum was made of the accumulation of <20 newly opened florets per day for >15 days. The total number of florets per capitulum was 210 to 330, 220 to 290, and 160 to 240 for the two pumila double cultivars Kumamotonokagayaki and Snowball and for the pompon cultivar Purple Zem, respectively. The numbers of tubular florets were negatively correlated with the numbers of ray florets. Ornamentally superior capitula, which were the basic capitulum types for pumila doubles and pompons, had more ray florets and fewer tubular florets than the ornamentally inferior capitulum (the single-flowered type). Results indicate that maintaining a high percentage of plants with double-flowered capitula may decrease seed yield.