Some plants are monoecious and some are dioecious. Some plant have flowers which are mixture of the above three categories. For example, andromonoecious plants have both hermaphrodite and male flowers etc.
Allen studied sex chromosomes in plants in the liverwort. Male gametophytes had 7 autosomes and an Y chromosome while the female gametophyte had 7 autosomes and an X-chromosome. After the union of the sperm and egg a diploid zygote, sporophyte, is formed which through reduction division, produces the haploid spores carrying only one of the sex chromosomes. These mature into male and female gametophytes. Mechanism of sex determination is relatively better known on dioecious plant species and they are essentially similar to those found in animals. These mechanisms are the same three types as described earlier.
1. Environmental sex determination
Sex determination in some plants is greatly affected by the environment. Equisetum plants grown under optimum conditions develop as females, while those grow under adverse conditions become males. In many other plants, example., melons, cucumber, cannabis, etc., the sex of flower is affected by temperature, day length, ethylene, gibberellic acid, Ca++, Mg++ ions etc. Treatment with ethylene or GA3 promotes the production of female flowers. In cannabis, a treatment with GA3 induces the development of only female flowers in male plants. In papaya, many male plants start producing bisexual flowers after a year or so; the reason for such a change is unknown. In general, environment has pronounced effect much more on male plants than on female plants, the latter being almost immune to such effects.
2. Chromosomal sex determination
Majority of the plants are monoecious and therefore have no question of sex determination. Many dioecious plants do not show distinct sex chromosomes. But some species have the XY type of sex determination, and a few are known with XO and ZW methods as in mammals.
Allen(1940) published a list of species in which sex chromosomes had been reported. Westergard(1950) prepared a sex wise list of plant species that are heteromorphic chromosomally. For some diocious plant species, the heteromorphic XY chromosome as obtained in different plant species is listed below.
Chromosome constitution of two sexes in some dioecious plant species.
1. Male heterogametic(XX female, XY male): Example, Melandrin album.
2. Male heterogametic(XX female, XO male): Example, Vallisneria
3. Male heterogametic(Male with extra chromosome): Example, Phoradendron flavescens
4. Female heterogametic(XY female, XX male): Example, Fragaria elatior
5. Compound chromosomes(XX female, XY1Y2 male): Rumex acetosa
6. No sex chromosomes(gene controlled): Ribes alpinum
7. No sex chromosomes(In male, translocation heterozygosity): Example, Viscum fischeri