Sexuality in plants – he or she? or both?

Most people would refer plants to female or she and mostly describe them as a symbol of femininity, resilience, and fertility in literacy because regardless of their sex, they are capable of producing seedlings or baby plants either through sexual or asexual reproduction. Similar to animals, plants have genders. However, it is not as clear and straightforward as compared to animals and humans. The variations of sex in plants are extensive which are reflected as flexibility throughout the plant kingdom. Possibly a result of co-evolution with pollinators in responding to the constantly changing environments. Flowers, as the reproductive organs that usually have bright colored petals, are actually morphologically modified leaves. Though the beauty itself is fascinating, the purpose is utilitarian. In order to attract pollinators, sometimes they seduce insects by mimicking the shape of insects and even producing insect hormones for a fake romance. Successful self- or cross-pollination leads to the formation of seed, more specifically, the result of the fusion of male and female gametes, which are sperm cells from the anther in the stamen and eggs from the ovary in the pistil. With an emphasis on flowers, fruits, and seeds, this article does not cover spore-producing plants like ferns and liverworts as their life cycles are more complicated which involves the alternation of generations or commonly known as metagenesis. Nevertheless, they have male and female parts like other higher plants.

Monoecious plant species have both male and female reproductive organs on the same individual plant. The flowers can be either perfect flowers or male and female imperfect flowers. Plants like pumpkin and kiwano (horned melon) have separate male and female flowers but on the same plant. Therefore, successful pollination must be conducted by pollinating the female flower from a male flower on the same plant with the help of bees or ants. The flowers are unisexual, imperfect, and incomplete, which means a lack of one of the sex organs, and the morphological difference is obvious. The male lacks pistil but has single or multiple protruding stamens coming out of the flower, whereas the female flower does not have stamen but a stigma and a swollen ovary shown in the shape of early fruit like pumpkin flower.

Hermaphrodite plants have both male and female reproductive organs on the same flower, the perfect flowers that represent intersexuality. Most plants are hermaphrodites including many grasses and dicots such as roses, lilies, and tulips. The advantage of being hermaphrodite is that self-pollination is possible with its own stamen and stigma, meanwhile not necessary to have colorful petals to attract pollinators. Pollination can be done through wind or water. Some of the most important crops such as wheat, soybean, rice, and grapes are self-pollination hermaphrodites and do not have showy attractive flowers.

However, self-incompatibility is widely observed in nature which is the mechanism that prevents pollen from fertilizing flowers of the same plant. Self-incompatibility is often observed in plant families as Solanaceae and Rosaceae. Some plant species also develop special flower structures or different flowering timing of male and female flowers to avoid self-pollination. For example, the male flower of corn is on the very top of the plant which is not immediately accessible for its female flowers. Sunflower has a large inflorescence that consists of rows of male and female flowers, but the development of the staminate stage is earlier than the pistillate stage.

Dioecious plants have male and female reproductive organs on two separate individuals, whereby categorized as male and female plants, which is similar to humans. The male plant produces flowers only have stamen, whereas the female plant that produces flowers only have stigma. Cross-pollination between male and female plants will produce viable seed. The flowers usually offer nectar as a reward, and with bright-colored petals to attract pollinators like bees and butterflies to pollinate from a plant to another plant. Dioecious fruit trees usually require a nearby tree of the opposite sex for fruiting, and the tree that ultimately bears fruit is the female one, such as apple, pear, and peach. Dioecious plants such as fig (Ficus carica L.) require specific pollinators, some certain types of wasps only found in certain areas, and it could be the major constraint for the areas that have no such specific pollinators. Another example of a dioecious plant is Nepenthes sp. in which some of them are endangered and facing extinguish because male and female plants are not flowering at the same time. Some other well-known dioecious plants include holly, asparagus, dates, mulberry, ginkgo, persimmons, spinach, kiwi, cannabis, willow, et al.

The phenomenon that a plant changes its sex is referred to as environmental sex determination (ESD). Also observed in reptiles like crocodiles, snakes, lizards, and turtles, in which gender can be manipulated by altering the temperature of the incubator. This is useful in population management to maintain and balance the ratio of gender to avoid extinction. For instance, incubation of turtle eggs below eighty-two Fahrenheit will result in male turtles. In plants, it is referred to as gyndioecy in which certain flowering plant species are found to have both female and hermaphrodite plants coexist within a population. Some people believe it is the evolutionary intermediate stage between hermaphrodite plants and dioecious populations.

Gynodioecious plants can change their sex depending on the environment. Papaya, a gynodioecious tropical fruit tree that is botanically known as Carica papaya, can produce seeded fruits by pollination, or seedless fruit without pollination. However, male papaya remains as male during hot weather but can change to female after topping. The flexibility is likely bestowed by their special nascent sex chromosomes, which offers a unique opportunity to reveal the mystery of sex-changing. A papaya tree could be a male tree (♂) that represented by X and Y chromosome, a female tree (♀) that has two X chromosomes, hermaphrodite (⚨) that consists of X and a slightly different Y chromosome (which denoted as Yh), hermaphrodite that blooms only female flowers, and hermaphrodite that blooms only male flowers. The phenomenon that gender changes to another sex in response to their environment is known as sex lability, or gender diphasy. The sex of diphasic plants could be switching between male and female, one sex during a time and then the other sex becomes dominant from season to season.

Flowering is the most dedicated, complicated, energy-consuming, and yet most short-period among all physiological events in plants. The floral development has been partially revealed by the ABC modle including a set of functional overlapped genes, which is seamlessly clocked up with multiple signals in a dedicated way. Plant hormones including auxin, gibberellin, cytokinin, and ethylene are reported involved in sex determination and differentiation, which offers possibilities for manipulation. Sex conversion could be achieved for some plant species in lab conditions and in fields. Hermaphrodite is unlikely to occur naturally but can be induced in plant tissue culture using silver nitrate which is an ethylene inhibitor. Male plants are important for crop improvement for breeding, which can be intentionally induced on a female plant and crossbreed other female plants. Successful pollination aiming for high yield can be achieved with control of the ratio of female to male as female plants are often favored in crop production. Naturally or gibberellin-induced stimulative parthenocarpy, which is the production of seedless fruits without pollination is so far one of the most important practices in the fruit industry. Examples of parthenocarpy including seedless banana, pineapple, orange, grape, watermelon, cucumber, and tomato.

Further reading:

Geber, M. A., Dawson, T. E., & Delph, L. F. (Eds.). (2012). Gender and sexual dimorphism in flowering plants. Springer Science & Business Media.