February is Valentine’s month, but also that of starting to feel impatient about gardening and all the yummy veggies and fruits that will come through the season. Those two things (Valentine’s, and fruits and veggies) are actually really connected, and in today’s post, we will dive into how that is so.
Valentine’s and fruits and veggies are connected? I don’t get it…
Have you asked yourself how those fruits you see are formed? In fact, this has a lot to do with love (well, plant love) and partner compatibility… quite a bit like Valentine’s couples! Like humans, plants have special organs they use to reproduce, which are all in the flowers (Figure 1).
When plants reproduce, their offspring are their seeds. Until here all is nice and good, however, a plant’s problem is that because it can’t move, if all the seeds it produces were to just to fall right under it, the mother plant would soon be surrounded by her offspring, which would then compete with each other and herself. To deal with this problem, and because plants are cool, they have evolved some super-smart strategies to help their seeds be dispersed: fruits! (Figure 2)
In wild plants (and many cultivated ones), the fruit is the packet that will carry or disperse the seeds, and among all fruits, the fleshy ones are actually a ‘bait’ for seed dispersers. In fact, these fleshy parts are usually sweet and nutritious, and thus attract animals that eat this ‘seed packet’ (often along with the seeds). However, because an animal’s digestive system is built the way it is, eating the fruit will usually also mean dispersing the seeds in a different place, when those seeds leave the animal’s body. So, you see, plants having offspring and fruits are directly related, like Valentine’s and my veggies and fruits!
Oooh… so, plants can only make fruit if there has been pollination?
Making a fruit requires a lot of energy from the plant (think about all those sugars and colors that go into that delicious tomato!). This means that it usually is the case that fruits will only form if pollination has happened and seeds have formed. So, if one wants a garden to produce fruits and vegetables, it is very likely that pollination and seed formation will need to happen for it to be productive.
Plants produce seeds and are pollinated in many different ways (see this blog post for examples), but it is pretty common that plants will produce more seeds and larger fruits if they receive pollen from a plant individual that is distantly related. In fact, as for many animals, crossing among too closely-related individuals can lead to genetic diseases and poor health. And this is why pollinators are so important; they allow pollen from different individuals to be transferred among plants of the same species and allow for healthy seeds.
Is this why I sometimes need many similar plants to produce fruit?
Yes! Many groups of plants have developed reproductive strategies that advantage fertilization by pollen from plants that are more distantly related over those from themselves or a very closely-related individual. We say that these plants need cross-pollination for producing seeds (and fruit).
On the other hand, there are some plants that are able to make fruits while receiving pollen from themselves or from closely-related individuals. Because these plants are equally exposed to the health problems associated with producing seeds with a closely-related individual, they have developed other strategies that can help them reduce the occurrence of such an event; for example, their seeds can disperse very long distances.
What are the plants that need cross-pollination?
Apples, pears, almonds, pistachio, some cherries, apricots, figs, and paw-paws are all plants that need to receive pollen from another plant of the same species to make fruits. Further, note that for many of these plants it is not just sufficient to have another plant of the same species; this plant has to be of a different variety and has to be present at a relatively short distance.
Unlike those mentioned above, tomatoes, bell peppers, eggplants, grapes, currants, raspberries, and gooseberries can self-pollinate, and thus do not require other surrounding plants of the same species to produce fruits.
Many plants are able to produce fruit by self-pollinating but are far more productive if they are cultivated along with other plants of the same species. This is the case of blueberries, huckleberries, and persimmon.
Finally, some plants, like kiwi, have male and female flowers on different individuals. In these species, although the only fruit-bearing individuals are the female plants, the fruit will only form if a male is present close by.
To help disperse their seeds, some plants have evolved pretty extreme strategies!
By Anahí Espíndola, Assistant Professor, Department of Entomology, Universit of Maryland, College Park. See more posts by Anahí.