GARDENING: Only the strong survive the ‘June (fruit) drop’
Free Press Gardening Columnist
Now comes the season for “June drop.” Even though this happens in May, July and July, most people refer to the time their apples and pears drop a portion of their fruit as June drop. I was reminded of this season of the year when walking my dog in our neighborhood park. The female honeylocust trees were dropping tiny seed pods. The male honeylocust trees where hanging onto their catkins; but the one-half-inch pea pod-like fruit of the female trees were bombarding us as they were shed by the thousands by their mothers.
June Drop occurs when the trees are unable to maintain the number of fruit they set earlier in the year. When the tree is stressed and can’t provide the new fruit (fruit in this regard refers to anything that is trying to set seed) with the nutrients necessary for development; when pollination hasn’t occurred; or fertilization of the proper number of seeds has not been possible. Pollination problems can occur when conditions are not appropriate for bees, beetles, flies and other insects to fly and the pollen is not transferred to the female receptive organ. Fertilization is when the pollen grain “germinates” and the male genetic material penetrates the ovule and fuses with the female genetic material to form the seed. The continued development of the fruit typically requires a certain number of seeds to start development.
If you have ever seen a lopsided apple and cut it open you will find that the side that is misshapen is lacking seed due to improper fertilization. The dropping of fruit can be the result of the killing of the ovaries by chilling conditions. Sometimes you will find fruit with a band of rusty cells around its middle. This is due to frost damage and the killing of the young embryo before it can form seed. Most trees will drop these frost-damaged fruits due to the lack of seed development, but in the case of Bartlett pear, the fruit can still develop. Bartlett is one fruit that does not need pollination to set a crop. Wind can also cause fruit drop by drying out the flower parts before pollination or fertilization takes place.
As alluded to above, pollination is only part of the process required to form fruit, whether the fruit is an apple or a kernel on an ear of corn. The amount of time and energy needed for pollen to do its job of fertilizing the female megagametophyte is quite considerable. When a pollen grain lands on one strand of silk on an ear of corn, it has to germinate and produce a germ tube that extends all the way down the strand to the female part of the plant. This germ tube may need to be 10 inches in length to complete its task. When it reaches its goal, it releases two sperm nuclei into the ovule. One fertilizes the egg cell and the other fertilizes two female nuclei that form the endosperm. When you cut open a kernel of corn, you will find a tiny corn plant primed for growth. The endosperm is the starch material taking up most of the volume of the seed, and provides the energy for the tiny plant to grow until it can start producing its own food from its own leaves. The seed of orchids are exceptions; they have very little stored energy and depend on fungi to provide their energy needs during germination.
If you split open a bean seed, you will find a tiny plant with a rudimentary root and leaves. As with the corn seed, the majority of the bean, as with most other seeds, is filled with starch. The structures containing the food supply for the embryotic plant are called cotyledons. Corn and other grasses have only one cotyledon and thus are called monocots. Lily and orchids are also monocots. Beans contain two cotyledons and are thus referred to as dicots. Most of the vegetables you grow other than garlic, corn and a few others are dicots.
After fertilization, the seed and its surrounding tissue become a sink for nutrients and hormones. Sugars flow into the fruit. The amount of nutrient flowing to the fruit depends on the health of the fruit and this is determined in most cases by the number of seeds developing in the fruit. If the number of fruit is deficient due to lack of fertilization, the fruit is liable to drop from the tree. That is what was happening the other day when my dog and I were walking under the honeylocust trees in our local park.
Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu.
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