Does Flower Abortion Represent Lost Yield?
DENNIS B. ELGI
LEXINGTON, KENTUCKY
Soybean is notorious for its high levels of flower and pod abortion. Fifty percent abortion can occur in high-yield environments. This loss of reproductive structures is often considered ‘lost yield’ as in ‘yield would have been higher without abortion’. Is this the correct conclusion? Does the abortion of reproductive structures represent lost yield? Not necessarily!
Grain crops are wildly optimistic about their potential to produce yield. They routinely produce many more reproductive structures than needed and then they discard (abort) excess structures to bring seed number in balance with the capacity of the crop to produce yield. This process is very obvious in soybean, but it occurs in most grain crops, except, perhaps, for modern corn hybrids and sunflower. Being wildly optimistic allows the crop to always have enough flowers to capitalize on high-yield environments.
Abortion reins in the wild optimism so that the number of seeds is in balance with the productivity of the crop. If the crop produced too many seeds, they would be smaller than normal, which could, in extreme cases, affect market quality. Too few seeds would limit yield. Yield is determined by the productivity of the crop, so abortion to balance seed number and productivity does not represent ‘lost yield’.
A magic potion that simply stopped abortion would result in more seeds, but they would be smaller, and yield would be the same because productivity did not change. Such a potion was tested as a yield-enhancing growth regulator in the 1960’s. TIBA (2,3,5 triiodobenzoic acid) increased seed number when sprayed on soybean fields, but the seeds were smaller and there was no effect on yield. The growth regulator reduced abortion, but it did not increase crop productivity, so yield stayed the same.
Yield can be limited if the number of flowers and seeds is too small to accommodate the productivity of the crop (abortion is zero). There is nothing more tragic than not having enough seeds to translate the productivity of the crop into yield. This never happens in soybean, but it can happen in corn when the ear is completely filled to the tip. Well-filled corn ears are often taken as a sign of high yield, but they can also indicate that there were not enough flowers and seeds to accommodate the productivity of the crop. In Crop Physiology terms - yield was limited by the size of the sink (the number of seeds). A higher population (more seeds) might have produced even more yield.
Stress during seed set will increase abortion and reduce seed number and yield, but abortion is only a symptom. The problem is the stress (e.g., moisture stress) that reduced productivity, not abortion. The stress must be removed to reduce abortion and restore the lost yield.
The reproductive structures that abort are very crop specific. Flowers or small pods abort in soybean, but full-size pods seldom abort. Individual seeds in a pod may also abort (a 3-seeded pod may contain only 2 seeds). All of the tillers produced by a wheat plant may not survive to produce spikes and some floret primordia in the spike may fail to develop into fertile flowers. Flowers on the corn ear also abort, but other options for adjustment are limited on modern corn hybrids.
Teosinte, the wild progenitor of corn, tillered and produced multiple ears on the main stem, but tillering in modern hybrids is greatly reduced and only an occasional hybrid produces a second ear (although ear primordia are still present at all nodes below the ear node). The producer must increase plant population to increase flowers per acre and avoid the dreaded sink limitation. Consequently, plant population is much more important in corn than it is in grain crops that are more flexible (e.g., soybean or wheat).
Environmental conditions can directly increase reproductive failure. High temperatures can reduce pollination in corn and soybean, as can boron deficiency in wheat. Nitrogen deficiency or drought stress can cause pollen shed in corn to occur before the silks appear. In these situations, reproductive failure reduces seed number and directly affects yield, sometimes catastrophically. Fortunately, these disruptions are not common or widespread.
The abortion of reproductive structures is a normal part of the yield production process and is not necessarily an indicator of lost yield. It usually represents the adjustment of seed number to the capacity of the crop to fill the seeds, helping to insure the production of normal-sized seeds. ∆
DENNIS B. ELGI: Dept. of Plant and Soil Science, University of Kentucky