Why would any self-respecting farmer want to pry into the secret sex life of a wheat plant? Why, to make better decisions of course! Decisions on early-season nutrition and need for and timing of pest control measures are reasons to pay closer attention to what’s going on in the bedrooms of the wheat plant.
We know that nutrition is extremely important to a growing fetus, be it animal or human, why would we expect it to be any different for a seed? Understanding when a seed is initiated, how it develops and how susceptible it is to stress helps us feed and maintain as many spikelets and florets as possible, then fill them to the max to achieve your target yield goals.
Directions for more and more input products, especially fungicides provide us with timings such as apply at early flowering. Scouting recommendations for Orange Wheat Blossom midge call for regular monitoring of wheat fields between heading and flowering, both to identify a wheat midge infestation and properly time the application of appropriate control products. What about fusarium head blight, a serious disease of wheat where the optimum stage for fungicide application is during early flowering (GS 61 or 10 to 15 per cent flowering). Then there is ergot. What is meant by anthesis? What and when, exactly, is flowering, what should I be looking for, etc., requires an understanding of wheat plant biology.
ANATOMY OF A WHEAT PLANT
First, we need to get a bit of terminology straight. The spike is commonly referred to as the head or ear. Each seed is produced after pollination of a floret and a group of florets is termed a spikelet. The anthers are the male parts or the reproductive system that produce the pollen and are attached to the base of the ovule by a filament. The anthers and filament as a unit are termed the stamens. The female parts are the stigma that receives the pollen and the ovule at the base of the stigma that once fertilized becomes the seed.
A wheat plant has the genetic potential to produce eight to 12 florets per spikelet in the centre of the spike and six to eight at the basal and distal spikelets of the spike. There is one spikelet per rachis node and by the time pollination is complete there are only three to six florets per spikelet. The death of florets and spikelets during development is high because of stress and shortages of nutrients. The number of kernels per plant is established 20 to 30 days before flowering to 10 days after anthesis. Each spike generally produces 20 to 30 spikelets, fewer if growing conditions are extremely poor.
FERTILIZATION OF WHEAT
1)By the booting stage (BBCH* GS 41-49), the reproductive parts start to become visible. While in the boot, the anthers are small and green but pollen production is well underway by the end of boot. The stigma is immature and the ovule is tiny. Note: At Agri- Trend Agrology, we are converting all our growth stage discussions to the BBCH system. The BBCH system attempts to simplify plant staging by following the same principles for all crop plants as well as weeds.
2)By GS 50, the tips of the awns are visible, the ovules, immature anthers and feathery stigma start to expand. (see Photo 1) By GS 60, the head is fully emerged. The stigma continues to grow and becomes sticky in preparation for arrival of the pollen. As the anthers mature, pollen pores at the tips of the anthers become visible. These pollen pores open when the pollen is mature and pollen bubbles out in a process called pollination. The point at which pollen is released from the anther is termed anthesis. Pollination starts in the florets located near the middle of the spike about three to four days after head emergence. The complete pollination process takes three to six days per spike. Because each plant has several spikes at differing maturities due to tillering, the pollination process for the whole plant lasts about 10-12 days (GS 61 to 69).
3)Once the pollen is released, it sticks to the feathery stigmatic branches and from here, the pollen forms a pollen tube down the stigma. Fertilization occurs when the genetic material from the pollen combines with the genetic material in the ovary and an embryo results.
4)After the anthers have released all their pollen, the anther filament elongates and pushes the anther outside the glumes (see Photo 2). The ejection of the anther, which becomes visible on the exterior of the floret is commonly termed flowering. At this point, the floret is about one to two days postanthesis. The anthers are spent and the embryo is starting to grow.
PUTTING KNOWLEDGE TO USE
How does knowing all the intimate details of wheat reproduction help me on the farm? By maximizing the four ways to increase crop yields without changing genetics and land area:
Grow more spikes: More heads per plant or more plants per square foot.
Grow more seeds: Increase survival of florets and spikelets, usually related to a shortage of carbohydrates, nutrients and stress.
Grow bigger seeds: Timing of nutrient availability is critical for both establishing reproductive potential and filling of that potential after fertilization.
Or a combination of all three.
None of this, in my opinion, is possible without soil and tissue testing along with a good scouting program to provide optimal plant heath. Don’t wait for the telltale signs of nutrient deficiency before you take action, by the time you see symptoms, embryos and potential has been lost. Load the crop with nutrients early in the season, by the four to five leaf stage (GS 14 to 15) to support both the vegetative and reproductive growth process, and protect it against pests and stresses through its life.
It is critical to have good nutrition to support the rapid demand for carbohydrates and nutrients from the boot stage until 10 days after anthesis to prevent loss of reproductive capacity and fill the seed. Once the crop is past the four to five leaf stage, it is difficult to correct large macro nutrient deficiencies required for protein synthesis and optimal plant growth. Additionally, since the seed is packed with starch during filling, adequate photosynthesis is necessary to complete seed fill, and this is largely facilitated by the flag and penultimate leaves. Maintaining these leaves in a healthy state along with good late season nutrient levels until complete seed fill is critical to producing large nutrient dense seed.
STAGING AND MIDGE
The Orange Wheat Blossom midge females lay their eggs on the wheat spikelet. The females usually emerge from the soil and start laying their eggs as at near the end of head emergence (GS 59). If the emergence of the females coincides with head emergence, the risk is high. If the females emerge after anthesis (remember this is one to two days before you see the anthers outside the wheat head), the risk is reduced depending on population levels. The risk is lower because the wheat embryo develops very rapidly after fertilization. However, if females lay their eggs before anthesis, even the tiny larva can do significant damage to the very small embryos, thus the importance of understanding the reproductive process in wheat.
Scouting needs to start shortly after spikes begin to emerge from the boot (GS 53-54) and continue until anthers are visible on at least the primary head and first tiller.
If threshold levels of female midge are detected before many anthers are visible on the spike (GS 61-63), control measures need to be applied.
STAGING AND FHB
The spent anthers are the major avenue through which the fusarium spores infect the growing seed, so if the conditions are suitable from just prior to wheat head emergence through to head fill, infection will occur.
The triazole fungicide chemistry used to control/suppress FHB is upwardly systemic in the plant. Because of this mode of action, the spores that germinate on the anthers are killed when the hyphae penetrate the anther. Any germinating spores coated with the fungicide during application will also be killed.
Therefore, applying these fungicides when 75 per cent of the main stem heads have emerged (GS 57) and/or are at 10 per cent flowering (GS 61) maximizes their effectiveness. The latest effective stage is 50 per cent flowering (GS 65), as the fungicides can stop fungal growth but only to a point. Once significant infection has occurred, the fungicide response from a yield/ quality perspective is minimal. The products work better by preventing fusarium infection.
Knowing what is going on in the wheat head during fertilization helps us to anticipate anther ejection and optimal timing for fungicide application. Phil Parker, agri-coach at Neepawa, Man., says, “In my experience, applying the triazole fungicide just prior to significant anther ejection allows the fungicide to move into the head systemically and gives better control/suppression.” Early is better than late but you can be too early.
Ergot infection process mimics a pollen grain growing into an ovary during fertilization. Infection requires that the fungal spore have access to the stigma, so ergot fungus is only able to infect a wheat flower, if for some reason, it stays open longer because of poor nutrition or cloudy, cold, wet weather. Copper deficiencies can delay flowering and triggers pollen sterility, causing the floret to remain open longer. Crops with a high proportion of tillers will flower unevenly and be exposed to infection for a longer period. Furthermore, herbicide injury can also delay maturity and cause sterility. The ovary is the only part of the plant susceptible to infection and fertilized ovaries are resistant to ergot infection. Therefore, ensuring proper nutrition and adequate copper, boron, etc. prior to and during the flowering process, along with good herbicide selection early in the season, will greatly reduce the potential of ergot infection by reducing pollen sterility and the likelihood of open florets.
Understanding what is going on in a plant, whether it is in the bedroom or the root cellar helps us make better, more informed decisions. It helps us sort through the myriad of theories, practices and products; choose the ones that make the most sense and increase the chances of maximizing returns on inputs and reducing risk throughout the years.