Micronutrient fertilizers have a significant effect on the reproductive capability of plants. These nutrients promote the activity of specific metabolic pathways and enzymes involved in the flowering process and can significantly increase the amount of flowering-related phytohormones in the plant. In a study of strawberries, the authors compared the effects of various foliar micronutrient sprays on blooming and fruiting. All of the strawberry plants were treated with the recommended dose of fertilizer (RDF) in the soil, and the different variables in the foliar spray were CaCl2, ZnSO4, and FeSO4, ranging in concentrations from 0.4-0.8%. The authors found that many of the different micronutrient sprays significantly increased the fruiting parameters relative to the control group, but the most effective treatment was 0.6% ZnSO4, which produced an average of 43.90 flowers per plant, 41.53 fruits per plant, and an average yield of 1.17 kg per plant, compared to the control group which had an average of 35.90 flowers per plant, 31.53 fruits per plant, and yielded 0.69 kg per plant.

In a greenhouse study of gerbera, an ornamental flower, the authors individually compared foliar treatments of ZnSO4 and FeSO4 in concentrations of 0.2 and 0.3%, as well as ZnSO4 and FeSO4 in combination at a concentration of 0.2%. These authors found that the combined foliar spray of Zn and Fe at 0.2% boosted growth the most and led to an increase in many flowering parameters. The plants in the Zn + Fe treatment produced buds earlier, at 48.11 days to appearance, vs. 60.89 days in the control group. The Zn + Fe treatment also had more flowers per plant (15.70) relative to the control (12.57) and more flowers per square meter (157.40) relative to the control (126.20). Similarly, in a field study of chamomile, the authors found that a foliar spray of 0.35% FeSO4 + ZnSO4 delivered at stages of stem elongation and flowering improved the flower yield by 46.4% (1963 kg/ha) compared to the control (1340 kg/ha).

Promoting the beneficial microbiota in the soil can be a great way to increase micronutrient availability without increasing fertilizer costs. There are many options to jump-start the microbial activity of soils. One simple method is preparing compost tea: soak some compost in a bucket of water with an air pump supplying oxygen. You will have a nutrient-rich broth teeming with beneficial microorganisms in a few days, which you can add to the soil.

For large-scale growers, commercially prepared inoculants may be more feasible. Tainio Spectrum™ + MYCO is designed with budget-conscious crops in mind, and it combines all of the powerful plant-supporting benefits of Tainio Spectrum™ and Mycorrhizal fungi in one convenient package. The beneficial soil-based microorganisms in Spectrum™ break down and release vital nutrients (such as phosphorus, nitrogen, calcium, iron, and more) stored in soil particles, making the nutrients more readily available to the plants in forms they can absorb. Plant growth-promoting microorganisms produce organic acids and amino acids, which dissolve and make available (chelate) mineral nutrients. Spectrum™ assists in the recycling of organic matter into an important substance known as humus, building richer, healthier soil.

For a more targeted approach, micronutrient fertilizers are a quick and effective way to increase bloom and fruit yield. Eden Blue Gold Iron is an iron nutrient derived from FeSO4 (Fe 4.63%). Blue Gold Iron maximizes immediate iron bioavailability and combines the iron with the flagship Blue Gold™ technology. It can be applied foliarly (according to label directions) without risk of phytotoxicity.

Eden Blue Gold Zinc is a zinc nutrient derived from zinc sulfate (Zn 5.24%). Blue Gold Zinc combines zinc with the flagship Blue Gold™ technology. Blue Gold Zinc provides bioavailable zinc, which is essential for cell multiplication and plays a vital role in water uptake, phytohormone activity, and the uptake of other nutrients. It can be applied foliarly (according to label directions) without risk of phytotoxicity.

Photo of cherry blossoms. Image from Wikipedia.

Photo of spring apple blossoms. Image from Flickr.

Arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) are known to affect plant growth in numerous ways. AMF have been shown to improve nutrient uptake through root colonization, which may improve micronutrient availability in the plant. PGPR can produce compounds that make nutrients more soluble and available to the plant. PGPR and AMF have also been shown to produce phytohormones such as IAA and gibberellins. In a study of strawberries, the authors tested the effects of AMF and PGPR soil inoculation on various fruiting parameters. The authors compared two uninoculated control groups (100% Recommended Dose of Fertilizer and 70% RDF) with inoculated groups of two Pseudomonas isolates and AMF + 70% RDF. The authors found that the combination of Pseudomonas + AMF resulted in the most improvement in growth parameters. The Pseudomonas + AMF group had an average of 13.6 flowers per plant, compared to 10.5 flowers per plant for the 100% RDF control group. The Pseudomonas + AMF group had a better percentage of fruits per flower, with 96.8% of flowers producing fruit, compared to 81.0% for the 100% RDF control group. This demonstrates that even with reduced fertilization, microbial inoculation can improve plant nutrient and phytohormone availability, improving reproductive capability.

The role of phytohormones in flowering must be considered. These phytohormones regulate every part of plant growth and development, and multiple hormones influence the flowering process, which may differ for each plant species. It is widely reported that high concentrations of gibberellic acid promote vegetative growth and inhibit reproductive growth. In contrast, other hormones like abscisic acid, jasmonic acid, and certain cytokinins promote reproductive growth and floral production. Many studies have documented that foliar applications of IAA and other phytohormones can encourage flowering and fruiting.

Gibberellic acid and other gibberellins have a complex and convoluted role in the flowering process. Recent investigations by Bao et al. (2020) have revealed that in Arabidopsis, a model plant for research, gibberellins are involved in the signaling hub that regulates flowering time. Gibberellin signaling creates feedback loops that link with other phytohormone signaling pathways, such as those of jasmonic acid, IAA, ethylene, and abscisic acid.

In many plants, including mango, floral production requires starch (carbohydrate) accumulation in the buds. Gibberellic acid increases the plant’s ability to transport carbohydrates synthesized in the leaves throughout the plant, promoting vegetative growth. For the proper starches to form, gibberellic acid concentrations must decline. Otherwise, these carbohydrates remain in the form of simple sugars.

In apples, trees that typically bear a heavy fruit crop every other year, gibberellic acid and other gibberellins also inhibit flowering. Heavy-flowering varieties of apples express high levels of abscisic acid, jasmonic acid, salicylic acid, and cytokinins. Zeaxanthin and jasmonic acid are known to act as promoter hormones, whereas many other hormones act as inhibitors of flowering. The number of apples grown one year influences the fruit quantity the tree will produce the following year. In a field study, Samuoliene et al. (2016) tested the effects of bud thinning in apples on the next year’s flowering and fruiting. The experimental groups were 12, 8, 4, and 0 inflorescences (clusters of flowers) per square centimeter of trunk cross-sectional area. The authors found that the highest crop load (12 inflorescences per) had the highest concentration of gibberellins (67.26 ug/g) and the lowest ratio of promoter hormones (zeaxanthin and jasmonic acid) to inhibitor hormones (1:21.4). The group thinned to 4 inflorescences per had a significantly lower concentration of gibberellins (15.95 ug/g) and a substantially higher ratio of promoters to inhibitors (1:4.7), indicating that a lower fruit load leads to a higher expression of promoter hormones. The following year, the thinned to 4 inflorescences per had a return bloom rate of 8.0 inflorescences per square centimeter of trunk cross-sectional area, whereas the group thinned to 12 inflorescences had a much lower return bloom rate (3.7). The above study shows that in alternate-bearing tree crops like apples, a heavy crop load one year leads to a greater expression of hormones inhibiting flowering the following year. Thinning buds can be a way to manage these hormone levels effectively, and doing so may allow crops like apples to be harvested yearly rather than every other year.

Similarly, in a study of sweet oranges, the authors found that branches that maintained all their mature fruits had 90% less flowering on average than branches that were defruited. The authors explain that mature fruits that stay on the branch begin to release inhibitory phytohormones, which can prevent fruiting from occurring the following year. Removing all the remaining fruits after harvest can help increase bloom.

In many plants, thinning buds or flowers can increase yield, even if they are not alternate-bearing crops. In a recent study of common buckwheat, the authors found that plants with 50-75% of their flowers removed had a significantly lower percentage of aborted embryos, which the authors hypothesize to be due to increased jasmonic and salicylic acid production. These thinned buckwheat plants also had a considerably higher yield of mature seeds.