High Yield Pollination and Fruit Set Enhancement

Insect pollination services significantly enhance global major crops in terms of fruit quantity and quality. They can also increase seed set in self-incompatible varieties (rapeseed, strawberries), increase seed set rate on self-incompatible ones such as rapeseed or strawberry plants and have an impact on nutritional content (apples and almonds).

In this webinar we will investigate the factors that affect flowering and how to maximize yield potential. The presentation will include:

1. Pollenizers + Pollinators

At temperate fruit crops, profitability hinges on producing commercially viable fruit in abundance; quality issues like storability and consumer appeal also play a pivotal role. Pollination-induced increases in fruit set can substantially boost yields but their mechanisms remain obscure.

Apples are North America’s most widely pollinator-dependent crop, relying on insect-mediated cross-pollination for up to 90% of fruit set. While apple cultivars can be self-compatible to some degree, most produce too little fertile pollen for self-pollination and require the transfer of pollen from another cultivar for optimal fruit set. Pollen from high yielding “pollenizer” varieties increases fruit set; however despite these benefits being reported there may still be variations in yield and quality among treatments.

The differences among treatments were attributed to differences in weather conditions and hybrid characteristics that influence their appeal for bees, in particular hybrids with more appealing nectar profiles tending to perform better in open pollination than less appealing varieties. Furthermore, pollinator activity levels also played a vital role with lower visitation frequencies leading to reduced yields and an increase in cull yield.

Achieving high yield requires an abundance of fertile pollen from pollinator visitation and proper fertilization in the flower. Fertilization involves multiple steps depending on flower onset and pollen availability in the environment.

Environment factors have an enormous influence on both pollen and seed production, including temperature, light intensity and nitrogen availability. These can limit fertilized flowers while simultaneously decreasing seed set. Furthermore, sterile pollen production could further limit seed set.

Control of these factors can be achieved with the help of compatible pollenizers, timely flowering periods and fertilizer applications. With regard to apples specifically, increasing visits by wild bees has proven more successful at increasing fruit set and yield than pollinator exclusion; additionally supplemental hand pollination treatments have shown to enhance storability.

2. S-Alleles and Compatibility

As its name implies, the S system provides an effective barrier against self-incompatibility. But this system doesn’t operate in a strictly linear fashion: pollen that fails to recognize an S-allele doesn’t immediately reject itself but can still move from plant to plant via non-self recognition (NSR), thus facilitating transference of compatible pollen between trees.

Therefore, one mutation to an S-locus gene can have profound ramifications on a plant’s functional characteristics. S alleles encoded by one chromosome can differ significantly in terms of amino-acid sequence that translates into protein; these differences (haplotypes) determine compatibility among other plants.

Even though a new allele can have a substantial effect on NSR and S-RNase expression, there are limits to how much variation is allowed at any one locus. The “rule of four” provides this guidance: alleles at any locus must differ by at least four-fold from one another if their effects increase or decrease activity of certain functions in plants; otherwise it would dominate its existing population and outcompete other alleles.

Kao’s team recently conducted research aimed at understanding these limitations by analyzing the relative abundance of different S-haplotypes across several pear populations. They discovered that alleles could be divided into two broad groups and each shared an ancestral allele some 4N generations prior. Grouping was achieved by comparing synonymous and non-synonymous changes amongst 59 most diverse S-haplotypes;

After using their findings to model how S-RNase expression responds to various pollination treatments in pear trees, the researchers then used this data to model S-RNase expression dynamics across unpollinated styles (1dpp, 2dpp and 3dpp) unpollinated styles were highly similar and represented baseline expression dynamics of both S-alleles. Expression patterns change with pollination treatment compatibility type. Semi-compatible pollination involves self-type pollen tubes reacting with each other to maintain a minimum level of S-RNase expression by activating transcription, while compatible pollination triggers increased S-RNase production when an influx of cross-type pollen tubes deplete the stylar S-RNase pool, leading to its depletion and activating transcription, leading to an enhanced activation of S-RNase expression.

3. Natural Pollination

As part of their natural foraging habits, insect pollinators visit many flowers during each bloom period. On each visit, some pollen from one flower is transferred to others and, under ideal conditions, these pollinated blooms will fertilize each other, producing fruit sets to complete its growth cycle.

Research has demonstrated that animal pollination increases crop yield in three quarters of world food crops (Dicks et al., 2016). Such increases are considered valuable enough that recommendations have been issued to support pollinators and create environments in which they can thrive.

At its core, crop production depends on multiple factors – genetics, weather and environmental conditions among them – which interact to influence animal pollination on crop yields and influence management strategies to achieve yield stability. Therefore, an in-depth knowledge of how different elements interact must be obtained in order to assess the effects of various strategies on yield stability.

This is especially important for perennial crops like passion fruit that depend on animal pollination for pollenation; thus, more accurate models would be most advantageous to assess their impacts on yield stability both spatially and temporally.

Recent research conducted in Brazil on Criolla mandarins found that when excessive honeybee visitation rates are reduced, both fruit set and quality increase significantly. The authors believe this positive result can be linked to more diverse native flower visitor assemblages associated with natural habitat conservation efforts on family farms.

Researchers tested the two most frequently applied fungicides in the field – AVG and 1-MCP – which have both been shown to delay fruit senescence, leading to enhanced fruit set, but their impact may differ depending on when they were applied phenologically.

This study’s results are in line with other investigations into the impact of using fungicides on the ripening process. For instance, it has been demonstrated that applying fungicides during early bloom phases can significantly decrease berry per tree numbers and ultimately improve yield (Abrol, Gorka, Ansari, Al-Ghamdi & Islamoglu, 2017).

4. Artificial Pollination

Artificial pollination is one of the most efficient ways to increase fruit and seed set and enhance quality crops, while simultaneously contributing to sustainable production and human food security. Unfortunately, its application is restricted by pollen availability – and as natural pollinators decline further, artificial pollination’s necessity becomes clear.

Animal-mediated pollination not only increases yields but can also boost their nutritional and sensory qualities (e.g. rapeseed and strawberries), as well as decrease malformations (such as in hickory trees and hazelnuts) or shorten storage times (like with figs).

Studies on artificial pollination have examined its effect on fruit and seed set among various crops. Examples include using stingless bees to cross-pollinate strawberries in greenhouses where their use increased yields while improving seed and fruit size by 29% relative to self-pollination; similarly, manual cross-pollination of eggplant (Solanum melongena) greenhouses showed 30% increases in fruit set, fruit size enhancement and quality over self-pollination.

Studies have also demonstrated the benefits of grafting pollinator-compatible cultivars to increase yields and fruit quality, leading to improved yields and nutritional outcomes [1,2]. Based on these discoveries, Edete offers artificial pollination services as part of their package for growers – its drones fly above fields applying pollen directly onto female flowers of trees for higher yields, quality, nutrition (Fuji apples and strawberries) [1-3].

Propensity score matching was used in this study to assess the effect of artificial pollination on cocoa production and smallholder welfare indicators of productivity, income, poverty and food security. Prior to matching, property scores were balanced to ensure variables had similar distribution in both groups. Results demonstrated that irrigation + artificial pollination treatment had greater effectiveness at improving welfare indicators than drought treatment alone; suggesting its adoption should be encouraged by governments, NGO’s, or any stakeholders interested in supporting cocoa farmers to enhance their livelihoods.