By Gal Sapir
In orchards growing Rosaceae varieties such as almonds, cherries, apples, pears, plums, and more, a mechanism called Gametophytic Self-Incompatibility (GSI) prevents self-fertilization. This means that different varieties must interplant in the orchard to ensure cross-pollination. Additionally, pollinating insects, mainly honey bees, are introduced to transfer pollen between the different varieties and ensure successful fertilization and fruit production. However, recent climatic changes like global warming and extreme weather events, along with agro-technical factors such as extensive pesticide use, have impacted the number and activity of bees worldwide. As a result, the cost of pollination has increased while yields have decreased. To address these challenges, efforts have been made to develop self-fertile varieties by “breaking” the self-incompatibility mechanism.
Self-pollinated varieties offer several advantages and disadvantages
Advantages:
1. Reduced dependence on pollinating insects: It is important to note that not all self-fertile varieties are the same in terms of self-fertility levels and fruit quality. Studies have shown that even in self-fertile varieties like ‘Independence’ (the most common self-fertile almond variety in California), a 60% decrease in fruit set percentages and a 20% decrease in yield were observed when trees were isolated from pollinating insects. While self-fertile varieties reduce dependence on pollinators, complete pollination independence is unlikely (1).
2. Planting a single variety: The ability to plant Monoblock of single variety is a significant advantage over the traditional method of interplanting different varieties in alternating rows for effective pollen transfer. Planting a single variety reduces labor costs and improves various agro-technical aspects. However, there are disadvantages to consider, which can negatively impact crop health and overall income.
It is worth mentioning that a single variety orchard could be feasible with a mechanized pollination solution that involves bringing cross pollen from distant orchards.
Disadvantages:
1. Yield: When comparing self-fertile varieties to main varieties like ‘Nonpareil’ almonds in the same geographical areas, a significant yield gap can be observed. In California, despite the increasing popularity of self-fertile varieties like ‘Independence’ which covered about 8% of the total planted areas of almonds and 23% of the new planting in 2022, but they still lag behind the commercial variety in terms of overall yield. Additionally, USDA data on the areas where self-fertile varieties are planted shows a constant decrease, indicating their poor economic performance compared to existing commercial varieties.
2. Establishing market suitability: Introducing a new variety into the market and establishing its commercial viability is a long-term process. The variety must excel in environmental conditions and meet market demands. Any new variety must surpass existing ones in terms of yield, fruit quality (size, taste, appearance), and ease of processing (cracking, cutting). Market conditions and price differences may favor established high-quality varieties like ‘Nonpareil’ over self-fertile varieties such as ‘Independence’ (2).
3. Self-fertilization versus cross-fertilization: In the plant kingdom, preventing self-fertilization mechanisms is widespread, as genetic diversity is favored through foreign pollen. Even when self-fertilization occurs, the resulting offspring may be weaker, and in stressful conditions, these fruits may be the first to fall. Examples from different species like avocados, litchi, and apples highlight the preference for foreign pollination and its impact on fruit development and quality.
Following are some examples illustrating this phenomenon:
A. Extensive research was conducted on avocados to investigate the offspring in various stages of development. The progeny ratio exhibited considerable variability, ranging from a clear majority of self-progeny during the initial phase before natural shedding to a starkly contrasting scenario after physiological fruit drop during the season, where almost all fruits resulted from foreign fertilization (3,4).
B. In the case of Litchi, a self-fertile species, as it possesses both male and female flowers on the same tree, research conducted in Israel revealed that planting a pollinator into an orchard enhances the percentage of fruit set and improves the overall yield, even though it can still yield a satisfactory commercial crop without a pollinating cultivar (5).
C. Among apple varieties, there are those known to have self-fertilization capability. One such variety, ‘Golden Delicious’, was previously cultivated in single-block units in Israel. Thorough research discovered that the number of seeds in the central area of the block was exceedingly low, with only a small percentage being self-fertilized. Most seeds originated from trees located tens to hundreds of meters away from the central sampling site within the orchard (6).
D. In a preliminary experiment conducted by the botanical team at EDETE in 2021, focusing on the ‘Matan’ variety in Ramat Magshimim it was observed that despite the variety’s inherent fertility (with or without bees), the addition of hand pollination increased the fruit set percentage by 28-42%.
4. Agrotechnical challenges: When new varieties enter the market, growers need to independently learn about the specific needs and problems associated with these varieties. Agrotechnical challenges often arise after the variety reaches maturity, which can take 10-15 years. This cautious approach to introducing new varieties is driven by the concerns and conservatism of breeders. For example, the ‘Independence’ variety faces issues like fruit retention on trees despite shaking, as documented in a study by Vicky Boyd (7).
Conclusions
While self-fertile varieties offer potential solutions to the pollination problem in orchards, there are still challenges to overcome. It requires a careful evaluation of the trade-offs between reduced reliance on pollinators, yield potential, market suitability, and agrotechnical considerations. As further research and development continue, growers can make informed decisions on the best approach for their specific orchard conditions and goals.
References
1. Sáez, Agustin, et al. “Bees increase crop yield in an alleged pollinator-independent almond variety.” Scientific reports 10.1 (2020): 1-7.
2. Parsons C. A closer look at the self-fertile Independence almond variety. Farm Progress June 14, 2017 5. Almond Board of California Honey Bee Best Management Practices for Almonds, 2018 Edition
3. Degani, C., R. El-Batsri, and S. Gazit. “Outcrossing Rate, Yield, and Selective Fruit Abscission in ‘Ettinger’ and ‘Ardith’ Avocado Plots.” Journal of the American Society for Horticultural Science 122.6 (1997): 813-817.
4. Goldring, Anat, Shmuel Gazit, and Chemda Degani. “Isozyme analysis of mature avocado embryos to determine outcrossing rate in a’Hass’ plot.” J. Amer. Soc. Hort. Sci 112.2 (1987): 389-392.
5. Raz, A., Goldway, M., Sapir, G., & Stern, R. A. (2022). “Hong Long” Lychee (Litchi chinensis Sonn.) Is the Optimal Pollinizer for the Main Lychee Cultivars in Israel. Plants, 11(15), 1996.
6. Schneider, D., Stern, R., Eisikowitch, D., & Goldway, M. (2001). Determination of the self-fertilization potency of ‘Golden Delicious’ apple. The Journal of Horticultural Science and Biotechnology, 76(3), 259-263.
7. www.wcngg.com/2020/05/14/self-fertile-almonds-gain-in-popularity
