By Gal Sapir
Xenia is a hidden aspect of pollination that has a direct impact on the growth and characteristics of seeds and fruits from fertilization to germination. It causes changes in the physical appearance of the seeds and fruits, which reflect the traits of the parent that provided the pollen.
These characteristics are not passed down to offspring. When the male and female reproductive cells unite, the pollen’s genotype can directly impact the maternal tissue surrounding the embryo and endosperm, including the seed coat and pericarp. This can result in variations in traits, color and quality of the hybrid fruit and seeds.
Research has shown that xenia is commonly found in fruit trees. It not only impacts the development of the endosperm and embryo after double fertilization but also causes variations in seed size and shape. In addition, it can affect the ripening time of the fruit, as well as its size, shape, color, flavor, and the presence of specific substances. Therefore, studying the xenia effect is highly significant in fruit production, where the fruit and seed are the primary targets for harvest. It can also serve as a theoretical reference for the pollination strategy of the cultivar, leading to increased yield and improvements in the internal and external qualities of the fruit. Furthermore, it contributes to the genetics, physiology, and breeding science of fruit trees.
Despite the studies that have been conducted on the observation and categorization of xenia, the classification remains incomplete. Due to the fact that humans consume fruits and seeds, the effect of xenia on phenotypic traits has been classified into two groups: xenia and metaxenia.
In Xenia, the pollen genotype has a direct impact on the seeds formed by the female parent after fertilization in the current year. This can result in differences in the shape, size, and color of the seeds. Additionally, Metaxania refers to the phenomenon where the pollen genotype affects the fruit’s shape, maturation period, size, color, flavor, and substance content in the current year. As a result, there can be variations in these traits.
The Xenia effect has been observed in numerous horticultural plants, including chestnut, maize, almonds, several citrus spp., hickory, kiwifruit, apple, pears, and blueberries. (1). For the purpose of focusing on the xenia effect with regard to seed number, I will use examples from both one-seeded and multi-seeded plants
When it comes to Almonds (Prunus dulcis), the traditional almond orchard relies on the compatibility and bloom overlap levels of cultivar combinations. However, recent studies on xenia in almonds have revealed that fruit size can be controlled by pollinating with larger seed pollinators, and harvest time can be controlled by using pollen donors from early maturing cultivars. (2)
In Apple trees (Malus domestica), the metaxania effect can influence the fruit’s diameter, firmness, sweetness (refractometric index or brix), and acidity levels. (3). Pollination in the dioecious plant, date palm (Phoenix dactylifera), has traditionally been done manually using pollen from a male plant. However, a recent study has shown that there can be significant differences, up to 75%, between different male plants. This study investigates the potential benefits of using pollen from different male plants, not just for increasing yield through better fertilization, but also for improving biochemical traits and ripening time. (4).
In Macadamia (Macadamia integrifolia), a type of nut that has a self-incompatibility mechanism, it’s clear that fruit production is boosted by cross-pollination in comparison to self-pollination. However, when assessing various male pollinators with distinct traits (such as seed size), the xenia effect will impact the size and recovery of the nut and kernel at maturity based on the origin of the male pollen. (5).
When considering Xenia in the complex process of pollination, precision mechanical pollination offers another advantage. Pollen can be collected and applied to the desired orchard without needing to synchronize flowering. This means that the male pollen donor plants can be grown in different climates and geographic conditions, unrelated to the cultivar that has been pollinated. This maximizes production in the main cultivar.
REF
- Yang, Q., Fu, Y., Liu, Y., Zhang, T., Peng, S., & Deng, J. (2020). Novel classification forms for xenia. HortScience, 55(7), 980-987.
- Kumar, K., and B. Das. “Studies on xenia in almond (Prunus dulcis (Miller) DA Webb).” Journal of Horticultural Science 71.4 (1996): 545-549.
- Bodor, P., M. Gaál, and M. Tóth. “Metaxenia in apples cv.’Rewena’,’Relinda’,’Baujade’as influenced by scab resistant pollinizers.” International Journal of Horticultural Science 14.3 (2008): 11-14.
- MARYAM, M., JASKANI, M. J., AHMAD, I., NAFEES, M., ALAFARI, H. A., FIAZ, S., … & UZAIR, M. (2023). Hybridization and Metaxenial Effects on Biochemical Attributes and Hybrid Development in Date Palm (Phoenix dactylifera L.) Hillawi. Cultivar. Turkish Journal of Agriculture and Forestry, 47(4), 553-566.
- Herbert, S. W., Walton, D. A., & Wallace, H. M. (2019). Pollen-parent affects fruit, nut and kernel development of Macadamia. Scientia Horticulturae, 244, 406-412.
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