Seed & Fruit Development
The number of seeds within a fruit is also tightly linked to fruit size and shape in species with multiple fused carpels. If only one ovule, or a subset of the total ovules, is fertilized, it can result in abnormal development close to the unfertilized ovules and fruit asymmetry (Figure 7c). Additionally, fruit size has been shown to be positively correlated with seed number in strawberry, kiwifruit, and apple. Generally, the more seeds a fruit contains, the bigger it is. The relationship between fertilization, seed development, fruit development, fruit size, and fruit shape explain why growers of some tree fruit crops (see Summary Chart) bring in supplemental pollinators to ensure that the maximum number of ovules within each flower are pollinated.
Carpel number determines the relationship between pollination, seed development & fruit development
A carpel is the structure which includes both the ovary and its associated ovule(s) in a flower. The number of carpels, and the degree of fusion between carpels, varies among plant species. The common tree fruit crops grown in California generally contain either a single carpel, or multiple fused carpels (see Summary Chart). For example, flowers of walnut, pistachio, and all crop species within the genus Prunus contain a single carpel (Figure 7a), while female kiwifruit flowers contain at least 30 fused carpels (Figure 7b).
After pollination and fertilization, carpels develop into the fruit tissue we eat (ovary) and the seeds within (ovules). Fruit development is initiated by growth regulating hormones produced by developing seeds. Because carpels ultimately develop into fruit tissue, the number of carpels in a flower determines the degree to which pollination and seed development is required to produce fruit. Flowers with one carpel only require fertilization of one of the two ovules to produce fruit. In contrast, species with multiple fused carpels require fertilization of a smaller proportion of the total ovules within a flower for fruit development. The growth regulating hormones produced by a subset of the total possible seeds are sufficient to initiate the development of fused carpels into fruit. As a result, normal fruit development is less dependent on seed development in species with multiple fused carpels.
Figure 7a,b. Illustration of a longitudinal section of a plum flower (a) with one stigma and style (blue) leading to a single ovule and carpel (blue and orange). Illustration of a female kiwifruit flower (b) with multiple stigmas and styles (blue) leading to multiple ovules and a fused carpel (blue and orange). Image source: USDA Handbook 495.