In the interest of genetic biodiversity, it is desirable for most flowering plants to be cross-pollinated. In most cases the anther supported by the stamen produces pollen. These structures are anatomically close to the pistil which receives pollen and transports it to the ovary at the base of the flower bud. This fertilizes the flower, it closes up and develops into a fruit. Asexual reproduction includes instances of self-pollination whereby the flower’s own pollen is transferred to the pistil by the wind, a bee, or a bird. However, some flowers have developed extremely long pistils to try to avoid pollination by the nearby stamen and anther. Of note, some pistils have developed the ability to determine whether the pollen is from a different flower from the same flower through a chemical process. This paper will look at these different methods and the effect of the pistil in promoting cross-pollination in fruit-producing plants.
Flowering plants have evolved various genetic mechanisms to circumvent the tendency for self-fertilization created by the proximity of male and female reproductive organs in the bisexual flower. One such mechanism is gametophytic self-incompatibility, which allows the female reproductive organ, the pistil, to distinguish between self-pollen and non-self-pollen (McCubbin 1996). Gametophytic self-incompatibility in certain plants is caused by unique S proteins, a unique class of RNases as both recognition molecules and cytotoxic to its own pollen to prevent inbreeding (McCubbin, 1996). Some species of fruit-bearing plants do not discriminate against their own pollination, however, it was observed that cross-pollinated flowers accounted for a higher percentage (12.6%) of fruit-bearing flowers than self-pollinators (1.52%) (Wang, Zhai, Wang, & Sun, 2011). This phenomenon suggests that there is an interaction between the pistil and the stigma affecting the recognition of foreign pollen in the bisexual flowering plant.
A literature search identified four sources relevant to the thesis of the effect of the pistil in recognizing and in some cases rejecting self-pollen. The phenomena are how this effect works and what is the impact on fruit-bearing plants. The exact mechanism of this is a matter of research. Gibbs et al. concluded from their research that extended floral longevity initiated with self-pollen tubes growing in the style indicates some kind of pollen tube-pistil interaction. Fruity set only in chase pollination up to three hours implies that self-pollen tubes either grow more slowly in the style or penetrate ovules more slowly on arrival at the ovary compared with cross-tubes. This agrees with previous observations indicating that the incidence of penetrated ovules is initially lower in selfed compared with crossed pistils (Gibbs, Bianchi, & Ranga, 2004).
As in the animal kingdom, genetic diversity is essential to any biological system. Orellana and Oquenwenmo studied the magnitude of insect-plant interaction based on the dynamics of breeding systems and floral biology and there effects on pollination intensity, fruit and seed set (2012, p. 1). They observed that 100% of the nine taxa of Solanum were self-pollinators in some instances. However, facultatively outbreeding in 12.5-75% of the species were cross-pollinated through insect. They concluded that insect pollinators complement self-pollination in Solanum. They provide cross-pollen which enhanced gene exchange and hybrids in natural population and lower inbreeding depression (Oyelana & Ogunwenmo, 2012).
A Chinese study of floral morphology and floral biology of Cynanchum otophyllum Schneid This flower is characterized by a staminal corona. The pollinia were lodged in sacs on each side of the stigma and needed pollen vector for fruit production (Wang et al. 2011). The flower has similar characteristics to bee-pollinated plants and honeybees were the main pollinators. Polinaria removal and pollen insertion rates were low at 5.4% and 0.45% respectively (Wang et al. 2011). The flowering span was about three months and the functional longevity of individual flowers was six to eight days (Wang et al. 2011). The extended period may have been related to the relatively low levels of effective pollinator activity. A high level of self-pollination is likely in this case. It was observed that the cross-pollinated flowers were 12.6% more likely to bear fruit with higher seed count when germinated in soil than self-pollinated flowers at 1.52% (Wang et al. 2011).
Throughout this study, the role of the pistil was evaluated along with a number of elements that would affect the self- and cross-pollination of fruit flowers. Most flowers would self-pollinate however as Wang et al. that self-pollinated flowers were less likely to produce fruit (Wang, 2011) and if they did the seed count would be lower than cross-pollinated plants (Oyelana & Ogunwenmo, 2012).
Another factor observed was the time a flower would stay open producing pollen and waiting for pollination. It was observed that in some instances the flower would stay open longer until it had been cross-pollinated (Wang et al. 2011, Oyelana & Ogunwenmo, 2012). This suggests that the pistil has some sort of interaction with the rest of the flower to determine when it has been pollinated and the nature of this interaction is worthy of study. In some instances the pistil grows to a length to attempt to stand above the rest to resist self-pollination. Gibbs et al. that in self-pollinators stamens grow more slowly to increase the distance between the pistil and the anther (Gibbs et al., 2004). It was further observed that the time to fertilize the ovary with self-pollination was longer than cross-pollination. This suggests that the flower is physically resistant to self-pollination in favor of cross-pollination as desirable.
Along with the physical proximity and anatomical features encouraging cross-pollination, there was information that certain elements of the pistil allowed it to recognize its own pollen and reject it over cross-pollination. It was observed that in sour passion fruit flowers that, “Stigmas of flowers 24 hours before anthesis are unable to discriminate compatible (genetically unrelated) and incompatible (genetically related) pollen grains” (Madureira, Pereira, Cunha, & Klein, 2012, p. 83). This suggests that there is an evolution to the role of the pistil as to when it can recognize compatible pollen.
The notion involved the influence of RNases on S proteins. This particular observation is fascinating because the genetic material of the pollen of one flower would be substantially similar to pollen from another plant. The differences would be very subtle but if there is an enzyme or protein that is able to detect this to prevent pollination is definitely worth studying. Further, this enzyme was not only capable of identifying these genetic differences but was also cytotoxic to the foreign pollen.
Pollination in fruit plants is critically important as it is an important part of agriculture. This paper researched the role of the pistil in the pollination process. As the receptor of pollen, the pistil plays a critical role in transporting that pollen to the ovary and fertilizing the flower. Insects are relied upon to cross-pollinate. In many cases the flowers will stay open longer waiting for cross-pollination. In some cases the pistil attempts to distance itself anatomically from the anthers to avoid self-pollination. In other instances the pistil has certain proteins that distinguish between its own pollen and other pollen. This is extremely interesting and worth further research because a protein that cannot only identify subtle genetic nuances and is also cytotoxic may have valuable insight into other branches of botany. The pistil plays a key role in genetic biodiversity in fruit-bearing plants, its mechanism both physically as well as on the cellular level is worth further inquiry.
Gibbs, P., Bianchi, M., & Ranga, N. T. (2004). Effects of self-chase and mixed self/cross-pollinations on pistil longevity and fruit set in ceiba species (Bombacaceae) with late-acting self-incompatibility. Annals of Botany, 94, 305-310.
Madureira, H. C., Pereira, T. N. S., Cunha, M. D., & Klein, D. E. (2012). Histological analysis of pollen-pistil interactions in sour passion fruit plants (Passiflora edulis Sims). Biocell, 36(2), 83-90.
McCurbin, A. (Director) (1996, April 30). How flowering plants discriminate between self and non-self pollen to prevent inbreeding. 133rd Annual Meeting of the National Academy of Sciences. Lecture conducted from National Academy of Sciences, University Park.
Oyelana, O., & Ogunwenmo, K. (2012). Floral biology and the effects of plant-pollinator interaction on pollination intensity, fruit and seed set in Solanum. African Journal of Biotechnology, 11(84), 14967-14981. Retrieved from http://www.academicjournals.org/Ajb/PDF/pdf2012/18Oct/Oyelana%20and%20Ogunwenmo.pdf
Wang, D., Zhai, S., Wang, B., & Sun, G. (2011). Floral structure and pollination in relation to fruit set in cynanchum otophyllum schneid. Systems Biology 2011 IEEE International Conference, 2011, 179-185. Retrieved from http://ieeexplore.ieee.org