Neutral influence of animal pollination in the common bean (Phaseolus vulgaris L., Fabaceae) production and seed germination

Abstract

The influence of animal pollination on the common bean (Phaseolus vulgaris L., Fabaceae) production is quite variable, but there is a tendency towards the predominance of strong self-fertility. Few studies tested for the influence of pollination on quality aspects of production and seed germination. In view of the global economic importance of the common bean, and the pollinator crisis that the planet is experiencing, the objective of this study was to investigate the influence of animal pollination in the amount and quality of fruits, seeds, and seed germination of the common bean, using four plantations of the ‘mulatinho’ variety in a region under semiarid climate of NE Brazil as a model. Our hypotheses are: 1) Flowers are visited mainly by bees; 2) The production of fruits, seeds and seed germination are favored by pollinators. Data were collected during the 2017 and 2018 plantings, when data on floral visitors were collected. Controlled crosses (spontaneous self-, cross- and natural pollinations) were performed and the number and morphometry of fruits, number, morphometry, and germination of seeds were compared between the experiments. Reproductive efficiency (RE) and pollen limitation index (PLI) were calculated. Number of formed fruits, measurements, weight, and germination resulting from spontaneous self-pollination showed higher values than those formed by natural and cross-pollination. In 2017, the RE was high (1.44) and the ILP was 0.14, indicating no pollen limitation. In 2018, RE was also high (1.8), but the ILP was 0.3, indicating pollen limitation. Overall, the frequency of visits on focal individuals was low. Conversely to what was expected, butterflies and moths were the most frequent floral visitors, although the flowers were also visited by bees, ants, flies beetles and spiders. None were considered pollinators because they did not touch the reproductive structures of the flower. The bee Xylocopa sp. was the only species capable of pollinating the flowers, as its size allows it to expose the reproductive parts present inside the keel when it lands on the flower to collect the floral resource. However, its frequency was very low, and it did not visit the focal plants. Apparently, low temperatures and high rainfall negatively interfered with pollination and, consequently, with the production. The common bean can be considered a relevant crop in the face of the current global pollinator crisis, as it does not depend on pollinators to produce food, the flowers are used by several pollinator groups and, additionally, it contributes to soil nitrogenating, reducing input costs for farmers.