Nectar and Pollination

 

Pollination, fruit set and ripening coming along  looking for advantages in crop production. For example, stigma of wild tomato normally grows above the anther cone, allowing cross-pollination in wild plants, although autogamy seems to be the reproduction system in the wild species. However, in the current commercial varieties, stigma grows at the same level than the anther cone, thus facilitating autogamy. Despite the fact that pollen shedding from anther could be a problem in  some varieties and culture conditions.

Pollen dispersing mechanisms have been considered as a prevalent force in floral evolution, acting mainly through three floral characters: nectar production; poricidal anthers; and, secondary pollen presentation. Such mechanisms enhance pollen dispersal by restricting pollen removal by individual bees during a series of visits (dispersing schedule).

 

On the other hand, honey bees are attracted to pollen and nectar, and specially to nectar sugars.  Several studies have stressed the importance of nectar volume, while others emphasize the importance of the concentrations or relative amounts of specific sugars, and it is worth to mention as homage to nectar specialists (Baker & Baker). Nectar is a mostly aqueous combination of a number of substances. Chief among these are sucrose, glucose and fructose. Other carbohydrates including arabinose, galactose, mannose, gentiobiose, lactose, maltose, melibiose, trehalose, melezitose, raffinose and stachyose have also been identified in nectars of some flowers. Various types of nectars can be ordered into three groups according to sugar content: sucrose prevalent, glucose and fructose prevalent, and almost equal amounts of sucrose, glucose and fructose. Some nectars also contain amino acids. All twenty of the normal amino acids have been identified in various nectars, and alanine, arginine, serine, proline, glycine, isoleucine, threonine and valine seem to be the most prevalent. Other substances reported in nectar include organic acids, terpenes, alkaloids, flavonoids, glycosides,, phenolics and oils.

 

Some plant species are more attractive to honeybees than others and, therefore, the proportions of the various species at any given time, play an important role in the relative attractiveness of each species. Flower attributes such as size, color, flower organs, and nectar guides on the petals, nectar volume, nectar composition, and amount of pollen are considered to be important factors attracting honeybees and  as such  can affect visitation. Recent studies indicate that the chemical components contributing to a flower’s fragrance also play an important role in the attractiveness of flowers to bees. The chemical composition of flower volatiles may also affect bee behavior. Olfactory signals are rapidly learned, indicating that foraging behavior results from the association of plant chemicals acting as chemosensory cues for the bees. Moreover, the fact that in some cases bees are more attracted to flowers with a meager level of nectar than those with high levels indicates that the olfactory signal(s) may be a more dominant factor controlling bee behavior.  Collectively,  these reports indicate that bee behavior is controlled by the integration of both perceived cues, such as color and/or  fragrance, and the amount of a reward, such as pollen and nectar. Current studies have focused on defining the importance of each factor, as well as the interaction between them, in the attractiveness to bees of flowers from various species. A high variation in the attractiveness of flowers from different species reflects diverse genetic variability.

There is an added difficulty to  pollination under greenhouse conditions due to the special micro-clima within it. Wind is not allowed to enter and any movement which in open air transport pollen grains, does not occur inside. Another important climatic factor is temperature, it affects either to the plants as to the bees. All these plant species are sensitive to temperature stress, mainly its sexual organs and many works have been published on the effects of temperature stress on pollen viability, seed and fruit set. On the other hand, temperature influences also bee activity and sometimes both actions are integrated on the responses. This is particularly important in  species such as melon or watermelon with male and female flowers (monoecious plant). It is true that some of these crops set fruit parthenocarpically (some cucumbers  and zucchini varieties), but this process not always happens to the particular variety which is doing well in the market. Fruit and seed set are specially dependent on successful pollination when the species under consideration cannot or must not be automatically selfed. This is true if the flowers have any adaptations for the avoidance of selfing, or temporal  separation of stigma and pollen presentation, or self incompatibility.

It is very remarkable the amount of nectar that produces the flowers of zucchini, specially the female ones, which can reach to 300 microlitres during the time that the flower is open. Taking into account that the volume of the nectar stomach of a honey bee (and also of a bumblebee), is as much as 100 microlitres, that means that only foraging for nectar in one female flower the bee can be satisfied. However, there are other attractiveness  like pollen grains, aroma cues, etc, which makes possible the cross pollination of the zucchini flowers. I enclose a picture of a bumblebee literally ‘bathing’ in nectar from the nectaries of a female zucchini flower.

 

José Manuel Guerra Sanz