Nectar, The Raw Product Of Honey
By Clarence Collison, PhD
Nectar, the sweet aqueous solution secreted by floral nectaries has the primary function of attracting and rewarding potential pollinators. While the mechanism of nectar secretion is not fully understood, it is well known that secretion is dependent upon the metabolic activity of the nectar tissue and several possible metabolic pathways have been described. Recent research has indicated that sugars are secreted exocytotically by means of vesicles that are derived from the endoplasmic reticulum of the secretory cells. Thus, nectar is secreted from the cells into the intercellular spaces from which it diffuses through the epidermis or stomata (pores) of the nectary.
The amount of nectar or sugar content that is secreted is dependent upon the plant itself and the environment. With some plant species, there is a relationship between the size of the nectary and quantity of nectar secreted. Other plant factors that may affect nectar quantity and quality include the: 1) sex of the flower; 2) position of the flower on the plant; 3) age of the flower; and 4) variety. Nectar secretion often ceases after pollination. When pollination fails to occur, then the length of the secretory period is usually extended. With many species, uncollected nectar may be reabsorbed by the nectary tissue.
The concentration of the nectar as it is secreted by the nectary is highly dependent on the anatomy of the vascular system supplying the nectary and on the sugar concentration in the phloem (food-conducting vessels) and/or xylem (water-conducting vessels) of the nectary vascular supply. Highly concentrated nectar which is characteristic of several plant species, essentially originates from phloem tissue whereas plants that produce high volumes of dilute nectar have limited phloem tissue and abundant xylem. The quality of the nectar secreted is essentially a function of the carbohydrate (sugar) supply to the nectary and indirectly related to photosynthesis (process by which green plants produce their own food). Most of the sugar of nectar probably comes from leaves fairly close to the flower. In herbaceous plants, the nectar sugar is likely to be of recent origin, whereas in trees and shrubs, it may also be derived from stored carbohydrates. It is highly probable that any factor which alters the rate of buildup or breakdown of the carbohydrate supply will influence nectar secretion. Sufficient sunlight is of primary importance in supporting a high level of photosynthesis.
Flowering is one of the later events in the life history of an annual plant and any factor to which the plant has been exposed prior to this time will, to some degree, influence flowering and nectar production. In general, conditions which impose no appreciable limitations on growth and which promote a reasonable balance between vegetative and reproductive development seem to support good nectar production.
Through the growing season, water is an important factor in the regulation of plant growth. Either a shortage or an overabundance of water may stunt plant growth and lead to poor nectar yields. During the secretory period, a lack of water may reduce the amount of sugar synthesized.
External factors influencing secretion are those of weather and soil. Weather is a complex of interrelated factors that are often difficult to separate into individual components. Temperature has received more attention than other factors and there is a difference of opinion regarding its importance. Records of daytime temperatures may reflect conditions of sunlight which in themselves cause wide variation in the nectar flow. Temperature affects many plant processes which are proceeding at the same time. A certain threshold temperature is necessary if secretion is to occur. Within normal limits, temperature variation probably has little influence on the amount of sugar which the plant synthesizes, but it has a very marked effect on the rate in which the sugar is consumed in growth, respiration, and other processes. Flower development is accelerated at high temperatures and the duration of secretory activity is probably shortened. Excessively high temperatures in combination with meager rainfall can lower nectar production by causing a moisture stress in the plant.
Atmospheric humidity does not affect nectar secretion directly, but has a pronounced inverse effect on nectar sugar concentration. As nectar is secreted, it undergoes a regulation of concentration until its vapor pressure comes to equilibrium with that of the atmosphere. Unless the humidity of the atmosphere is very high, the change will be a loss of water molecules to the air and an increase in nectar sugar concentration. Rates of increase in nectar sugar concentration can be extremely rapid in flowers in which the nectar is exposed. Evaporation is hastened by high temperature and rapid air movement across the nectaries.