Some Factors Affecting Nectar Secretion in Red Clover

In many cross-pollinated crop plants, pollination is dependent upon bee activity which is influenced in part by nectar availability and quality. If some means could be found to increase the amount of nectar secreted by the floral nectaries of these plants, beekeepers would be encouraged to keep more colonies of bees in the vicinity of the crops, and pollination and fruit set would be improved. The problem of nectar secretion, therefore, is of more than academic interest to the agronomist. The first comprehensive work published on nectar secretion was that of BONNIER (3) who made physiological, anatomical, and microchemical studies in a large number of plant species. Although considerable work has been done since Bonnier's time, no essentially new lines of investigation have been followed. Studies have been made of the influence on nectar secretion of temperature (5, 9, 11), rainfall (3, 10), soil water content (2), light (9), latitude and altitude (3), soil type (8), and atmospheric humidity (3, 12, 14, 15, 19, 20). Unfortunately, much of this work has been done with small samples and without adequate experimental control. As a result, much disagreement exists in the literature. Excellent anatomical studies of the passage of nectar from the nectary cells to the exterior have been made by DAUMANN (4) and RADTKE (13). Considerable interspecies variation in the visible mechanics of the process was noted. Little is known about the basic mechanism of nectar secretion. Two general theories are extant: 1. That sugar secretion and water secretion are distinct phases, water secretion being analogous to guttation and dependent on root pressure (18). 2. That secretion occurs as one phase, nectar being secreted in its final form as the result of a specific cellular activity (2, 13). BONNIER (3) has presented evidence for a positive relationship between secretion and root pressure. RADTKE (13), on the other hand, has shown that secretion can occur in the absence of root pressure in isolated flowers floated on sucrose solutions. Recently ARENS (1) has advanced an hypothesis in which secretion, absorption, and transport of solutes are treated as closely related phenomena