The Influence of Soil Physical Condition During the Flowering Period on Nectar Production in Snapdragon

Abstract

During the past few years the problem of nectar secretion has acquired increased significance because of its bearing on pollination and seed and fruit set. Although optimal conditions for nectar yield have not been clearly defined, there is evidence that an abundant supply of carbohydrate is important. A correlation between nectar yield and quantity of solar radiation has been found for red clover (7), and in horse chestnut yield has been shown to be related to the magnitude of the carbohydrate supply to the flower (9). Little is known about the mechanism. Secretion of nectar involves a transfer of sugars against a concentration gradient, and appears to be dependent on metabolic activity in the nectary tissues (6). Nectar concentration ranges characteristic of species have been shown by AGTHE (1) to be related to the proportions of xylem and phloem in the vascular supply to the nectaries. The manner in which volume and concentration are controlled within the individual plant, however, and the mechanism by which the necessary energy is supplied, are still obscure. JOHNSON (4) has noted that in New Zealand white clover consistently produces more nectar on sandy soils than on clay soils, except in drought years. This effect may be attributable in part to the action of such soil physical factors as moisture level, temperature, and aeration during the flowering period. The influence of these factors has been investigated in the present study under controlled conditions. Snapdragon (Antirrhinum majus L.) has been used as a test plant because of its high nectar production and suitability for winter greenhouse culture. Materials and methods Although it was appreciated that the soil environment during early plant growth may greatly influence nectar secretion by conditioning plant growth and composition, it was desired in this instance to limit the study to the relatively short-term effects during the flowering period. All plants, therefore, were given similar treatment prior to flowering. Hybrid F1 plants were used to avoid the masking of treatment effects by genetic variation. Flowers used for nectar assay were collected as soon as possible after reaching full size. The centrifuge method (8) was used for nectar measure-imlent. Flowers were inverted in calibrated glass capillary tubes after clipping off the upper part of the corolla and centrifuged for six minutes at several hundred x g. The sugar concentration of the centrifugate was read 645