Nectar secretion

Abstract: SUMMARY Secretory tissues occur in most vascular plants. Some of these tissues, such as hydathodes, salt glands and nectaries, secrete unmodified or only slightly modified substances supplied directly or indirectly by the vascular tissues. Other tissues secreting, for instance, polysaccharides, proteins and lipophilic material, produce these substances in their cells. The cells of secretory tissues usually contain numerous mitochondria. The frequency of other cell organelles varies according to the material secreted. In most glandular trichomes the side wall of the lowest stalk cell is completely cutinized. This prevents the secreted material from flowing back into the plant. The salt glands in Atriplex eliminate salt into the central vacuole of the bladder cell but, in other plants, the glands secrete salt to the outside. Different views exist as to the manner in which salt is eliminated from the cytoplasm. According to some authors, the mode of elimination is an eccrine one, while others suggest the involvement of membrane-bound vesicles. Nectar is of phloem origin. The pre-nectar moves to the secretory cells through numerous plasmodesmata present in the nectariferous tissue. Nectar is eliminated from the secretory cells by vesicles of either KR or dictyosomal origin. In some cases, both organelles may be involved but an eccrine mode of nectar secretion has also been suggested by some authors. Carbohydrate mucilages and gums are synthesized by dictyosomes but virtually every cell compartment has been suggested as having a role on the secretion of lipophilic substances. Most commonly, plastids are implicated in the synthesis of lipophilic materials but KR may also play a part. In some cases lipophilic materials may be transported towards the plasmalemma in the KR. Resin and gum ducts of some plants develop normally or in response to external stimuli, such as microorganisms or growth substances. Among the latter, ethylene is the most effective. During the course of evolution, secretory tissues seem to have developed from secretory idioblasts scattered among the cells of the ordinary tissues. Subsequently ducts and cavities developed and finally secretory trichomes.

Abstract: To understand the adaptive nature of floral nectar production it is necessary to determine for individual plants the associated costs and benefits in terms of growth and/or reproduction1-3. Nectar production may use up to 37% of a plant's available energy4,5 but might not affect growth or reproduction. I report here that removal of nectar from flowers of Christmas bells (Blandfordia nobilis) increased the plant's net nectar production but reduced its ability to produce seeds. To our knowledge this is the first demonstration that nectar production entails a cost to a plant in terms of growth and/or reproduction and that both the gains and costs associated with nectar production may be estimated in the same 'currency' (seeds). As a plant's nectar production increases, there should therefore be a trade-off between pollinator-mediated increases in numbers of fertilized seeds1-3 and decreases in seed number due to the costs of producing the nectar.