The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp.) II. Nectar composition, origin of nectar solutes, and nectary functioning

Abstract

Nectar was collected from the extrafloral nectaries of leaf stipels and inflorescence stalks, and phloem sap from cryopunctured fruits of cowpea plants. Daily sugar losses as nectar were equivalent to only 0.1-2% of the plant's current net photosynthate, and were maximal in the fourth week after anthesis. Sucrose:glucose:fructose weight ratios of nectar varied from 1.5:1:1 to 0.5:1:1, whereas over 95% of phloem-sap sugar was sucrose. [(14)C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [(14)C]glucose and [(14)C]fructose prior to or during nectar secretion. Invertase (EC 3.2.1.26) activity was demonstrated for inflorescence-stalk nectar but not stipel nectar. The nectar invertase was largely associated with secretory cells that are extruded into the nectar during nectary functioning, and was active only after osmotic disruption of these cells upon dilution of the nectar. The nectar invertase functioned optimally (phloem-sap sucrose as substrate) at pH 5.5, with a starting sucrose concentration of 15% (w/v). Stipel nectar was much lower in amino compounds relative to sugars (0.08-0.17 mg g(-1) total sugar) than inflorescence nectar (22-30 mg g(-1)) or phloem sap (81-162 mg g(-1)). The two classes of nectar and phloem sap also differed noticeably in their complements of organic acids. Xylem feeding to leaves of a range of (14)C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. (14)C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. (14)C-labelled glycine, serine, arginine and aspartic acid showed limited direct access to phloem and nectar, although labelled metabolic products were transferred and secreted. The ureide allantoin was present in phloem, but absent from both types of nectar. Models of nectary functioning are proposed.

Nectar chemistry

“…All are derived from sucrose translocated in phloem sap or synthesized in the nectary. The relative amounts of each are determined by nectary invertase which hydrolyses sucrose to glucose and fructose, before or during nectar secretion (Pate et al, 1985). The sharp dichotomy between sucrose and hexose nectars seen within genera such as Erica and Leucospermum (Barnes et al, 1995;Nicolson & van Wyk, 1998), with nectars at both extremes of the sucrosehexose continuum, is indicative of the absence or presence of invertase activity.…”

Nectar as food for birds: the physiological consequences of drinking dilute sugar solutions

“…Partial hydrolysis is evidently responsible for the mixed sugar composition of the majority of nectars (Baker andBaker 1982a, Baker et al 1998), but details of nectar secretion mechanisms and the location of nectary invertase are generally lacking (Nicolson 2002). Studies such as those of Pate et al (1985) are an exception.…”

An Insight in the Reproductive Biology of Therophilus javanus (Hymenoptera, Braconidae, and Agathidinae), a Potential Biological Control Agent against the Legume Pod Borer (Lepidoptera, Crambidae)

“…In fact, synovigenic parasitoids that do not feed on host are usually able to use sugar foods for oogenesis [18]. Notably, cowpea itself, the major crop hosting caterpillars of M. vitrata, secretes extrafloral nectar [39], which may provide an adequate source of sugar food for foraging female parasitoids. We expect similar extrafloral nectar to be present on other important, wild-occurring host plants such as Sesbania rostrata (Fabales: Fabaceae) and Tephrosia platycarpa (Fabales: Fabaceae), known to harbor important pod borer populations [40], which might also be visited by foraging T. javanus females.…”