Sinks for Plant Surplus Carbon Explain Several Ecological Phenomena

Plants engage in many processes and relationships that appear to be wasteful of the high-energy compounds that they produce through carbon fixation and photosynthesis. For example, living trees keep leafless tree stumps alive (i.e. respiring) and support shaded understory trees by sharing carbohydrates through root grafts or mycorrhizal fungal networks. Plants exude a variety of organic compounds from their roots and leaves, which support abundant rhizosphere and phyllosphere microbiomes. Some plants release substantial amounts of sugar via extra-floral nectaries, which enrich throughfall and alter lichen communities beneath the canopy. Large amounts of photosynthetically fixed carbon are transferred to root associates such as mycorrhizal fungi and N-fixing micro-organisms. In roots, some fixed C is respired through an alternative non-phosphorylating pathway that oxidizes excess sugar. Each of these processes is most prevalent when plants are growing under mild-to-moderate deficiencies or nutrients or water, or under high light or elevated atmospheric CO2. Under these conditions, plants produce more fixed carbon than they can use for primary metabolism and growth, and so have ‘surplus carbon’. To prevent cellular damage, these compounds must be transformed into other compounds or removed from the leaf. Each of the above phenomena represents a potential sink for these surplus carbohydrates. The fundamental ‘purpose’ of these phenomena may therefore be to alleviate the plant of surplus fixed C.

Nectar secretion in a dry habitat: structure of the nectary in two endangered Mexican species of Barkeria (Orchidaceae)

Barkeria scandens and B. whartoniana are endangered, endemic taxa from Mexico. They are epiphytes adapted to dry habitats. Since these plants are xerophytic, their flowers were investigated for structural adaptations to nectar secretion. The flowers of both species are structurally similar, and contrary to most claims for the genus, have functional floral nectaries comprising a nectary chamber and a narrow tubular cuniculus. Nectar is present in both these structures, and contains sugars and lipid-like compounds. The nectary tissue is composed of a single-layered epidermis overlying 1–2 layers of subepidermal secretory parenchyma. The outer tangential wall of the epidermal cells is thick and multi-layered, whereas the cuticle, which often shows blistering, is lamellate and possesses micro-channels. Lipid-like material occurs both between the microfibrils of the cell wall and in the micro-channels. Robust secretory tissue, thick cell walls, and lipid-like nectar components limit nectar evaporation. Moreover, the rigidity of the nectary potentially makes it possible for red-flowered B. scandens to switch from entomophily to ornithophily.

Altered feeding behavior and immune competence in paper wasps: A case of parasite manipulation?

Paper wasps (Polistes dominula), parasitized by the strepsipteran Xenos vesparum, are castrated and desert the colony to gather on plants where the parasite mates and releases primary larvae, thus completing its lifecycle. One of these plants is the trumpet creeper Campsis radicans: in a previous study the majority of all wasps collected from this plant were parasitized and focused their foraging activity on C. radicans buds. The unexpected prevalence and unusual feeding strategy prompted us to investigate the influence of this plant on wasp behavior and physiology through a multidisciplinary approach. First, in a series of laboratory bioassays, we observed that parasitized wasps spent more time than non-parasitized ones on fresh C. radicans buds, rich of extra-floral nectaries (EFNs), while the same wasps ignored treated buds that lacked nectar drops. Then, we described the structure and ultra-structure of EFNs secreting cells, compatible with the synthesis of phenolic compounds. Subsequently, we analysed extracts from different bud tissues by HPLC-DAD-MS and found that verbascoside was the most abundant bioactive molecule in those tissues rich in EFNs. Finally, we tested the immune-stimulant properties of verbascoside, as the biochemical nature of this compound indicates it might function as an antibacterial and antioxidant. We measured bacterial clearance in wasps, as a proxy for overall immune competence, and observed that it was enhanced after administration of verbascoside—even more so if the wasp was parasitized. We hypothesize that the parasite manipulates wasp behavior to preferentially feed on C. radicans EFNs, since the bioactive properties of verbascoside likely increase host survival and thus the parasite own fitness.

Studies on the Dipterocarpaceae of Borneo, II. Ant stipule-brood sites and extra floral nectary association in saplings of Shorea macrophylla [sect. Pachycarpae] in Sarawak, Malaysian Borneo

The presence of stipular and leaf blade extra floral nectaries and associated ant activity, including brood raising within stipules, is reported for saplings of Shorea macrophylla [sect. Pachycarpae] in Kuching Division, Sarawak.

The structure of nectaries in the genus Strombocactus (Cactaceae)

<p><strong>Background:</strong> Floral nectar is offered by the plant to its pollinators to promote cross-fertilization.</p><p><strong>Questions:</strong> Are floral nectaries morpho-anatomically similar among the taxa of <em>Strombocactus</em>? What sugars do nectaries offer their pollinators?</p><p><strong>Studied species:</strong> <em>Strombocactus disciformis</em> subsp. <em>disciformis</em>, <em>S. disciformis</em> subsp. <em>esperanzae</em> and <em>S. corregidorae,</em> during 2016-2018.</p><p><strong>Methods:</strong> Bud flowers and flowers in anthesis from each taxon of <em>Strombocactus</em>, were processed to be observed in SEM; other specimens were embedded in Paraplast and sectioned to histochemical tests. The concentration of nectar was measured using a refractometer.</p><p><strong>Results:</strong> The nectaries are similar among the three studied taxa, located below the stamens and reach the upper part of the ovary forming a ring in the hypanthium. An almost flat epidermis constitutes the nectary in <em>S. disciformis </em>subsp.<em> esperanzae</em>, although in <em>S. disciformis </em>subsp.<em> disciformis</em> is slightly bulked and papillae in <em>S</em>.<em> corregidorae, </em>a nectariferous parenchyma composed of metabolically very active cells, and a subnectariferous parenchyma of larger cells, associated with vascular bundles. The nectar is secreted through nectarostomata. The concentration of nectar was 8.65 ± 3.98, 16.12 ± 4.48 and 22.09 ± 7.42 °Brix for <em>S. disciformis</em> subsp. <em>esperanzae</em>, <em>S. corregidorae</em> and <em>S. disciformis</em> subsp. <em>disciformis</em>, respectively.</p><p><strong>Conclusions:</strong> All taxa of <em>Strombocactus</em> have an annular-type nectary that secretes a low volume of nectar and together with pollen are offered as rewards. Based on other floral characteristics observed, such as the diurnal anthesis, the infundibuliform shape and the color of the flower, we propose that bees pollinate these species.</p>