Floral nectary, nectar production dynamics and chemical composition in five nocturnal Oenothera species (Onagraceae) in relation to floral visitors

Abstract

Main conclusion The floral nectars were sucrose-dominant; however, nectar protein and amino acid contents differed, indicating that composition of nitrogenous compounds may vary considerably even between closely related plant species, irrespectively of nectary structure. Numerous zoophilous plants attract their pollinators by offering floral nectar; an aqueous solution produced by specialized secretory tissues, known as floral nectaries. Although many papers on nectaries and nectar already exist, there has been a little research into the structure of nectaries and/or nectar production and composition in species belonging to the same genus. To redress this imbalance, we sought, in the present paper, to describe the floral nectary, nectar production, and nectar composition in five nocturnal Oenothera species with respect to their floral visitors. The structure of nectaries was similar for all the species investigated, and comprised the epidermis (with nectarostomata), numerous layers of nectary parenchyma, and subsecretory parenchyma. Anthesis for a single flower was short (ca. 10–12 h), and flowers lasted only one night. The release of floral nectar commenced at the bud stage (approx. 4 h before anthesis) and nectar was available to pollinators until petal closure. Nectar concentration was relatively low (ca. 27%) and the nectar was sucrose-dominant, and composed mainly of sucrose, glucose and fructose. The protein content of the nectar was also relatively low (on average, 0.31 µg ml−1). Nevertheless, a great variety of amino acids, including both protein and non-protein types, was detected in the nectar profile of the investigated taxa. We noted both diurnal and nocturnal generalist, opportunistic floral insect visitors.

Pollination biology and breeding system in five nocturnal species of Oenothera (Onagraceae): reproductive assurance and opportunities for outcrossing

“…Our findings are consistent with those of the latter authors and demonstrate that, apart from prior selfing, visits by bees may lead to effective transfer of outcross pollen in the investigated species of Oenothera. Furthermore, in another, complementary project (Antoń et al 2017), we reported that the complex pattern of nectar production and nectar distribution along the inflorescences of these taxa may determine the movement of pollinators between individual plants, thereby promoting cross-pollination. These observations all indicate that the breeding system present in subsect.…”

“…It is generally agreed that the great requirement of pollinators for energy necessitates the uptake of much nectar (Nicolson 2007 and references therein). In the case of the species of Oenothera studied, insect visitors were attracted to the flower by both copious amounts of floral nectar (i.e., up to 26.8 mg/flower; Antoń et al 2017) and pollen (i.e., up to 6.4 mg/flower; Antoń and Denisow 2018) that were constantly accessible from the onset of anthesis (i.e., evening hours) until the start of flower senescence (i.e., early morning hours). It is also worth noting that it has been proposed for other species of subsect.…”

“…Flowers of the investigated species produce both pollen and nectar as floral rewards. Floral nectar is exuded by secretory structures (nectaries) located at the base of a long, tubular hypanthium (Antoń et al 2017). Pollen grains are connected to each other by viscin threads, and pollen is produced in eight large, elongated anthers that dehisce longitudinally (Szklanowska and Czubacki 2000;Antoń and Denisow 2018).…”

“…More recently, however, it has been demonstrated that several species of subsect. Oenothera occurring in Poland produce copious amounts of floral nectar (up to 26 mg/flower), and nectar production has been related to the preferences of floral insect visitors observed in the field (Antoń et al 2017). Although many papers have been published on members of subsect.…”

“…By contrast, a preliminary survey of floral visitors in members of subsect. Oenothera in southeastern Poland (Antoń 2015;Antoń et al 2017) revealed that hawkmoths do not visit these species. Instead, both nocturnal and diurnal insects (including moths and bees) frequently visit flowers of Oenothera, possibly indicating a more generalist pollination syndrome.…”