The Strombocactus nectary. Secretory tissue inside the flower

Three colours and a common site of secretion

The nominate subspecies of Strombocactus disciformis opens a white or yellowish-white flower, subsp. esperanzae a magenta flower, and S. corregidorae a yellow one. The funnel-shaped perianth opens during the day. Inside it, numerous stamens surround the style. Below lies the nectar-producing zone. Camacho-Velázquez and co-authors described the structure of this small tissue in 2019, combining light and electron microscopy, histochemical staining and nectar measurement. [Camacho-Velázquez et al. 2019, pp. 100–103]

For structural analysis, 20 buds and 20 open flowers were collected from at least ten individuals of each taxon. The material of S. corregidorae and the nominate S. disciformis was collected in Querétaro in February and March 2016–2017. Subsp. esperanzae came from the UNAM collection and the botanical garden in Cadereyta. Flower opening time was determined from observations of at least 30 flowers of each taxon. These were different parts of the project, each with its own material and measurement scope. [Camacho-Velázquez et al. 2019, pp. 101–102]

A ring in the hypanthium

The nectary lies in the lower inner part of the floral tube, below the attachment of the innermost stamens, around the base of the style. This tube is part of the hypanthium, the portion of the flower in which the perianth segments and stamens are inserted. The secretory tissue forms a ring. Its upper edge has free projections that are wavy in places. A small open space beside the style collects the secretion. The position of the nectary and the shape of the ring can thus be described independently of perianth colour. [Camacho-Velázquez et al. 2019, pp. 102–104, Fig. 2–3]

In older cactus terminology, the Strombocactus nectary was sometimes described as furrowed. The researchers in 2019 indicated that the innermost stamens form neither a closure nor an appropriate fold that would justify this description. The observed tissue retains a clear ring-shaped arrangement. The term hypanthial nectary specifies its position, and ring-shaped describes its form. These are two complementary ways of characterising the same structure. [Camacho-Velázquez et al. 2019, pp. 104–105]

Epidermis, parenchyma and vascular bundles

The nectary surface is covered by a single layer of thin-walled epidermal cells. Their shape differs between taxa. In subsp. esperanzae, they are relatively flat, in the nominate subspecies slightly convex, and in S. corregidorae more strongly bulging, forming small papillae. The cuticle on the surface has subtle striated ornamentation. Nectarostomata, modified stomata involved in the release of secretion, are distributed among these cells. [Camacho-Velázquez et al. 2019, pp. 102–103, Fig. 2]

Nectariferous parenchyma lies directly beneath the epidermis. Its small, fairly closely packed cells have dense cytoplasm, distinct nuclei, thin walls and vacuoles. Spaces occur between them. Beneath this layer is parenchyma with larger cells and less dense cytoplasm. It is accompanied by numerous bundles containing xylem and phloem. They run parallel within the hypanthium tissue, above the upper part of the ovary, and are oriented transversely towards the nectary. The deeper parenchyma thus connects the secretory zone with the vascular bundles. [Camacho-Velázquez et al. 2019, pp. 103–104, Fig. 3]

In the sections examined, the number of secretory parenchyma layers was approximately 10–12 in subsp. disciformis, 8–9 in subsp. esperanzae and 7–8 in S. corregidorae. These values describe the tissues observed. Nectar volume was too small to determine using the method employed, so these anatomical differences were not linked to a specified secretion rate. [Camacho-Velázquez et al. 2019, pp. 103–104]

Open nectarostomata

Large spaces beneath the nectarostomatal pores connect the surface with the internal tissue. In the buds examined, the pores were open before the flower had fully expanded, and also during its secretory phase. Residues of secretion on the surface remained visible even after preparation of the material for electron microscopy. Together, the observations identify these pores as the route through which nectar exits. [Camacho-Velázquez et al. 2019, p. 103, Fig. 2–3]

In their functional description, the authors relate nectarostomata to stomata that have lost the usual regulation of opening and closing. The observed sections document the state of the pores at selected flower stages. The kinetics of guard-cell opening and closing were not measured. The functional conclusion is based on the visible state of the pores, their position and the residues of secretion. [Camacho-Velázquez et al. 2019, pp. 103, 106]

What staining reveals

PAS staining detected insoluble polysaccharides in the walls and parts of the cytoplasm of the parenchyma, while naphthol blue staining indicated proteins. The secretory tissue stained more intensely than the deeper parenchyma. Dense cytoplasm and distinct nuclei support the interpretation that the cells were active. The stains locate specific groups of components. The image is a spatial map of the detected components, and the rate of sugar transport requires physiological measurement. [Camacho-Velázquez et al. 2019, pp. 102–104, Table 1, Fig. 4–6]

In the two nectary layers themselves, Lugol's test did not detect starch. Starch was found in the parenchyma of the style and other, non-secretory parts of the hypanthium. Observation under polarised light confirmed identification of the grains. Their ordered structure produces an image of bright arms called a Maltese cross. The authors link the distribution of starch to a possible storage function of neighbouring tissues. A positive oil red reaction occurred in the cuticle, while this staining did not reveal lipid reserves in the parenchyma examined. The distribution of these components distinguishes the actual secretory zone from the surrounding floral tissues. [Camacho-Velázquez et al. 2019, pp. 103–104, Table 1, Fig. 4–6]

The authors interpreted the appearance of living cells retaining their structure as consistent with merocrine secretion. In this process, secretion leaves the cell while its integrity is maintained. The histological data support this interpretation. [Camacho-Velázquez et al. 2019, p. 106]

Measuring nectar at a specified time

In February 2018, nectar was collected in the UNAM collection at 15 hours from fifteen flowers of different individuals of each taxon. The droplets were analysed with a refractometer. The instrument measures the refractive index and reports the result on the degrees Brix scale. This scale is referenced to sucrose solutions, and in nectar the reading characterises the concentration of dissolved substances. Determining the mixture's composition requires chemical analysis of individual sugars and other compounds. [Camacho-Velázquez et al. 2019, pp. 102, 104, 107]

The values represent one sampling time in a specific collection. They do not determine daily secretion or the flower's total energy reward, because its full volume and composition were not measured. [Camacho-Velázquez et al. 2019, pp. 102–104, 107]

A reward for a flower visitor

Pollen in open anthers and nectar at the base of the stamens provide two available rewards. Daytime opening, the funnel-shaped flower and its colour led the authors to hypothesise pollination by bees. Such a set of characters helps identify likely visitors. Establishing a pollinator also requires observations of pollen transfer to the stigma and seed set after its visits. These experiments were not performed in the nectary study. [Camacho-Velázquez et al. 2019, pp. 104, 107]

In the 2018 seed study, the authors recorded frequent bee visits to the nominate S. disciformis and less frequently visited flowers of S. corregidorae. The latter species had a lower ratio of mean seed number to ovule number. Insufficient pollination was considered, but no detailed comparison of visitation numbers and the effects of a single visit was presented. The material of subsp. esperanzae was produced after hand pollination under collection conditions. These three situations differ in their biological scope. [Camacho-Velázquez et al. 2018, pp. 90–91, 93–94, 100]