Strombocactus seeds. Aril, water and the beginning of growth

A seed from a rocky canyon

On a limestone wall, a small Strombocactus stem occupies a crevice containing fine soil. The fruit releases seeds so small that details of their coat become clear only under a microscope. Establishment of a plant in such a place requires successive events, from fertilisation of the ovule through seed movement and radicle emergence to seedling survival. The 2018 study by Camacho-Velázquez and co-authors combined observations of seed development with flotation and germination experiments. Its material represented S. corregidorae and both subspecies of S. disciformis. [Camacho-Velázquez et al. 2018, pp. 89–91]

POWO continues to accept two species. The name Chichimecactus corregidorae, present in some more recent literature, is treated here as a synonym of Strombocactus corregidorae. In the comparison of three taxa, diagnostic characters include flower colour, spine persistence and details of the seed coat. [2-WWW][2-WWW]

Origin of the pale appendage

The ovule remains connected to the placenta in the ovary by the funiculus. Tissues of the future seed develop within the ovule, followed by the embryo after fertilisation. In both subspecies of S. disciformis, local cell divisions beneath the epidermis of the terminal portion of the funiculus form an additional projection. It appears very early, before the first visible divisions of the zygote. It subsequently grows into a horseshoe shape, gradually surrounding the lower part of the seed. Successive embryo stages allowed this sequence to be reconstructed, providing developmental evidence for the origin of the appendage beyond its mature appearance. [Camacho-Velázquez et al. 2018, pp. 91–93, Fig. 2–4]

Its origin from the funiculus determines the name of this structure. It is an aril, or arillus. Older descriptions of Strombocactus also used the terms strophiole or elaiosome. In the terminology adopted by the authors, the first refers to an appendage arising in the region of the ovule's raphe. Elaiosome describes a structure providing food for animals that carry seeds. The names therefore describe different properties. Development of the S. disciformis aril was documented in the funiculus, without involvement of the ovule integuments. [Camacho-Velázquez et al. 2018, pp. 91, 97–98]

Aerenchyma, parenchyma containing large air spaces, differentiates inside the growing appendage. The mature aril is pale and membranous. No reserves were demonstrated in it that would justify treating it as a food reward for ants. The authors observed ants of the genus Atta carrying fruits. This event involves the whole fruit, and the role of the aril in the transport of individual seeds in the field was not established. [Camacho-Velázquez et al. 2018, pp. 92–93, 98]

The aril and seed surface

The embryo and principal seed layers develop similarly in all three taxa. In S. corregidorae, the projection of the funiculus that forms the aril does not develop. Growth of the coat near the micropyle, the small opening in the ovule integuments, also proceeds differently. The outer layer later catches up with and covers the inner layer. In the mature seed, the region of the micropyle and hilum, the scar marking the attachment of the funiculus, remains exposed. [Camacho-Velázquez et al. 2018, p. 92, Fig. 5]

Electron microscopy revealed further differences. In S. corregidorae, the outer walls of the coat cells are flat, with fine relief that is reticulate in places. In the nominate S. disciformis, the cells are strongly convex, particularly on the dorsal side of the seed. Subsp. esperanzae has flatter projections and a more distinct reticulate pattern in the cuticle. Both subspecies share the aril, while details of the seed surface also allow comparison between them. [Camacho-Velázquez et al. 2018, pp. 92–93, Fig. 7]

Measurements included 32 seeds of each taxon, four from each of eight fruits. Mean length without the aril was 0,41 mm in subsp. disciformis, 0,46 mm in subsp. esperanzae and 0,48 mm in S. corregidorae. The aril increased total length to a mean of 0,53 and 0,57 mm in the first two taxa. Inside the seed, much of the space is occupied by an embryo with a broad hypocotyl, small cotyledons and a short radicle. The hypocotyl is the part of the axis below the cotyledons that forms a substantial portion of the young globose seedling after germination. [Camacho-Velázquez et al. 2018, pp. 90, 92–93, Fig. 4–6]

Air in the aril and coat cells

In the flotation experiment, 20 seeds of each taxon were placed in 265 ml of distilled water, in five replicates. All remained on the surface even after 25 days. The result also included S. corregidorae, which lacks an aril. Buoyancy is therefore associated with several seed characters. In all taxa, cells of the hard coat contained empty spaces. In both subspecies of S. disciformis, these were supplemented by spaces within the aril and a different surface relief. [Camacho-Velázquez et al. 2018, pp. 91–94, 99]

A separate treatment with a small amount of detergent reduced the proportion of seeds remaining on the surface. The authors also examined the response to submersion. A jet of 0,3 ml of water pushed a seed towards the bottom of the vessel, and for seeds that reached the bottom, the time taken to rise through an eight-centimetre water column was measured. This comparison used 40 different seeds of each taxon. [Camacho-Velázquez et al. 2018, pp. 91, 94]

Seeds of both subspecies with an aril returned faster than seeds of S. corregidorae. The taxa compared differed simultaneously in the aril, the shape of coat cells and cuticular ornamentation. The result demonstrates a property of the whole seed, and the contribution of each individual character requires a separate experiment. Anatomy and buoyancy support the hypothesis of transport by water flowing over rock. The study did not measure dispersal distance or the contribution of this pathway to recruitment in natural populations. No separate germination test was performed after the seeds had spent 25 days on the water surface. [Camacho-Velázquez et al. 2018, pp. 91, 94, 98–100]

Germination at a specified temperature and light regime

The germination experiment used eight replicates of 30 seeds of each taxon. They were placed on moist filter paper in dishes, at 25°C with a twelve-hour photoperiod. The seeds were not scarified, meaning their coats were not mechanically damaged, or disinfected. Daily monitoring continued for 30 days. Germination was defined as emergence of the radicle tip beyond the seed coat. [Camacho-Velázquez et al. 2018, p. 91]

Differences in the final proportion of germinating seeds and mean germination time were not statistically significant in this experiment. The properties of the tested batches were therefore similar despite the presence of an aril in two taxa. The comparison involved different taxa, so the effect of the aril itself remains to be tested separately. Under the conditions used, no coat barrier or dormancy requiring a treatment to break it became apparent. [Camacho-Velázquez et al. 2018, pp. 94, 100]

How the seed opens

The first radicle emerged approximately two to three days after water uptake began. A circumferential crack forms near the hilum and micropyle, separating a small lid in the coat. This is called an operculum. The radicle pushes this lid aside and, in seeds with an aril, also passes through the aril and may curve as it does so. The growing hypocotyl widens a second, longitudinal crack. As development continues, the coat opens into two irregular parts. [Camacho-Velázquez et al. 2018, p. 94, Fig. 8]

The radicle is short and broad, with numerous root hairs appearing close to its tip. The hypocotyl expands radially. Small cotyledons cover the shoot meristem, the tissue that gives rise to new parts of the shoot. The cotyledons and hypocotyl become green within two to three days of germination. Stomata appear early in the hypocotyl epidermis, and small tubercles and areoles later differentiate near the apex. In this way, the embryo occupying almost the entire seed becomes an independent globose plant. [Camacho-Velázquez et al. 2018, pp. 94–95, Fig. 8]

From numerous seeds to an established seedling

The material of S. corregidorae and the nominate subspecies of S. disciformis came from single natural localities in Querétaro. Subsp. esperanzae was represented by plants from two botanical gardens, and seeds were obtained after hand pollination. The ratios of developed seeds to ovule numbers, calculated from the means, were 52,01%, 74,20% and 63,87%, respectively. The difference concerned the proportion of potential seed production realised. High germination in all three batches shows that the seeds produced retained a high capacity for development in the test used. [Camacho-Velázquez et al. 2018, pp. 90–91, 93–94, 100]

The experiment measured the capacity to germinate under controlled conditions. Population recruitment subsequently involves seedling establishment and survival in the rocky habitat. These later stages lie outside the scope of the test described. Seeds of subsp. esperanzae, obtained in botanical gardens after hand pollination, retained the ability to germinate and provided material for propagation in a collection. [Camacho-Velázquez et al. 2018, pp. 90, 100]