Flowers on opposite sides of a stem. Light and reproduction in Myrtillocactus geometrizans

One plant and different developmental conditions

A branched plant produces many repeated organs. Each branch occupies its own compass position, and a flower growing on it develops in a particular microenvironment. The cactus studied has bisexual flowers. Each flower contains anthers producing pollen and an ovary containing ovules. This makes it possible to compare investment in male and female functions within a single individual. Differences between its northern and southern sides exemplify within individual variation, meaning variation among organs belonging to the same plant [Aguilar-García et al. 2018].

The relevant reference is the position of ribs and branches on which reproductive structures form. Researchers compared flowers and buds with contrasting exposure and then measured their properties. This design describes the spatial organisation of flowering. Observing more fruits on the southern side of a crown concerns their distribution. Explaining the pattern biologically requires measurements of available light, stem physiology and flower structure. Successive studies of M. geometrizans cover different parts of this chain of relationships [Aguilar-García et al. 2018] [Ponce-Bautista et al. 2017].

Buds from opposite sides of the crown

The floral study was conducted at Barranca del Conejo near Zapotitlán Salinas in the Mexican state of Puebla. In February 2014, researchers selected 15 plants, followed in February 2015 by another 15. Two mature buds were collected from a northern branch and two from a southern branch of each individual. This produced 120 buds from 30 plants. A similar developmental stage was selected using bud size and colour. The comparison therefore involved material of comparable developmental age collected from differently exposed parts of the crown [Aguilar-García et al. 2018].

Collecting closed buds allowed pollen to be counted before it was released from the anthers. Sampling after opening and insect visits would make the remaining quantity depend partly on earlier removal. Anthers were counted in each bud, and pollen grains were counted in selected anthers. Total pollen production was estimated by multiplying the mean number of grains per anther by the number of anthers. Ovaries were dissected separately to count ovules and measure organ dimensions. Individual means were compared between the two sides, retaining the connection between samples from the same plant [Aguilar-García et al. 2018].

More pollen with similar ovule numbers

Southern buds had more anthers on average and more pollen grains within an individual anther. Both components increased the estimated number of grains per whole bud. The authors reported approximately 18% greater pollen production in southern buds. Anther length and width remained similar in the statistical comparison. The result shows that external anther dimensions alone poorly describe their contents. Two anthers of similar size can contain different numbers of grains, and total flower production also depends on how many such organs are present [Aguilar-García et al. 2018].

The female response was more restricted. Ovaries in southern buds were longer, whereas ovary width and ovule number did not differ significantly between orientations. The last trait is particularly relevant when interpreting resource allocation. Greater pollen production with similar ovule numbers increases the ratio between those quantities. In this experiment, southern flowers therefore showed a greater relative investment in pollen production. This indicator describes reproductive structure and allocation. Establishing the actual contribution of self pollination and pollination by other plants requires separate pollination experiments [Aguilar-García et al. 2018].

Pollen viability and germination

Pollen quality was considered through several separate properties. Diameter described grain size. Staining and shape were used to classify grains as viable or nonviable. Germinability was tested by the emergence of a pollen tube, and tube length described growth over a defined interval. The tube is a structure extending from a germinating grain that participates in delivering sperm cells to an ovule. Each measurement addresses a different stage. A positive staining result and development of a tube on a culture medium have different experimental meanings [Aguilar-García et al. 2018].

The germination test involved 20 flowers from 20 plants, with ten flowers representing each orientation. In this case, samples came from different individuals because flower availability prevented comparison of both sides of every plant. Pollen was placed on a culture medium and incubated for 30 hours at 30 °C. A grain was classified as germinated when its tube reached at least the length of the grain diameter. The mean proportion germinating was 25.5% for the northern side and 35% for the southern side. These results describe a particular laboratory procedure using pollen collected in April 2015 [Aguilar-García et al. 2018].

The pollen size measurement used another sampling design. In 2014, an additional set of buds was collected from 15 plants. For each orientation, 30 grains from a single anther were measured, producing 450 measurements in total. A mean was then calculated for each anther, and the two sides were compared at plant level. Numerous microscopic measurements improved the description of each sample, while the number of independent individuals remained determined by the field collection. Grain size was assessed after chemical preparation of the material. This matters when comparing these values with measurements of fresh pollen or pollen preserved differently [Aguilar-García et al. 2018].

Grain size and pollen tube growth

Pollen from the southern side had a larger diameter and longer tubes in the laboratory experiment. Mean tube length was approximately 197 µm for northern pollen and 366 µm for southern pollen. The authors discuss an explanation involving resources stored in a grain. A greater supply of available carbohydrates could support germination and tube elongation. The study measured pollen size and behaviour, whereas carbohydrate content in individual grains remains part of the proposed mechanism. This distinction connects the observed result with physiology without attributing unperformed measurements to the experiment [Aguilar-García et al. 2018].

The pollen results indicate differences in the potential of male function under the test conditions. In nature, the path from producing a grain to fertilisation contains further stages. Pollen must reach a suitable stigma, germinate within floral tissues and participate in fertilising an ovule. The 2018 study did not trace offspring paternity or the success of individual grains under natural pollination. A longer tube on a culture medium is therefore a specific physiological result. Its consequences for seed number and a plant’s contribution to the next generation require a separate field assessment [Aguilar-García et al. 2018].

Light as a possible source of differences

A study of three M. geometrizans populations linked fruit numbers with photosynthetically active radiation and the overnight increase in stem acidity. Higher values of these indicators were associated with greater spring fruit production. Acidity provided an indirect measure of carbon gain. The findings support an explanation in which differing illumination creates differences in resource availability among plant parts. Relationships nevertheless varied among sites and observation dates. Their scope covers the populations and season actually measured [Ponce-Bautista et al. 2017].

In the pollen study, most measured properties had higher values on the southern side. The authors interpreted this pattern as consistent with greater resource availability in more favourably illuminated plant parts. The experiment nevertheless used a comparative design. Entire branches were not experimentally assigned different exposures, and assimilate movement into each flower was not traced. The resource hypothesis therefore connects the measured gradient of traits with independent knowledge of stem illumination. The directly supported result remains clear. Within one population, flower position was associated with floral structure and pollen properties [Aguilar-García et al. 2018].

From floral traits to reproductive outcome

The further study published in 2022 combined branch phenology, floral traits, visiting animals and pollination experiments. Southern branches retained the mature-fruit phase for longer. Male function began and ended earlier in their flowers, while overall flower opening and stigma receptivity lasted longer on the northern side. This timing altered the duration of contact between a flower and visiting insects [Aguilar-García et al. 2022].

The researchers compared natural pollination, spontaneous self-pollination in enclosed flowers, hand pollination with pollen from the same plant and pollen from other individuals. Success was assessed as seeds per fruit and the fraction of estimated ovules producing seeds. Outcross pollen produced more seeds than hand self-pollination. The authors identified a mixed mating system with predominant outcrossing. Under natural pollination, the two orientations achieved similar values of both success measures [Aguilar-García et al. 2022].

Comparison of the 2018 and 2022 papers also reveals variation in the trait gradient itself. In the later study, southern buds again contained more anthers and more pollen per whole bud. Differences in grain size and pollen per individual anther were statistically inconclusive, while ovule number was higher on the southern side. Individual components of reproductive investment can therefore differ in their responses across successive samples [Aguilar-García et al. 2018] [Aguilar-García et al. 2022].

Longer availability of northern flowers and a similar overall visitor community help frame explanations for comparable seed production. The authors also considered redistribution of resources among retained structures and the effects of flower-damaging beetles. These hypotheses connect outcomes from different measurements. The direct finding is that differences in structure and flowering rhythms coexisted with similar reproductive success [Aguilar-García et al. 2022].