Neoraimondia and Thelocactus. Flower access, foreign pollen and fruit set

Two populations and two types of trial

Studies of Neoraimondia herzogiana were conducted from October to December 2013 in Wara Wara near Aiquile in Bolivia, at 2084 m elevation. The plants grew in a dry inter-Andean valley. Arzabe and coauthors compared flowers enclosed in fine mesh with flowers freely accessible to animals. Excluding visitors in this way allows their association with fruit set to be tested.[Arzabe et al. 2018, pp. 180–183]

In 2018, Delgado-Ramírez and coauthors studied a population of Thelocactus leucacanthus near La Estación in the municipality of Peñamiller, in Querétaro, Mexico, at 1290 m elevation. The publication identified the plants as T. leucacanthus subsp. schmollii. POWO now includes this name in T. leucacanthus. The experiment involved hand transfer of self-pollen and foreign pollen, and its result applies to this local population with purple-carmine flowers.[Delgado-Ramírez et al. 2021, pp. 793–794; POWO, checked on 11 October 2026][2-WWW]

The tubular flower of Neoraimondia

Flowers of N. herzogiana were tubular with two series of thick, succulent pink tepals. Anthers and the stigma were pale yellow. The authors detected no perceptible scent. Anthesis, the period when a flower is open and functional, included both day and night. Over such a broad interval, animals may reach the reproductive organs at different times.[Arzabe et al. 2018, pp. 181–184, Tables 1–2]

On 30 plants, one bud per plant was assigned to covering and one to opening freely. After losses, the study sample numbered 57 flowers. Fruit set was recorded in 86,7% of flowers accessible to animals and 3,7% of covered flowers. The authors identified fruit set by ovary enlargement. The difference documents a strong dependence of this stage on visitor access. The study did not measure seed number or subsequent germination.[Arzabe et al. 2018, pp. 181–183, Figure 4]

The N. herzogiana trials did not involve hand transfer of self-pollen or foreign pollen. Low fruit set under cover may result from limited spontaneous pollen delivery to the stigma or from properties of the reproductive system. These trials leave the physiological mechanism unresolved.[Arzabe et al. 2018, pp. 181–183, 186]

Insect contact with floral organs

Observers recorded a visitor only when it touched reproductive organs. They followed 30 flowers of each of the two species studied for six days, in four daily half-hour periods between 5.30 and 22.00. In N. herzogiana, Vespidae wasps accounted for 37,5% of recorded visits, honeybees for 25%, and Scarabaeidae and Tenebrionidae beetles for 21,9% and 9,4% respectively. Ants accounted for the remaining visits.[Arzabe et al. 2018, pp. 181–185, Table 3]

Contact with anthers or the stigma indicates the possibility of pollen transfer. Contact frequency describes the contribution of insect groups to observed visits. This study did not measure pollen deposited on the stigma after an individual visit. The proportions of wasp and bee visits therefore do not directly determine these insects' contributions to seed production.[Arzabe et al. 2018, pp. 182–186]

A few hours in the life of a Thelocactus flower

In the Querétaro population, T. leucacanthus flowers opened on a single day. Following 27 flowers showed anthesis at approximately 10.30–15.00. Measurements of 30 flowers gave a mean length of 36,04 ± 4,02 mm and a diameter of 51,21 ± 11,67 mm. Dispersion was reported as the standard deviation. This short period brings together flower opening, pollen availability and visits by diurnal insects.[Delgado-Ramírez et al. 2021, pp. 794–795, Table 2, Figure 2]

Anthers released pollen at the start of anthesis, and stigma receptivity appeared later. This is incomplete protandry, an earlier start to the male phase. The phases partly overlapped in time. The authors found no clear spatial separation between the stigma and outer stamens in this population. Earlier anther maturation may limit some contacts of self-pollen with the stigma, while the hand-pollination experiment separately tested its capacity to induce fruit set.[Delgado-Ramírez et al. 2021, pp. 795–796, Table 2, Figure 2]

Self-pollen and foreign pollen in five treatments

Pollination trials were performed in May 2018. In the outcrossing treatment, anthers were removed, the flower was covered and a mixture of fresh pollen from eight other plants was applied at about 12.30. In the self-pollination trial, pollen from the same plant was applied. Separate treatments tested fruit set under cover alone and apomixis, seed formation without fertilization, after anther removal and covering. Fruits were collected about 45 days after the trials.[Delgado-Ramírez et al. 2021, pp. 794, 797–798]

Foreign pollen enabled fruit set, while both self-pollination treatments produced none. The result supports recognition of this population's sexual reproduction as dependent on outcrossing. The experiment did not separate biochemical self-incompatibility from embryo loss associated with mating between closely related individuals. No significant difference in seed number was found between fruits from freely accessible flowers and those pollinated with foreign pollen. Pollen transfer to flowers after anther removal was not a supplementary-pollination trial on intact open flowers, so it did not directly measure pollen limitation under natural conditions.[Delgado-Ramírez et al. 2021, pp. 797–798, 801–802]

Pollen and a small amount of nectar

During 55 hours of observation in May, June and October, the authors assessed visits to about 50 flowers in four clusters. Small Lasioglossum bees of the subgenus Dialictus appeared most often, at 4,2 visits per flower per hour. Frequencies of 2,6 were recorded for Pseudopanurgus and 2,2 for Exomalopsis. Analysis of pollen on insect bodies supplemented observations of their contacts with floral organs. Diadasia bees carried much pollen from this cactus despite less frequent visits.[Delgado-Ramírez et al. 2021, pp. 794, 798–800, Tables 4–5]

The nectary formed a ring at the bottom of the hypanthium, the lower flower part surrounding the pistil, below the stamen bases. Preparations revealed stomata, small cells near vascular bundles and larger cells deeper inside. Starch, polysaccharides and proteins were detected in the tissue, and lipids mainly in the cuticle. The authors were unable to collect nectar with a 10 µl capillary. The size of the nectar reward therefore remained unmeasured. Bee behaviour indicated pollen as the main observed reward.[Delgado-Ramírez et al. 2021, pp. 796–797, Figures 3–4]

The description of the flower and pollination trials connects its structure with reproductive outcome in a particular population. In N. herzogiana, the sharp decline in fruit set under cover demonstrates the significance of animal access. In the studied T. leucacanthus, hand transfer of foreign pollen additionally isolates the significance of its origin. Both experiments concern local plants and specified observation periods.[Arzabe et al. 2018, Arzabe et al., pp. 181–183; Delgado-Ramírez et al., pp. 794, 798][Delgado-Ramírez et al. 2021, Arzabe et al., pp. 181–183; Delgado-Ramírez et al., pp. 794, 798]