Reproductive Biology in the Possible Last Healthy Population of Parodia rechensis (Cactaceae): Perspectives to Avoid Its Extinction

Abstract

All 32 Brazilian species of Parodia Speg (Cactaceae) occurring in Rio Grande do Sul State are considered threatened, according to the IUCN criteria. Until 2021, Parodia rechensis (CR) was known by only two small populations. However, a new population with over 400 individuals was discovered in 2021, prompting the study of its reproductive biology as a way to promote its conservation. Anthesis, breeding system, and natural pollination were studied in the field. The breeding system was studied by applying controlled pollination treatments to plants excluded from pollinators (bagged). Germination features were studied at the Seed Bank of the Porto Alegre Botanical Garden under controlled temperatures (20, 25, and 30 °C). The anthesis is diurnal and lasts for up to four days. The flowers offer pollen as the sole resource to the pollinators. The study species is unable to set fruit and seed without the agency of pollinators and has self-incompatible (unable to set fruit and seeds when pollinated with pollen of the same individual) characteristics that can considerably restrict its reproduction. Native bees of Halictidae and Apidae (Hymenoptera) are the main pollinators, with a smaller contribution of Melyridae (Coleoptera) and Syrphidae (Diptera). Natural fruit set is moderate (≤64%, per individual), but the species presents vegetative growth, producing several branches from the mother plant. Seeds showed the optimum germination rate at 20 °C and an inhibition of 75% in germinability at 30 °C. Our findings suggest the need to manage the species' habitat to guarantee the permanency of the plants and healthy populations of pollinators as well. Our findings raise concerns about the germination and establishment of new individuals in the context of rising temperatures caused by climate change. Suggestions for the possible management of the extant populations are made.

Keywords: breeding system; cactus; conservation; mellitophyly; pollination.

Figure 1

General appearance of Parodia rechensis in its habitat. (a,b) Flowers with orange perianth elements. (c,d) Flowers with yellow perianth elements. (e) Plants of both phenotypes in the environment.

Figure 2

Parodia rechensis habitat. (a) Area of the type population discovered in 1968. (b) New population discovered in 2021.

Figure 3

Average frequency of pollinator interaction with Parodia rechensis flowers at 60-min intervals.

Figure 4

Insect pollinators in Parodia rechensis. (a) Augochlora sp. (Halictidae); (b) Ceratina sp. (Apidae); (c) Syrphidae; and (d) Melyridae.

Figure 5

Boxplot graphics showing the variation in height (a–c), width (d–f), and number of branches (g–i) of Parodia rechensis during 12 months. In yellow is the group exposed to the sun, and in gray is the shaded group. T1: initial measurement (October/2022); T2: period between October/2022 and April/2023; T3: period between April/2023 and October/2023. Significant p-values are indicated in their respective boxplots. (ns): non-significant p-value at 5%.

Figure 6

Temperature in Caxias do Sul, Brazil, between 1961 and 2024. Mean maximum temperature for October–January (blue); historical mean maximum temperature (orange) (INMET, 2024).

Figure 7

Parodia rechensis location. (a) Rio Grande do Sul state map with phytogeographic domain borders, following IBGE (Instituto Brasileiro de Geografia e Estatística). (b) general aspect of Mixed Ombrofilous Forest, an Atlantic Rainforest biome; (c) general aspect of Pampa grasslands.