Morphology, secretion composition, and ecological aspects of stipular colleters in Rubiaceae species from tropical forest and savanna

Abstract

Colleters are secretory structures that produce and release mucilage or a mucilage-resin mixture protecting meristems and young structures against desiccation, herbivores, and pathogens. The secretions may vary in colleters of same or different types, indicating that the functionality of colleters may be more specific than previously thought. In this study, we compared 17 Rubiaceae species from savanna and forest environment focusing on colleter secretions and its ecological role. First, we evaluated the morphology, distribution, and histochemistry of stipular colleters using light and scanning electron microscopy. Additionally, we investigated the phenology, microclimate, and the proportion of damaged apices in the savanna and forest species. We recorded standard-type colleters, variable in distribution and size, in 14 of the 17 studied species. The secretion varied from predominantly hydrophilic, mixed to predominantly lipophilic. During the budding period, secretion covered the vegetative apices. Savanna species had a prevalence of lipid secretion in habitats with higher luminosity, which had a lower proportion of damaged apices. In contrast, forest species occurred in habitats with lower luminosity and had a higher proportion of damaged apices, in general with the absence of lipids in the colleters. These results highlight that colleters with similar morphology clearly differed in secretions among species, especially between species from savanna and forest, in which the colleters appear potentially associated with protection against irradiation in savanna, but not in the forest environment.

Colleters in American Spermacoceae Genera (Rubiaceae): Morphoanatomical and Evolutionary Aspects

“…The presence of chlorenchyma is recorded for the first time in Borreria, Diodia, Galianthe, Manettia, and Oldenlandia. Tresmondi et al (2015) mentioned this tissue in stipules of Richardia grandiflora, and we also observed it in stipules and calyx of Richardia brasiliensis.…”

“…The presence of colleters on the internal surface of the calyx and stipules is common in other tribes of the Rubiaceae, even in Rubioidae (e.g., on the stipules of Psychotria L., tribe Psychotrieae; Miguel et al 2009;Vitarelli and Santos 2009;Tresmondi et al 2015). The latter pattern is also found in Cephalanthus occidentalis L. of the tribe Naucleeae (Cinchonoideae; Rutishauser 1983) and in species of Gardenieae (Ixoroideae), such as Randia L. (Judkevich et al 2015;Tresmondi et al 2015). In other Gardenieae, the co-occurrence of colleters inside the stipules and calyx is very common, such as in Gardenia J. Ellis (Dave et al 1988;Low and Wong 2007), Cordiera A.…”

“…Rich. (Barreiro and Machado 2007;Tresmondi et al 2015), Rosenbergiodendron Fagerl. (Gustafsson 1998), and Tocoyena Aubl.…”

“…(Gustafsson 1998), and Tocoyena Aubl. Tresmondi et al 2015). According to Robbrecht (1988), the distribution of colleters on the internal surface of stipules is perhaps the primitive condition.…”

“…We found green colleters on stipules and calices in six of the analyzed species. The presence of chlorenchyma has been mentioned in stipular colleters of a few species of Rubiaceae (Tresmondi et al 2015), in the stalk of colleters of Allamanda L. of Apocynaceae (Ramayya and Bahadur 1968), and in the developing colleters of Macrocarpaea obtusifolia, Gentianaceae (Dalvi et al 2014). It has been mentioned that the presence of chlorenchyma in the latter species might be related to the production of the precursors of the secreted compounds.…”