Abstract
It is well known that trichomes protect plant organs, and several studies have investigated their role in the adaptation of plants to harsh environments. Recent studies have shown that the production of hydrophilic substances by glandular trichomes and the deposition of this secretion on young organs may facilitate water retention, thus preventing desiccation and favouring organ growth until the plant develops other protective mechanisms. Lychnophora diamantinana is a species endemic to the Brazilian 'campos rupestres' (rocky fields), a region characterized by intense solar radiation and water deficits. This study sought to investigate trichomes and the origin of the substances observed on the stem apices of L. diamantinana. Samples of stem apices, young and expanded leaves were studied using standard techniques, including light microscopy and scanning and transmission electron microscopy. Histochemical tests were used to identify the major groups of metabolites present in the trichomes and the hyaline material deposited on the apices. Non-glandular trichomes and glandular trichomes were observed. The material deposited on the stem apices was hyaline, highly hydrophilic and viscous. This hyaline material primarily consists of carbohydrates that result from the partial degradation of the cell wall of uniseriate trichomes. This degradation occurs at the same time that glandular trichomes secrete terpenoids, phenolic compounds and proteins. These results suggest that the non-glandular trichomes on the leaves of L. diamantinana help protect the young organ, particularly against desiccation, by deposition of highly hydrated substances on the apices. Furthermore, the secretion of glandular trichomes probably repels herbivore and pathogen attacks.
Highlights
In nature, the trichomes can exhibit an enormous diversity in relation to the morphology, origin, size, location, timing of activity and function; often the trichomes are distinguished as glandular and non-glandular, according to their functions, mainly the capability to secrete (Werker 2000)
The trichomes are commonly related to the protection of the young organs: the glandular trichomes release repellent compounds against herbivores and pathogens (Werker 2000; Siebert 2004; Gopfert et al 2005; Machado et al 2006) or produce hydrophilic substances related to protection against desiccation (Bruni et al 1987; Fahn 1990; Paiva and Martins 2011); and non-glandular trichome acting as a mechanical barrier against excessive light, water loss and extreme temperatures (Fahn and Cutler 1992; Turner 1994; Werker 2000)
We describe for the first time an unusual way for water retention on the stem apices of L. diamantinana by the partial degradation of the cell wall of the terminal cells in the non-glandular trichomes
Summary
The trichomes can exhibit an enormous diversity in relation to the morphology, origin, size, location, timing of activity and function; often the trichomes are distinguished as glandular and non-glandular, according to their functions, mainly the capability to secrete (Werker 2000). The trichomes are commonly related to the protection of the young organs: the glandular trichomes release repellent compounds against herbivores and pathogens (Werker 2000; Siebert 2004; Gopfert et al 2005; Machado et al 2006) or produce hydrophilic substances related to protection against desiccation (Bruni et al 1987; Fahn 1990; Paiva and Martins 2011); and non-glandular trichome acting as a mechanical barrier against excessive light, water loss and extreme temperatures (Fahn and Cutler 1992; Turner 1994; Werker 2000). More studies on the structure and function of non-glandular trichomes, or covering trichomes are necessary to understand their complete adaptive significance in young organs of plants that are exposed to intense solar radiation and drought
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