Seasonality and mycorrhizal colonization in three species of epiphytic orchids in southeast Mexico

Vincenzo Bertolini,Javier Valle Mora,Anne Damon,Jesús Cruz-Blasi

doi:10.1590/0102-33062014abb3436

Vincenzo Bertolini, Javier Valle Mora + Show 2 more

Open Access

https://doi.org/10.1590/0102-33062014abb3436

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Abstract

Orchids establish symbiosis with Rhizoctonia mycorrhizal fungi, forming the characteristic pelotons within the cells of the root cortex. Under natural conditions, terrestrial and epiphytic orchids have different levels of dependence upon the fungal symbiont, although various authors have mentioned that once orchid plants reach maturity the interaction becomes weaker and intermittent. Recent evidence shows that in some epiphytic orchid species mycorrhization is constant and systematic. In three species of wild orchids from southeast Mexico, we show that mycorrhization is systematically present in roots of different ages, in the wet and dry seasons. We demonstrate that the volume of the root that is colonized depends upon the quantity of rainfall and the diameter of the root, and that rainfall also determines the presence of fresh, undigested pelotons. In very thin roots, mycorrhizal colonization occupies a considerable proportion of the cortex, whereas in thicker roots the proportion of the volume of the root cortex colonized is lower.

Highlights

The nutritional role of mycorrhizae in the Orchidaceae has been well documented for temperate, terrestrial species, and especially those native to North America, Europe and parts of Asia and Australia (Rasmussen 1995), including photosynthetic species with partial mycotrophy and obligate mycoheterotrophs that depend upon the symbiosis to complete their life cycle (Leake 1994; Gebauer & Meyer 2003; Julou et al 2005; Girlanda et al 2006)
Analysis of mycorrhization intensity in function of the length of the roots (L, M and S), which is related to root age, demonstrated significant differences according to the season
New roots are usually formed toward the end of the dry season, in anticipation of the coming rains, and new root formation marks the end of a rest period typical of epiphytic orchids even in tropical climates

Summary

Introduction

The nutritional role of mycorrhizae in the Orchidaceae has been well documented for temperate, terrestrial species, and especially those native to North America, Europe and parts of Asia and Australia (Rasmussen 1995), including photosynthetic species with partial mycotrophy (mixotrophy) and obligate mycoheterotrophs that depend upon the symbiosis to complete their life cycle (Leake 1994; Gebauer & Meyer 2003; Julou et al 2005; Girlanda et al 2006). It has been assumed that, for epiphytic orchids, dependence upon mycorrhizae is probably not obligate during the first stages of germination, for three reasons: some orchid seeds contain small drops of lipids and protein in the embryo; the rapid assimilation of simple nutrients in in vitro culture media; and after imbibing water, and when exposed to light, the seeds rapidly pass to a photosynthetic state (Arditti 1992). Studies on symbiotic germination in vitro demonstrate dependence upon the mycorrhizae for the differentiation and development of each life stage, and that growth ceases and the plant eventually dies in the absence of a mycorrhizal symbiont (Zettler et al 1998; Markovina & McGee 2000; Pereira et al 2005a)

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