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Crassulacean Acid Metabolism in Two Species of Orchids Infected by Tobacco Mosaic Virus‐orchid Strain and/or Cymbidium Mosaic Virus

Abstract:

The effect of viral infections on the Crassulacean Acid Metabohsm (CAM) was determined in two orchid species. Plants of a Sophrolaeliocattleya (SLC) commercial hybrid were inoculated with either the orchid strain of the Tabacco Mosaic Virus (TMV‐O), with Cymbidium Mosaic Virus (CybMV) or with a mixture of both viruses. Plants of Epidendrum elongatum, a wild terrestrial orchid, were inoculated either with TMV‐O or with TMV‐O and CybMV simultaneously. Virus infected plants started to show a systemic mosaic on the leaves 3 to 4 years after inoculation, the symptoms being more appreciable in SLC than in E. elongatum. Double inoculation with TMV‐O and CybMV proved to the lethal for the SLC plants. TMV‐O infection caused 58 % and 24 % reduction in nocturnal leaf titratable acidity (TA) in SLC and E. elongatum, respectively. TMV‐O also changed the daily pattern of leaf nonstructural carbohydrates typical of CAM plants. A 50 % decrease in TA at dawn and a mid‐morning peak of sugar accumulation were measured in leaves of doubly inoculated E. elongatum plants. In SLC CybMV infection inhibited CAM and induced an accumulation of glucans in the leaves. Cytoplasmic agglomerations of CybMV, but not of TMV‐O, were detected in SLC mfected cells, whereas TMV‐O particles were clearly observed in the cytoplasm of E. elongatum. Infection by TMV‐O or CybMV in SLC caused an increase in chloroplasts volume and distortion of the grana due to high glucan accumulation. In contrast, TMV‐O infection in E. elongatum induced a lesser degree of damage in the cell ultrastructure. Copyright © 1993, Wiley Blackwell. All rights reserved