Abstract
Vascular epiphytes represent almost 10% of all terrestrial plant diversity. Despite the extensive research on the functional ecology and challenges of epiphytic growth, there is still very little known on how exposure to mechanically induced stress affects the growth and development of epiphytes. Therefore, this study investigated the effect of such mechanical stress on the growth and biomass allocation of epiphytic bromeliads. Juvenile plants of two species were subjected to two types of mechanical stress in the greenhouse—permanent displacement and temporary, recurring mechanical flexing. ANOVAs were used to test possible treatment effects on growth, root–shoot ratio, root diameter, and root area distribution ratio. Contrary to previous studies on herbaceous plants, these bromeliads showed little to no change in root and shoot properties in either species. The root–shoot ratio increased in disturbed Guzmania lingulata plants, but not in Vriesea sp. Treatment effects on growth were inconsistent: a stress effect on growth was significant only in the first 2 months of the experiment in G. lingulata, whilst none of the stress treatments negatively affected growth in Vriesea sp. All disturbed plants showed some degree of curvature on their stems and leaves against the area of stress to obtain an upright position. This was probably related to the maintenance of a functional tank. This study provides quantitative and qualitative data to understand thigmomorphogenic responses of bromeliads to mechanical stress. Future studies could include field surveys to quantify on-site mechanical stresses and the corresponding morphological changes in vascular epiphytes.
Highlights
Mobile animals can respond to stressful events such as predator attacks either by fighting against it or by fleeing to a more favourable environment
In G. lingulata, there was a significant two-way interaction between treatment and time (F (10, 320) = 3.71, where the numbers in brackets are the degrees of freedom in the numerator and denominator, respectively, p \ 0.001)
The permanent and temporary recurring mechanical stress applied to the plants in our experiment simulated stress scenarios that an epiphyte may face in its natural habitat, e.g. wind in exposed tree crowns or falling debris deflecting the plants from their upright growth
Summary
Mobile animals can respond to stressful events such as predator attacks either by fighting against it or by fleeing to a more favourable environment. Such responses are physically impossible despite being exposed continuously to different stressors like (1) environmental stresses Koyro et al 2012; Hasanuzzaman et al 2013), (2) growth stresses, i.e. increment of dead weight or having an inclined stem (Gril et al 2017) and (3) mechanically induced nonpermanent stresses when the plants’ aerial parts are moved by wind or water currents in flow-dominated habitats (Harder et al 2006; Read and Stokes 2006). The degree of thigmomorphogenesis displayed by Phaseolus vulgaris is positively related to the amount of wind, an arguably adaptive response that minimizes damage by wind gusts (Hunt and Jaffe 1980); the overall alteration in the plant allometry reduces the effective canopy frontal area exposed to wind and reduces drag (Rudnicki et al 2004; Vollsinger et al 2005)
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