Abstract
In its natural habitat, the one-leaf plant Monophyllaea horsfieldii (Gesneriaceae) shows striking postural changes and dramatic loss of stability in response to intermittently occurring droughts. As the morphological, anatomical and biomechanical bases of these alterations are as yet unclear, we examined the influence of varying water contents on M. horsfieldii by conducting dehydration–rehydration experiments together with various imaging techniques as well as quantitative bending and turgor pressure measurements. As long as only moderate water stress was applied, gradual reductions in hypocotyl diameters and structural bending moduli during dehydration were almost always rapidly recovered in acropetal direction upon rehydration. On an anatomical scale, M. horsfieldii hypocotyls revealed substantial water stress-induced alterations in parenchymatous tissues, whereas the cell form and structure of epidermal and vascular tissues hardly changed. In summary, the functional morphology and biomechanics of M. horsfieldii hypocotyls directly correlated with water status alterations and associated physiological parameters (i.e. turgor pressure). Moreover, M. horsfieldii showed only little passive structural–functional adaptations to dehydration in comparison with poikilohydrous Ramonda myconi.
Highlights
In contrast to the neighbouring rainforest ground flora, Malaysian limestone outcrops are characterized by higher wind, rain and light exposures and by periodic low water availabilities due to thin, rapidly draining soil layers [1,2]
We qualitatively demonstrated that the posture and mechanical stability of M. horsfieldii are influenced by water status alterations, and that they are unable to recover from relative water contents (RWCs) below 10% [7]
We found that the median cell areas of the majority of tissues are positively correlated with the RWC of the hypocotyl (Pearson’s product–moment correlation, Ep: r = 0.25; Par: r = 0.84; Vbr: r = 0.36; Pac: r = 0.88; figure 2b)
Summary
In contrast to the neighbouring rainforest ground flora, Malaysian limestone outcrops are characterized by higher wind, rain and light exposures and by periodic low water availabilities due to thin, rapidly draining soil layers [1,2]. Oftentimes, these habitats are inhabited by members of the Gesneriaceae family (gesneriads), such as Monophyllaea horsfieldii R.Br. M. horsfieldii shows striking morphological and structural alterations (i.e. distinct hypocotyl bending) in response to moderate water stress, which are fully recoverable after anew irrigation [7]. Such phenomena are typical for desiccationtolerant plants, which possess the ability to survive cellular water contents below 10% for several weeks or months until they rapidly regain normal function upon rehydration [10,11]
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