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Q62 - Habitat-Linked Hydraulic Vulnerability in Ranunculus Species

Theoretical B Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Water Transport & Transpiration

To understand the effect of desiccation on herbaceous plants and their responses, scientists conducted a study on three Ranunculus species in their natural habitats, including R. bulbosus in dry meadow, R. lanuginosus in humid meadow, and R. acris in both habitats. They measured leaf water potential and hydraulic conductance of these species in response to dehydration (Fig.Q62). Xylem staining experiment on R. acris in dry habitat was used to estimate loss of conductivity due to embolism. An estimated 50% loss of xylem hydraulic conductivity occurred at -2MPa or less owing to embolism. Previously, leaf hydraulic vulnerability studies found 50% reduction in leaf hydraulic conductance between -1 and -1.8 MPa in perennial grasses and at -1.8 MPa in woody plant species.

Figure Q.62 Leaf hydraulic conductance (Lhc) of Ranunculus species/populations in response to dehydration. Solid and dashed vertical lines indicate, respectively, fitted 50% and 88% leaf hydraulic conductance losses.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. All species were very vulnerable to water stress. In species with narrow ecological amplitude, the drought-exposed R. bulbosus was less vulnerable to desiccation than the humid habitat R. lanuginosus.
B. Herbaceous species would be more vulnerable to water stress than woody species and perennial grasses, but also would show interspecific and intraspecific adjustments in hydraulic vulnerability based on the water availability of their respective habitats.
C. The leaf hydraulics method employs hydraulic conductance including both xylary and extraxylary pathways.
D. The effect of drought in these plant species is found to be a loss of leaf hydraulic conductance at moderate water potential based on extraxylary pathways rather than embolism formation.
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Question reproduced from IBO 2016, Theoretical Paper B, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)