Stretched to the limit : leaf tensile properties and lignin content of resurrection plants

Leaf tensile strength was measured for four resurrection plants, Craterostigma wilmsii Engl, Xerophyta schlecteri (Baker) N.L. Menezes, Xerophyta humilis (Baker) T. Durand & Schinz and Sporobolus stapfianus Gandoger, as well as two desiccation-sensitive controls, Zea mays L. and Arabidopsis thaliana (L.) Heynh. (ecotype Columbia) at full hydration and after dehydration, both on the plant (naturally-dried) and rapidly off the plant causing death (flash-dried). In the desiccation-tolerant plants, leaf tensile strength was higher in the monocots than the dicots at full hydration. Three different mechanisms of cell protection occur in resurrection plants on drying: cell-wall folding, packing vacuoles with non-aqueous solute or a combination of the two. Tensile strength in C. wilmsii ( dicot) increased when naturally-dried but decreased when flash-dried, possibly due to the nature of the drying mechanisms (wall folding). The, leaf tensile strength of the Xerophyte species, both monocots, increased when naturally dried and when flashdried. Xerophyte species pack their vacuoles during desiccation. S. stapfianus, a grass which uses a combination of wall folding and vacuole packing, had the highest tensile strength possibly due to its unique architectural structure. Differences in leaf architecture, in terms of lignin content, were examined using light microscopy after histo-chemical staining for lignin, which showed that monocotyledons had a higher percentage of lignin per unit leaf cross-sectional area than dicotyledons. A regression analysis revealed that leaf tensile strength and lignin content were positively correlated in fully hydrated leaves I but no relationship existed between lignin content and naturally dried leaves. This may be due to variations of protective mechanisms induced during desiccation by the four resurrection plants. Notching was observed in X schlechteri, behaving differently to grasses which are notch-insensitive, possibly due to large lignin contents on the outer edges of the leaves