Is sclerophylly of Mediterranean evergreens an adaptation to drought?

Abstract

SUMMARYThe potential role of sclerophylly (leaf hardness and rigidity) in the control of leaf dehydration and rehydration was investigated in two sclerophylls {Viburnum tinus and Ilex aquifolium) and two non-sclerophylls (Hedera helix ssp. helix and Sambucus nigra). After leaves were dehydrated in the pressure chamber, water transport from the apoplast (mainly consisting of xylem conduits and mechanical cells) to symplast was detected 15 min from pressure release, in terms of a spontaneous increase in leaf water potential (!F,). This !P, increase was much larger in sclerophylls than in non-sclerophylls.Positive pressures applied to leaves simulated the tensions developing in the leaf apoplast under water stress conditions, causing water to be expelled from leaf xylem conduits and mechanical cells and transferred to the leaf symplast, thus leading to symplast rehydration and to the consequent W^ increase.No correlation was found between the leaf modulus of elasticity at full turgor or the degree of sclerophylly (in terms of the ratio of leaf d. wt to surface area) and the characteristic rehydration time of the leaves, i.e. between the two main parameters expressing the rigidity of the leaf blade and the rate of leaf rehydration. However, when changes in !F, were measured as a function of leaf water deficit (RWD), equal W^s corresponded with larger RWDs during leaf rehydration than during leaf dehydration in the two non-sclerophylls. In particular, the two sclerophylls showed rehydration of their leaf apoplast and symplast completely and simultaneously. By contrast, the two non-sclerophylls showed a persisting water loss, localized, it is likely, in their xylem conduits and mechanical cells.In other words, the two sclerophylls did not recover from water loss more rapidly than non-sclerophylls but they recovered from xylem cavitation more completely. The major elasticity of the cavitation strain shown by the two sclerophylls studied was interpreted as of advantage to plants subjected to diurnal large drops in W^ followed by nocturnal recovery. This is the case in Mediterranean sclerophylls growing in areas characterized by high humidity of the air condensing on the soil at night. The same mechanism of cavitation recovery, however, would be useless in very xeric areas.The hypothesis is advanced that sclerophylly of Mediterranean species may derive from similar anatomical structures developed in species formerly adapted to more humid environments which later migrated to more arid zones.

Influence of the ectomycorrhizas formed by Tuber melanosporum Vitt. on hydraulic conductance and water relations of Quercus ilex L. seedlings

“…The higher LSM recorded in I-versus NI-seedlings suggests that plants invested a larger amount of photosynthetic products in the accumulation of cell wall materials in the leaf rather than in seedling growth. Although the functional significance of sclerophylly is still matter of debate [23,25,28,34,41], the sclerophyllous habit is typical of all Mediterranean evergreens and has been recently interpreted as a factor improving leaf rehydration after water stress release [34]. In this respect, mycorrhizal symbiosis might improve the drought recovery of Q. ilex after the scarce summer rainfalls typical of the Mediterranean areas.…”

Low leaf‐level response to light and nutrients in Mediterranean evergreen oaks: a conservative resource‐use strategy?

“…systems (Turner, 1994 ;Aerts, 1995 ;Salleo et al, 1997). Leaves, stems, and roots all respond to environmental stimuli in order to control plant development, but it is the metabolic responses of leaves to different environmental stresses that largely regulate the growth and development of both shoots and roots (Dickson & Isebrands, 1991).…”

Refilling of Embolized Vessels in Young Stems of Laurel. Do We Need a New Paradigm?1

“…Only one branch per plant was subjected to pressurization in the pressure collar to minimize rehydration of tissues after release of water from embolized stems: Water released in one stem should travel to all stems, roots, and soil soon after embolism formation. Water released by embolisms can cause a rehydration of ⌿ L of 100 to 200 kPa during bench-top dehydration of excised branches (Dixon et al, 1984;Salleo et al, 1997). Intact pressurized twigs were enclosed in black plastic bags with the entire plant to minimize transpiration until they were cut off and tested for PLC.…”

“…Such a narrow safety margin (Sperry, 1995) is intrinsically dangerous for plant survival under adverse environmental conditions but might be of some advantage in buffering leaf water status (Dixon et al, 1984;Salleo et al, 1997) and in inducing stomatal closure (Sperry, 1995).…”