Leaf Thickness as Key to the Contrasting Water and Nutrient Relations of Eight Arid-Climate Species, Including Water-Loss Resistance

Byron B. Lamont; Neil S. Eccles; Heather C. Lamont

doi:10.53941/plantecophys.2025.100011

Abstract

Shrubs (families Asteraceae, Lamiaceae, Aizoaceae and Zygophyllaceae) in the succulent karoo of the southern Namib Desert survive an annual rainfall < 150 mm per annum but vary greatly in their vegetative morphology, so we hypothesised that they must possess a range of structural and physiological traits to resist perennial drought. Eight Namib species were assessed for their leaf structural properties [e.g., thickness (z), specific leaf area (SLA)], water storage capacity [e.g., relative water content (RWC)] and water potential (ψ) over 12 months or when severed from the parent plant in the field and laboratory, water-use efficiency (WUE) via δ¹³C content, and metabolite (N, P) and osmotic ion (Na⁺, K⁺) contents. Four species were considered (1) orthophylls/semi-succulents, and (2) four were succulents (succophylls), (3), two of these exhibiting CAM-type photosynthesis, and (4) six with C3-type photosynthesis. Succophylls were distinguished by their thicker leaves, lower SLA, presence of water-storing parenchyma, higher levels of ‘utilizable’ water, slower rates of water loss, higher/less variable ψ, and higher (Na⁺ + K⁺), N and P contents/leaf-area, δ¹³C and WUE. Water-loss resistance (WLR)—the change in RWC resulting from a given change in water potential (ΔRWC/Δψ) when subjected to drought conditions—was twice as high in the succophylls as the non-succulents under both laboratory and field conditions, with the latter showing twice the level of osmotic adjustment for a given drop in RWC. Leaves of the CAM species stored most water, decreased their ψ overnight, showed least rates of water loss, and had the highest N/P contents/area and WUE. Cations may serve an osmotic balancing function among succophylls, whereas high N and P/area may help maintain metabolic functions when transpiration is limited. δ¹³C/WUE relationships were functions of photosynthetic type, N/P contents/area, and (especially) leaf thickness. The opposing water relations of the four groups centre around their different trait combinations for accessing, utilizing and storing water. The special structural and physiological properties of succophylls need to be recognized when developing any general theory about the water relations of plants.

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