Contrasting water, dry matter and air contents distinguish orthophylls, sclerophylls and succophylls (leaf succulents)

Differences in leaf texture (hardness, thickness) distinguish orthophylls (soft leaves), sclerophylls (hard leaves) and (semi)succophylls (water-storing leaves). Texture is controlled by dry matter, water and air contents. Our aim was to a) identify the best index of succulence, b) assess how these three components vary with leaf type, and c) derive bounds for these properties among the four main leaf-texture classes. Eight contrasting species from the Namib Desert, South Africa were assessed for their leaf area (A), thickness (z), dry mass (D), saturated water content (Q), and relative volume of dry matter, water and air to derive various indices of leaf texture. Q/A (= Q_V•z), where Q_V is saturated water storage per unit volume of leaf and z is leaf thickness is an ideal index of succulence. Specific leaf area (SLA) is more suitable as an index of hardness (SLA^-1 = D/A) but only among non-succulents. Rising leaf specific gravity among sclero-orthophylls is due to replacement of air by dry matter but water among succophylls. Collation of 13 worldwide studies showed that orthophylls can be distinguished by a Q/A ≤ 0.45 mg water mm^-2 leaf surface from succophylls with Q/A ≥ 0.9, such that there is a divergent relationship among plants regarding their water-storing properties. Semi-succophylls can be defined as having a Q/A > 0.45 to < 0.9, and sclerophylls can be separated from orthophylls by a SLA ≤ 10 mm² mg^-1 dry mass. The distribution of these leaf texture classes may vary greatly within, and especially between, local floras.