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, 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 Q/A ≤ 0.45 mg water mm^-2 leaf surface from succophylls with ≥ 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.

Fieldwork

Leaves were collected from eight species growing wild at Groenriviersond, 500 km north of Cape Town, South Africa (30º 51' S, 17º 34' E). The species were selected to cover the full range of textures among perennials at the study site [specific leaf area (SLA) ranged from 2 to 20 mm² mg^-1, Lamont and Lamont 2000]. They were Pteronia onobromoides (Asteraceae, shrub to 50 cm tall, hard-leaved), Salvia lanceolata (Lamiaceae, shrub to 1 m tall, soft-leaved), Eriocephalus africanus (Asteraceae, shrub to 1 m tall, soft-leaved), Stoeberia utilis (Aizoaceae, syn. Mesembryanthemaceae, ground creeper, succulent), Ruschia fugitans (Aizoacae, syn. Mesembryanthemaceae, ground creeper, large-leaved succulent), Zygophyllum morgsana (Zygophyllaceae, shrub to 50 cm, semi-succulent), Othonna cylindrica (Asteraceae, shrub to 40 cm, succulent) and Senecio aff. sarcoides (Asteraceae, undershrub, small-leaved succulent). Nomenclature is as given in Eccles et al. (1999) and, from hereon, only the genus names are used. Leaves of all species were iso(bi)lateral and sessile (except Salvia).  As this is a shrubland, all species were growing in the open so that differences in microclimate would have had no role in affecting the results. They varied from apparently sclerophyllous to highly succulent. On a water mass content per unit volume basis, Q_v, two species were in the range 40-50%, three were 60-70%, and three were 80-95% (Lamont and Lamont 2000). Thus, the water-storing properties of the eight species studied formed a well-defined gradient that proved ideal for testing the hypotheses outlined here.

The study site lies in the southern portion of the Namib Desert. The vegetation is part of the succulent karoo and consists of clumps of climbers to woody shrubs up to 2 m tall (Eccles et al. 2001). The soil is red aeolian sand overlying an impenetrable silcrete hardpan at about 2 m depth. Rainfall was 79 mm in the year of the study although fog and dew are regular occurrences (Fradera-Soler et al. 2021).

Laboratory work

Current season’s mature twigs (100–150 mm long) were removed from side branches of 6–8 plants of each species by cutting under water predawn. They were kept with their ends in water at 17.5–20.5ºC and covered with plastic bags for 1–4 days in the laboratory to promote full hydration. They were then recut under water and their pressure-volume relations determined following the protocol of Radford and Lamont (1992). The balancing pressure was achieved with a digital pressure chamber, model 1003, PMS Instruments, Corvallis, OR, USA. In order to obtain turgid (saturated) mass as needed for this study, wet weight values of twigs were extrapolated to Y  = 0, i.e., full turgidity. Ten mature, full-sized leaves were removed from other stems, and these plus the original supporting twigs used were weighed, frozen at -16ºC to rupture the cells and hasten drying, dried at 72ºC for 48 h and reweighed. From this, turgid mass of the twigs was used to obtain leaf turgid mass (60–95% of total mass for individual twigs).

Midpoint thickness of 10 leaves from three plants was determined with callipers. Projected area (A) was obtained by placing 30 leaves or more diagonally on the conveyor belt of an area meter (Li-Cor 3000, Lincoln, NK, USA). Adjustments were made for the shape of leaves and their volume (V) determined geometrically (Lamont et al. 2015): five were cylindrical (V = p/4z•A) where z was diameter, two were laminate (V = z•A where z was thickness) and one was subulate (V = mean z•A), all lacking midribs. SLA [A/D = (D_V•z)^-1, where D = dry leaf mass and D_V = dry leaf density on a volume basis, Witkowski and Lamont 1991] was adjusted for leaf shape in the same way, and D_V and Q_V (dry matter and saturated water mass per unit leaf volume) were based on these measurements.

Volume of dry matter was determined by removing all, and only, mature leaves from six twigs, bulking and macerating after oven-drying as above to pass through a 1.1 mm mesh, then twice through a 0.3 mm mesh. The powder was then moistened with a wetting agent (1% Tween 20) in distilled water to form a thick paste. A cork borer (internal diameter 3.58 mm) was pushed into the paste, to produce an initial firm cylinder 20–40 mm in length. It was then placed on a paper tissue to absorb water over plastic sheeting on a fibrocement base. An iron rod of diameter 3.50 mm was pushed into the borer and tapped with a small hammer about 30 times, until water no longer squeezed out of the bottom. The pressure applied was up to 5.1 kg cm^-2 but it was usually about 2.1 kg cm^-2. The cylinder of compressed paste was forced out with the rod, and the ends cut with a razor blade as required to produce a perfect cylinder and its length and width determined with callipers. 3–5 cylinders were obtained per species. They were dried at 65ºC for 40 h and kept in a desiccator until weighing.

Properties assessed

Knowing D_v (D/V) and volume of dry matter per unit dry mass (V_D/D), the contribution of dry matter volume – essentially cell walls, but including protein and most solutes) – to total volume [(V_D/D)(D/V) = V_D/V = F_D] and air, F_a = (1 – F_D) were calculated. Thus, colloidal protein and other non-soluble substances were put with structure rather than cytoplasm or vacuole when estimating volume fractions (as in Roderick et al. 1999b). Some solutes may not have been adsorbed or held back by the cell-wall components during compression, but, even if some were lost, their contribution to volume would be negligible (< 0.01% of dry matter according to our estimates). Formulae for specific gravity (r) based on either non-air leaf volume (r_Q+D) or total leaf volume (r_l) (see Table 1) were as given in Roderick et al. (1999a).

Z Wang et al. (2022) fitted a single relationship between SLA and Q/D for over 3,000 species distributed throughout most of the world’s vegetation types, especially in China and North America, that did not appear to include succophylls (confirmed in a later figure). We therefore decided to compare Q/D with SLA (A/D) (as did they among many other structural and ecophysioloical properties of the species), whose slope produces Q/A – our preferred index of succulence, with all data sets which did include succulents that we could locate: ours and 12 others (Table 3). These were identified by feeding in the keywords: water content, succulent, SLA, into Google Scholar®. This yielded about 200 papers that were inspected to see if they provided data on Q/M and SLA, or they could be obtained from leaf dry matter (LDMC) or water content on a turgid mass basis. LDMC = D/(D + Q) that was inverted and 1 subtracted to give Q/D. Q/(Q + D) values were inverted, 1 subtracted, and re-inverted to give Q/D. Some data were obtained by measuring the length of bars in figures with callipers (to 0.05 mm) and converted to the indices of interest using the scale on the axis. Some data sets had to be rejected as their units were clearly incorrect and the solution was not obvious, while some listed papers proved impossible to obtain.

Half the papers used fresh mass to calculate Q that is misleading as it should be based on turgid mass if it is to be treated as a property of the plant rather than as a proximate response to the conditions of the day. No attempts to at least harvest the plants predawn were evident (see above for how turgid mass can be obtained, including extrapolation from pressure-volume curves). In this case, Q/D was multiplied by 1.1 (i.e., water content increased by 10%) to adjust for the error unless it was grown in hydroponics or water content exceeded 90% when any error would be small. For similar reasons, plants subjected to drought stress treatments were ignored.  In passing, we note that many species had leaves with a Q_M of only 50% or less and wonder how successful the claims of turgid mass were.