Relationship between specific leaf area, leaf thickness, leaf water content and SPAD-502 readings in six Amazonian tree species

The aim of this work was to assess the effect of leaf thickness, leaf succulence (L(S)), specific leaf area (SLA), specific leaf mass (W(s)) and leaf water content (LWC) on chlorophyll (Chl) meter values in six Amazonian tree species (Carapa guianensis, Ceiba pentandra, Cynometra spruceana, Pithecolobium inaequale, Scleronema micranthum and Swietenia macrophylla). We also tested the accuracy of a general calibration equation to convert Minolta Chl meter (SPAD-502) readings into absolute Chl content. On average, SPAD values (x) increased with fresh leaf thickness (FLT [mu m] = 153.9 + 0.98 x, r (2) = 0.06**), dry leaf thickness (DLT [mu m] = 49.50 + 1.28 x, r (2) = 0.16**), specific leaf mass (W(s) [g (DM) m(-2)] = 6.73 + 1.31 x, r (2) = 0.43**), and leaf succulence (L(S) [g(FM)] m(-2) = 94.2 + 1.58 x, r (2) = 0.19**). However, a negative relationship was found between SPAD values and either specific leaf area [SLA (m(2) kg(-1)) = 35.1 - 0.37 x, r (2) = 0.38**] or the leaf water content (LWC [%]= 80.0 - 0.42 x, r (2) = 0.58**). Leaf Chl contents predicted by the general calibration equation significantly differed (p < 0.01) from those estimated by species-specific calibration equations. We conclude that to improve the accuracy of the SPAD-502 leaf thickness and LWC should be taken into account when calibration equations are to be obtained to convert SPAD values into absolute Chl content.