The chlorophyll meter (SPAD-502) is widely used to estimate chlorophyll content, but non-uniform chloroplast distribution can affect its accuracy. This study aimed to assess the effect of photon fluence (F, irradiance x time of illumination) in leaves with different chlorophyll content and determine the effect of chlorophyll a/b on SPAD values of four tropical tree species (Croton draconoides Müll. Arg., Hevea guianensis Aubl., Hymenaea courbaril L. and Matisia cordata H.B.K.). There were also determined calibration equations for the chlorophyll meter and assessed the effect of F on SPAD values between 07:00 h and 17:00 h. Calibration equations were obtained after determining leaf chlorophyll content in the laboratory. Increases in F with time caused a reduction in SPAD values in species with a high chlorophyll content, with reductions of 20% in M. cordata and 10% in H. guianensis. Leaves of C. draconoides and H. courbaril had lower chlorophyll content and showed no changes in SPAD values with increase in F. The chlorophyll a/b ratio increased with SPAD values and the SPAD/chlorophyll relationship was best described by an exponential equation. It seems that F may affect SPAD values in leaves with high chlorophyll content, probably due to non-uniform chloroplast distribution at high irradiance. This indicates that SPAD values tend to be more accurate if recorded early in morning when irradiance is low.
Accumulated irradiance; chlorophyll; photosynthetic pigments
