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 [µm] = 153.9 + 0.98 x, r 2 = 0.06 ** ), dry leaf thickness (DLT [µm] = 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.

Mesophyll conductance (g m ) is essential to determine accurate physiological parameters used to model photosynthesis in forest ecosystems. This study aimed to determine the effects of time of day on photosynthetic parameters, and to assess the effect of using either intercellular CO 2 concentration (C i ) or chloroplast CO 2 concentration (C c ), on maximum carboxylation velocity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), V cmax . We used Amazonian saplings of Myrcia paivae and Minquartia guianensis. Photosynthetic parameters were measured using an infrared gas analyzer (IRGA); g m was determined using both gas exchange and chlorophyll (Chl) a fluorescence and gas-exchange data alone. Leaf thickness (L T ) and specific leaf area (SLA) were also measured. Air temperature, relative humidity or understory light did not correlate with g m and on average daily IRGA-fluorometer-determined g m was 0.04 mol(CO 2 ) m -2 s -1 for M. paivae and 0.05 mol(CO 2 ) m -2 s -1 for M. guianensis. Stomatal conductance (g s ), g m , electron transport rate (J F ), and light-saturated net photosynthetic rate (P Nmax ) were lower in the afternoon than in the morning. However, no effect of time of day was observed on V cmax . L T and SLA did not affect any of the examined parameters. IRGAdetermined g m was almost the double of the value obtained using the IRGA-fluorescence method. V cmax values determined using C c were about 25% higher than those obtained using C i , which highlighted the importance of using C c in V cmax calculation. Decline in P Nmax at the end of the afternoon reflected variations in g s and g m rather than changes in V cmax . Diurnal variation in g m appeared to be associated more with endogenous than with atmospheric factors.

In juvenile trees growing at the rainforest understory, light is the most limiting factor for growth. It has been assumed that stomata quickly respond to light irrespective of the physical conditions prevailing before leaf illumination. Nevertheless, so far this issue has not been addressed for saplings of Amazonian tree species. The aim of this study was to determine how stomatal conductance (g s ) and photosynthetic parameters of Amazonian saplings respond to diurnal variation in the physical environment and to rainfall seasonality. Light-saturated net photosynthetic rate (P Nmax ) and g s at light saturation (g smax ) were measured in the dry (August) and rainy (January) season of 2008 in saplings of 10 Amazonian tree species (Minquartia guianensis, Myrcia paivae, Protium apiculatum, Guatteria olivacea, Unonopsis duckei, Rinorea guianensis, Dicypellium manausense, Eschweilera bracteosa, Gustavia elliptica, and Tapura amazonica). At the forest understory, variables of the physical environment were measured. Rainfall seasonality did not affect P Nmax and g smax , nor was the effect of species on P Nmax and g smax significant (p>0.05). The g s and P Nmax increased as the forest understory became brighter and warmer; as a result, P Nmax and g smax were higher at midday than early in the morning or in the afternoon. However, contrary to expectations, neither changes in air vapor pressure deficit nor air CO 2 concentration at the forest understory affected stomatal opening. More investigation is needed to elucidate the role of environmental factors in modulating stomatal movements in juvenile trees growing beneath the dense canopy of tropical rainforests.

RESUMOA disponibilidade de luz é um dos fatores que mais limita a fotossíntese de árvores juvenis no sub-bosque da floresta. Os objetivos deste estudo foram determinar como os parâmetros fotossintéticos (velocidade máxima de carboxilação da Rubisco -Vc-max e as taxas máximas de transporte de elétrons -Jmax) respondem à irradiância, temperatura e horário do dia no interior da floresta. Os dados foram coletados nos meses de setembro a novembro de 2010, entre 08h00 e 16h00, em condições ambientais de temperatura (27 ± 2ºC) e umidade relativa do ar (70 ± 3% Ausência de efeito do horário do dia é importante para a folha poder utilizar com alta eficiência os feixes de luz direta que ocasionalmente atingem o sub-bosque da floresta. Palavras-chave: limitação não estomática; colimitação da fotossíntese; variação diurna. ABSTRACTLight availability is one of the main factors that limits photosynthetic rates in saplings in the forest understory. The aims of this study were to determine how photosynthetic parameters (maximum carboxylation velocity of Rubisco -V c-max and maximum rate of electron transport -J max ) respond to irradiance, temperature and diurnal variations in the physical environment. Gas exchange data were collected between 08h00 and 16h00, and from September to November (2010) under ambient conditions, temperature of 27± 2 ºC and relative humidity of 70 ± 3%. CO 2 saturated photosynthesis (A pot ) was measured under light saturation and [CO 2 ] of 2000 μmol mol -1 . Data were subjected to analysis of variance and then the Tukey test was used for mean separations. V c-max and J max correlated with photosynthetic rates at [CO 2 ] of 380 μmol mol -1 (A 380 ) and A pot . The increase in intercellular CO 2 concentration (C i ) led to increase in photosynthetic rates, up to the point of photosynthesis limitation by the electron transport rate. Below the collimation point (300-569 μmol mol -1) photosynthetic rates were limited by V c-max . Increases in irradiance and temperature in the forest understory led to increasing the values of A 380 , A pot , V c-max and J max . However, the effect of time of day on

RESUMO Leituras do SPAD-502 em resposta a fluência de fótons em folhas com diferentes teores de clorofilaO clorofilômetro (SPAD-502) é amplamente utilizado para estimar teores de clorofila, mas a falta de uniformidade na distribuição dos cloroplastos pode afetar sua precisão. Os objetivos deste estudo foram avaliar o efeito da fluência de fótons (F = irradiância x tempo de iluminação) em folhas com teores diferentes de clorofila e determinar o efeito da relação clorofila a/b nos valores do SPAD em quatro espécies de árvores tropicais (Croton SPAD-502 readings in response to photon fluence in leaves with different chlorophyll contentThe 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.