Biomass, growth and ecophysiological responses of young plants of bertholletia excelsa Bonpl. Subjected to different levels of irradiance/Biomassa, crescimento e respostas ecofisiologicas de plantas jovens de bertholletia excelsa Bonpl. Submetidas a diferentes niveis de irradiancia

Brazil nut (Bertholletia excelsa Bonpl.) is a light-demanding species, because in natural forest, the species depending of formation of clearings for reach reproduction size and in forest plantation, showing fast initial growth when exposed to high levels of irradiance. However, the ecophysiological traits of this species under contrasting conditions of irradiance were not investigated. In this study, in addition to growth traits, we investigated for first time the degree of plasticity of ecophysiological response of Bertholletia excelsa when subjected to contrasting irradiance environments. Young plants of Bertholletia excelsa were cultivated under three conditions of irradiance: low (20-300 [micro]mol [m.sup.-2] [s.sup.-1]), moderate (800-1000 [micro]mol [m.sup.-2] [s.sup.-1]) and high (1900-2100 [micro]mol [m.sup.-2] [s.sup.-1]). We analyzed the growth traits, gas exchange, chloroplastid pigment contents and, the end of experiment, the accumulation and partitioning of biomass. Young plants of Bertholletia excelsa showed the highest values of biomass, growth and photosynthesis when exposed to environments of moderate and high irradiance. Low irradiance condition stimulated more biomass partitioning for shoot and chloroplastid pigment contents. Bertholletia excelsa showed physiological plasticity under contrasting conditions of irradiance, the largest growth and biomass accumulation in environments of moderate and high irradiance were promoted by better photosynthetic performance, whereas the positive carbon balance under low irradiance is ensured by investment in structures of interception and energy harvesting. Keywords: Brazil nut; gas exchange; initial growth; tropical tree. A castanheira-da-amazonia (Bertholletia excelsa Bonpl.) e uma especie exigente de luz, pois em floresta nativa, a especie depende da formacao de clareiras para alcancar o tamanho reprodutivo e em plantios florestais demonstra rapido crescimento inicial quando exposta a altos niveis de irradiancia. No entanto, os aspectos ecofisiologicos dessa especie sob condicoes contrastantes de irradiancia nao foram investigados. Nesse estudo, alem das caracteristicas de crescimento, objetivou-se analisar pela primeira vez o grau de plasticidade das respostas ecofisiologicas de Bertholletia excelsa quando submetida a ambientes de irradiancia contrastantes. Plantas jovens de Bertholletia excelsa foram cultivadas em tres condicoes de irradiancia: baixa (20-300 [micro]mol [m.sup.-2] [s.sup.-1]), m