Water relations and hydraulic architecture in Cerrado trees : adjustments to seasonal changes in water availability and evaporative demand

O objetivo deste estudo foi determinar os ajustamentos na morfologia e fisiologia que permitem arvores das savanas neotropicais do Brasil Central (Cerrado) de evitar deficits hidricos e de manter um balanco hidrico interno praticamente constante apesar das variacoes sazonais da precipitacao e no deficit de saturacao do ar (D). A precipitacao na area de estudo e fortemente sazonal, com cerca de cinco meses praticamente sem chuva durante os quais D e duas vezes maior aos valores medidos na epoca chuvosa. Como consequencia da flutuacao sazonal das chuvas e de D, o potencial hidrico do solo muda substancialmente, nos primeiros 100 cm do solo, mas permanece quase constante abaixo de 2 m de profundidade. A arquitetura hidraulica e os parâmetros relacionados a relacoes hidricas das arvores do Cerrado se ajustaram durante a estacao seca para evitar o deficit hidrico crescente e assegurar a homeostase nos valores minimos de potencial hidrico foliar ψL e na perda total diaria de agua pela planta (iso-hidria). O comportamento iso-hidrico das arvores do Cerrado foi o resultado de uma diminuicao da superficie foliar total por arvore, um forte controle estomatico das perdas por evaporacao, um aumento na condutividade hidraulica especifica da folha e na condutância hidraulica foliar e um aumento da quantidade de aguas retirada dos reservatorios internos do caule, durante a estacao seca. A eficiencia no transporte de agua aumentou, nas mesmas proporcoes, nas folhas e nos ramos terminais durante a estacao seca. Todos estes ajustamentos sazonais foram importantes para a manutencao de ψL acima de limiares criticos, com isto contribuindo para uma reducao na formacao de embolismos nos ramos e ajudando a evitar a perda de turgor em tecidos foliares durante a epoca seca. Esses ajustes permitem que os ramos das especies lenhosas do Cerrado operem bem distanciados do ponto de disfuncao catastrofica para a cavitacao, enquanto as folhas operam proximas e sofrem embolismos em uma base diaria, especialmente durante a estacao seca.

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