Palavras-chave
fotossíntese C4
mecanismo de descarboxilação
déficit hídrico.
Como Citar
Resumo
Neste trabalho testou-se a hipótese de que o estresse hídrico reduz a fotossíntese da cana-de-açúcar impondo limitações de origem estomática e mesofílica e modificando a via de descarboxilação nas células da bainha do feixe vascular. Para tanto se mediu as trocas gasosas, a fluorescência da clorofila a, a atividade das enzimas envolvidas na descarboxilação e na fixação do CO2 na fotossíntese – fosfoenolpiruvato carboxilase (PEPC), ribulose-1,5-bisfosfato carboxilase/oxigenase (Rubisco), enzima málica dependente de nicotinamida adenina dinucleotídeo fosfato (NADP-ME), enzima málica dependente de nicotinamida adenina dinucleotídeo (NAD-ME) e fosfoenolpiruvato carboxiquinase (PEPCK). Com a redução do potencial hídrico foliar devido à baixa disponibilidade de água, houve redução na assimilação de CO2, na condutância estomática, na transpiração e na eficiência do uso da água, bem como na eficiência fotoquímica do fotossistema II. A descarboxilação de CO2 na cana-de-açúcar ocorreu com a participação das três descarboxilases; todavia, a atividade PEPCK foi mantida sob estresse hídrico, enquanto as atividades da NADP-ME e NAD-ME foram reduzidas. A Rubisco teve sua atividade diminuida sob estresse hídrico, enquanto que a PEPC não foi afetada. Após o período de reidratação do substrato, as plantas recuperaram-se atingindo valores de fotossíntese muito próximos dos apresentados em plantas bem hidratadas. Como conclusão, a fotossíntese em cana-de-açúcar sob deficiência hídrica é limitada pela disponibilidade de substrato devido ao fechamento estomático, assim como pela menor atividade fotoquímica e menor atividade da Rubisco. O déficit hídrico também evidenciou a flexibilidade do mecanismo de descaboxilação de CO2 nas células da bainha do feixe vascular, com aumento da contribuição relativa da enzima PEPCK.
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