Metabolic Responses of Luffa Roots to Long-term Flooding

Abstract

Summary Luffa ( Luffa cylindrica Roem. cv. Cylinder # 2), a flood-tolerant species, can accommodate itself to flooding, caused by prolonged monsoon rain. In the present paper, metabolic responses of luffa roots to experimental flooding were evaluated. Upon flooding, the respiratory capacity of flooded roots decreased to 60 % of the nonflooded level. The activity of pyruvate decarboxylase (PDC, EC 4.1.1.1) and alcohol dehydrogenase (ADH, EC 1.1.1.1) increased rapidly and steadily. After 7 d of flooding, almost 50-fold increases in both enzymes indicated that strong ethanolic fermentation capacity was induced in response to long-term flooding. Nevertheless, the ethanol level showed no substantial increase, but declined after an initial rise at day one. A high ADH/PDC ratio in activity may be of importance to avoid accumulation of toxic levels of acetaldehyde. In the other fermentative pathways, the activity of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), the key enzyme catalyzing malate synthesis, showed no measurable difference between nonflooded and flooded roots. However, NADP-malic enzyme (NADP-ME, EC 1.1.1.40), catalyzing the decarboxylation of malate, exhibited a markedly increased activity in flooded roots as compared with nonflooded controls, and the level of malate in flooded roots dropped to 40 % of the control level. These results are inconsistent with a current metabolic theory of flooding tolerance. The activity of lactate dehydrogenase (LDH, EC 1.1.1.27) rose slightly in response to flooding, and an initial rise of lactate was observed in flooded roots.