Florinia E. Merca

Adaptation to flooding in upland and lowland ecotypes of Cyperus rotundus, a troublesome sedge weed of rice: tuber morphology and carbohydrate metabolism

BACKGROUND AND AIMS In recent years, Cyperus rotundus has become a problem weed in lowland rice (Oryza sativa) grown in rotation with vegetables in the Philippines. As the growth of C. rotundus is commonly suppressed by prolonged flooding, the ability of the weed to grow vigorously in flooded as well as upland conditions suggests that adapted ecotypes occur in these rotations. Studies were conducted to elucidate the mechanisms that permit C. rotundus to tolerate flooded soil conditions. METHODS Upland and lowland ecotypes of C. rotundus were compared in terms of growth habit, carbohydrate reserves and metabolism, and activities of enzymes involved in alcoholic fermentation - alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC). KEY RESULTS The lowland ecotype has much larger tubers than the upland ecotype. Prior to germination, the amylase activity and total non-structural carbohydrate content in the form of soluble sugars were greater in the tubers of lowland plants than in those of upland C. rotundus. At 24 h after germination in hypoxic conditions, PDC and ADH activities in the lowland plants increased, before decreasing at 48 h following germination. In contrast, ADH and PDC activities in the upland plants increased from 24 to 48 h after germination. CONCLUSIONS Tolerance of lowland C. rotundus of flooding may be attributed to large carbohydrate content and amylase activity, and the ability to maintain high levels of soluble sugars in the tubers during germination and early growth. This is coupled with the modulation of ADH and PDC activities during germination, possibly to control the use of carbohydrate reserves and sustain substrate supply in order to avoid starvation and death of seedlings with prolonged flooding.

Morphological and physiological responses of lowland purple nutsedge (Cyperus rotundus L.) to flooding.

Comparing a lowland and an upland ecotype of Cyperus rotundus, the former had greater carbohydrate reserves in tubers, thicker roots and stems with larger air spaces and, under hypoxia, it maintained relatively lower activities of alcohol dehydrogenase and lactate dehydrogenase.

Differences in responses to flooding by germinating seeds of two contrasting rice cultivars and two species of economically important grass weeds

Barnyard grasses are serious weeds in direct seeded rice. We assessed the effectiveness of using controlled flooding for its control using two rice cultivars and two barnyard grasses contrasting in flood tolerance during germination. Flooding with 100 mm water after seeding suppressed barnyard grasses; but delaying flooding by 2-4 days was ineffective. Flooding increased the activity of alcohol dehydrogenase and pyruvate decarboxylase; the increase was higher in the tolerant rice cultivar but similar in both barnyard grasses. Aldehyde dehydrogenase activity increased only in flood-tolerant types of rice and weeds, but not in flood-sensitive types, implying potential role in tolerance.