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

Abstract

Crop productivity is largely affected by abiotic factors such as flooding and by biotic factors such as weeds. Although flooding after direct seeding of rice helps suppress weeds, it also can adversely affects germination and growth of rice, resulting in poor crop establishment. Barnyard grasses (Echinochloa spp.) are among the most widespread weeds affecting rice, especially under direct seeding. The present work aimed to establish effective management options to control these weeds. We assessed the effects of variable depths and time of submergence on germination, seedling growth and carbohydrate metabolism of (i) two cultivars of rice known to differ in their tolerance to flooding during germination and (ii) two barnyard grasses (Echinochloa colona and E. crus-galli) that commonly infest rice fields. Flooding barnyard grasses with 100-mm-deep water immediately after seeding was effective in suppressing germination and growth. Echinochloa colona showed greater reductions in emergence, shoot and root growth than E. crus-galli. Delaying flooding for 2 or 4 days was less injurious to both species. Echinochloa colona was also more susceptible to flooding than the flood-sensitive rice cultivar 'IR42'. The activity of alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) in rice seedlings was increased by flooding after sowing but with greater increases in 'Khao Hlan On' compared with 'IR42'. The activity of ADH and PDC was enhanced to a similar extent in both barnyard grasses. Under aerobic conditions, the activity of ADH and PDC in the two barnyard grasses was downregulated, which might contribute to their inherently faster growth compared with rice. Aldehyde dehydrogenase activity was significantly enhanced in flood-tolerant 'Khao Hlan On' and E. crus-galli, but did not increase in flood-sensitive E. colona and 'IR42', implying a greater ability of the flood-tolerant types to detoxify acetaldehyde generated during anaerobic fermentation. Confirmation of this hypothesis is now being sought.

Keywords: Alcohol dehydrogenase; Echinochloa colona; Echinochloa crus-galli; aldehyde dehydrogenase; anaerobic germination; barnyard grass; direct-seeded rice; fermentative metabolism; pyruvate decarboxylase; rice weeds..

Figure 1.

Percent germination of E. crus-galli, E. colona, ‘Khao Hlan On’ and ‘IR42’ submerged in 0, 5, 10 and 20 mm of water immediately after seeding. Vertical bars indicate l.s.d. at P < 0.05.

Figure 2.

Percent germination of E. crus-galli, E. colona, ‘Khao Hlan On’ and ‘IR42’ submerged in 0 and 100 mm of water immediately after seeding. Vertical bars indicate l.s.d. at P < 0.05.

Figure 3.

Shoot length (A) and root length (B) of E. colona, E. crus-galli, ‘IR42’ and ‘Khao Hlan On’ germinated under 100 mm of water. Controls were kept aerobic (0 mm flooding). Data were taken at 7 DAS and vertical bars indicate l.s.d. at P < 0.05.

Figure 4.

Shoot length (A) and root length (B) of E. colona, E. crus-galli, ‘IR42’ and ‘Khao Hlan On’ germinated under aerobic conditions for 3 days followed by flooding under 100 mm of water. Controls were kept aerobic (0 mm flooding). Data were taken at 7 DAS and vertical bars indicate l.s.d. at P < 0.05.

Figure 5.

Starch concentration (% DW) of (A) ‘Khao Hlan On’ and ‘IR42’ and (B) E. crus-galli and E. colona from 0 to 14 days after sowing under aerobic and submerged (100 mm) conditions. Vertical bars indicate l.s.d. at P < 0.05.

Figure 6.

Soluble sugar concentration (% DW) of (A) ‘Khao Hlan On’ and ‘R42’ and (B) E. crus-galli and E. colona from 0 to 14 days after sowing under aerobic (0 mm) and flooded (100 mm) conditions. Vertical bars indicate l.s.d. at P < 0.05.

Figure 7.

Activities of PDC (A), ADH (B) and ALDH (C) enzymes during germination under aerobic (0 mm) and flooded (100 mm) conditions. The graphs on the left represent activities in ‘Khao Hlan On’ and ‘IR42’ and the graphs on the right represent E. crus-galli and E. colona. Vertical bars indicate l.s.d. at P < 0.05.

Figure 8.

Immunoblot raised against ALDH2 in (A) ‘Khao Hlan On’ and 'IR42′ and (B) E. crus-galli and E. colona. Samples were sown in the soil flooded with 100 mm of water and harvested daily for 9 days for rice and 8 days for weeds. Black arrows indicate the bands detected in each genotype for ALDH2 proteins. (C) Immunoblot raised against ALDH2 in ‘Khao Hlan On’ (left) and ‘IR42’ (right).