Mechanisms associated with tolerance to flooding during germination and early seedling growth in rice (Oryza sativa)

Abstract

Background and aims: Flooding slows seed germination, imposes fatalities and delays seedling establishment in direct-seeded rice. This study describes responses of contrasting rice genotypes subjected to flooding or low oxygen stress during germination and discusses the basis of tolerance shown by certain cultivars.

Methods: In one set of experiments, dry seeds were sown in soil and either watered normally or flooded with 10 cm of water. Seedling survival and shoot and root growth were assessed and seed portions of germinating seedlings were assayed for soluble sugars and starch concentrations. The whole germinating seedlings were assayed for amylase and peroxidase activities and for ethylene production. Activities of enzymes associated with anaerobic respiration were examined and gene expression was analysed separately with seeds germinating under different amounts of dissolved oxygen in dilute agar.

Key results: Flooding during germination reduced survival but to a lesser extent in tolerant genotypes. Starch concentration in germinating seeds decreased while sugar concentration increased under flooding, but more so in tolerant genotypes. Amylase activity correlated positively with elongation (r = 0.85 for shoot and 0.83 for root length) and with plant survival (r = 0.92). Tolerant genotypes had higher amylase activity and higher RAmy3D gene expression. Ethylene was not detected in seeds within 2 d after sowing, but increased thereafter, with a greater increase in tolerant genotypes starting 3 d after sowing. Peroxidase activity was higher in germinating seeds of sensitive genotypes and correlated negatively with survival.

Conclusions: Under low oxygen stress, tolerant genotypes germinate, grow faster and more seedlings survive. They maintain their ability to use stored starch reserves through higher amylase activity and anaerobic respiration, have higher rates of ethylene production and lower peroxidase activity as germinating seeds and as seedlings. Relevance of these traits to tolerance of flooding during germination and early growth is discussed.

Fig. 1.

Survival of flooded rice seedlings of (A) ten genotypes in expt I, (B) four genotypes in expt II, 21 d after seeding and flooding with 10 cm water. Vertical bars indicate l.s.d. at P = 0·05.

Fig. 2.

Shoot (longest leaf) and root lengths of seedlings of flooding-tolerant and -intolerant genotypes of rice measured 21 d following sowing under control and flooding in 10 cm of water. Data are means of ten genotypes in expt I. Values in parentheses are percentage reductions in length relative to the length in the control. Vertical bars indicate l.s.d. at P = 0·05.

Fig. 3.

Lengths of (A) shoots (longest leaf) and (B) roots of two flooding-tolerant and two flooding-sensitive rice genotypes following dry seeding and flooding in 10 cm of water in expt II. Vertical bars indicate l.s.d. at P = 0·05.

Fig. 4.

Changes in soluble sugar concentrations in germinating seeds of two flooding-tolerant and two flooding-sensitive rice genotypes sown in soil under either (A) control conditions or (B) flooding in 10 cm water in expt II. Vertical bars indicate l.s.d. at P = 0·05.

Fig. 5.

Changes in starch concentration in germinating seeds of two flooding-tolerant and two flooding-sensitive rice genotypes sown in soil under either (A) control conditions or (B) flooding in 10 cm water in expt II. Vertical bars indicate l.s.d. at P = 0·05.

Fig. 6.

(A) Amylase activity in seeds germinating in flooded soil during the first 3 d after sowing in expt II (vertical bar indicates l.s.d. at P = 0·05) and (B) reverse-transcript (RT-PCR) analyses of amylases (RAmy1A, RAmy3C, RAmy3D and RAmy3E) and sucrose synthases (Sus1 and Sus3) using RNA extracted from germinating embryos of cultivars ‘Khaiyan’ and ‘IR42’ under hypoxia (0·03 mol O₂ m⁻³), air (0·25 mol O₂ m⁻³) and anoxia (<0·003 mol O₂ m⁻³) treatments for 0, 12, 24, 48 and 72 h.

Fig. 7.

Association of amylase activity 3 d after sowing with (A) seedling survival measured 21 d after sowing and with (B) shoot and root length 5 d after sowing. Data are means of two tolerant cultivars (‘Khaiyan’ and ‘Khao Hlan On’) and two intolerant (‘FR13A’ and ‘IR42’) genotypes sown in soil and flooded in 10 cm water in expt II.

Fig. 8.

Enzyme activity of (A) pyruvate decarboxylase (PDC) and (B) alcohol dehydrogenase (ADH) in flooding-tolerant and flooding-sensitive genotypes during germination under aerated and hypoxic conditions. (C) Expression of Pdc and Adh in germinating seeds incubated in air, hypoxic or anoxic conditions. Data are from expt VI and vertical bars in (A) and (B) indicate l.s.d. at P = 0·05.

Fig. 9.

Ethylene production by rice seedlings after sowing and flooding in soil in expt III. Measurements were made 3, 4, 6, 7 and 9 d after sowing. Data for days 3 and 7 are based on one measurement per treatment. Data for day 9 are means of three replicate measurements (l.s.d. at P = 0·05 is 11).

Fig. 10.

(A) Peroxidase activity in germinating rice seeds incubated in nitrogen-bubbled stagnant agar solution for 5 d to maintain hypoxic condition similar to flooded soil in expt IV. (B) Relationship of peroxidase activity with shoot length after 5 d in agar solution in expt IV, and (C) with seedling survival 21 d after sowing and flooding in soil in expt II. The vertical bar in (A) indicates l.s.d. at P = 0·05.

Fig. 11.

Proposed mechanisms involved in enhancing germination and seedling growth under low-oxygen stress in rice. A detailed description is given in the Discussion.