Evangelina S. Ella

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

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.

PHYSIOLOGICAL BASIS OF SUBMERGENCE TOLERANCE IN RAINFED LOWLAND RICE ECOSYSTEM

Abstract The rainfed lowland rice ecosystem is affected by not only water deficit but also excess water. Nearly half of the ecosystem is prone to submergence damages caused by flash flooding. Although the rice plant is well adapted to aquatic environments, it is unable to survive if it is completely submerged in water for an extended period. Germplasm improvement for submergence tolerance is likely to be the best option available for alleviation of the submergence damages, other than construction of extensive drainage systems which require a large investment. The germplasm improvement may be successful if physiological mechanisms for submergence tolerance are well understood and floodwater environments are characterized. The plant response to submergence stress varies greatly depending on conditions of floodwater such as light intensity and concentrations of O 2 and CO 2 . This suggests that various physiological mechanisms are involved for survival from the submergence stress. This paper reviews the current understanding of submergence tolerance in rice plants from physiological aspects and then discusses possible strategies to improve the tolerance.

Enhancement of submergence tolerance in transgenic rice overproducing pyruvate decarboxylase.

Summary Transgenic rice ( Oryza sativa L.) lines were produced through transformation with rice pdc1 gene coding for pyruvate decarboxylase (PDC), one of the enzymes involved in alcohol fermentation. The over-expression of PDC was used not only to assess the role of alcohol fermentation but also to produce lines with enhanced metabolic capacity under anaerobiosis conferring submergence tolerance to these lines. Tillers of confirmed T 0 transgenic lines showed higher PDC activities and ethanol production compared to the untransformed control. Consequently, ethanol production of tillers of T 0 transgenic plants was positively correlated with survival after submergence. This is the first known report of transformation of an economically-important crop resulting in increased submergence tolerance.

Importance of active oxygen-scavenging system in the recovery of rice seedlings after submergence

Abstract Active oxygen species (AOS) can damage cells by mutation of nucleic acids and derivatives, dysfunction of proteins, oxidation of lipids to peroxides that make membranes leaky. Excessive illumination during recovery of submerged rice seedlings may induce an oxidative stress because of abnormal amount of AOS such as hydrogen peroxide (H 2 O 2 ), superoxide and hydroxyl radicals. Though submergence-tolerant FR13A and -intolerant IR42 both have comparable H 2 O 2 production during recovery, FR13A had less lipid peroxidation and had maintained a considerably higher level of ascorbate antioxidant during recovery than IR42. Only glutathione reductase (GR) activity had significantly different levels between the two cultivars with greater level in FR13A. The high levels of ascorbate and GR activity ensure a better operation of ascorbic acid–glutathione cycle that helps detoxify H 2 O 2 more efficiently in FR13A. However, in IR42 where this cycle is expected to operate at much slower rate because of limited ascorbate and GR activity, H 2 O 2 becomes readily available for lipid peroxidation, producing more MDA. Our data reveal the involvement of active oxygen-scavenging system during recovery of submerged rice seedlings under excessive illumination.

Adaptation to flooding during emergence and seedling growth in rice and weeds, and implications for crop establishment

Direct seeding is replacing transplanting in rice. Early flooding suppresses weeds but selective action is compromised by the sharing of flood-tolerance traits. Understanding adaptive traits in both species is therefore a prerequisite for developing direct seeding systems that control weeds while leaving rice seedlings relatively unharmed.

Seed pre-treatment in rice reduces damage, enhances carbohydrate mobilization and improves emergence and seedling establishment under flooded conditions

Priming rice seeds (soaking followed by drying) or soaking just before sowing improved emergence from flooded soil, reduced membrane damage from ROS and hastened carbohydrate mobilization. Most benefit was to lines with a superior ability to germinate in flooded soil even when untreated.

Gas film retention and underwater photosynthesis during field submergence of four contrasting rice genotypes

Summary The flood-tolerant genotype FR13A retains leaf gas films and its capacity for underwater net photosynthesis, whereas gas films are lost faster and photosynthesis declines markedly in sensitive genotypes.

Comparison of adaptability to flash flood between rice cultivars differing in flash flood tolerance

Abstract The mechanism of flash flood tolerance was investigated by using a flash flood-intolerant rice cultivar (Oryza sativa), IR42 and a tolerant one, FR13A. The photosynthetic rate of the rice seedlings from both types of cultivars decreased during submergence, whereas the photosynthetic rate of FR13A remained higher than that of IR42. The decrease in photosynthesis in IR42 was partially due to a higher inhibition of the activities of ribulose-1,5-bisphosphate carboxylase, fructose-1,6-bisphosphatase, and glyceraldehyde-3-phosphate dehydrogenase, than in FR13A. After submergence receded (hereafter referred as “desubmergence”), both types of submerged rice cultivars experienced oxidative damage. However the oxidation of the lipids and proteins in FR13A was maintained at a lower level than that in IR42 after desubmergence. On the other hand, the superoxide dismutase activity in FR13A increased more than that in IR42 after desubmergence. Moreover, the content of total ascorbate (reduced and oxidized ascorbates), a main antioxidant increased in FR13A, while in IR42 the content remained low after desubmergence. It is assumed that the photosynthetic activity during submergence may be related to flash flood tolerance. Moreover, flash flood tolerance requires a rapid stimulation of antioxidant systems after desubmergence.

Specific Gravity of the Grain — a Factor to Consider in Rice Tissue Culture

Summary Development of the rice grain explant affects its response in tissue culture. Grading the grains based on specific gravity to differentiate development improved callus production and plant regeneration. Grains with specific gravity greater than 1.2 consistently gave the highest percent callus production, average weight of callus per callus-producing grain and plant regeneration. Statistical data showed that the variability on callus production and plant regeneration was high in the ungraded grains but was lessened by grading the grains based on specific gravity before use as explants in tissue culture.