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1.  Adaptation to flooding during emergence and seedling growth in rice and weeds, and implications for crop establishment 
AoB Plants  2012;2012:pls019.
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.
Background and aims
Direct seeding of rice is being adopted in rainfed and irrigated lowland ecosystems because it reduces labour costs in addition to other benefits. However, early flooding due to uneven fields or rainfall slows down seed germination and hinders crop establishment. Conversely, early flooding helps suppress weeds and reduces the costs of manual weeding and/or dependence on herbicides; however, numerous weed species are adapted to lowlands and present challenges for the use of flooding to control weeds. Advancing knowledge on the mechanisms of tolerance of flooding during germination and early growth in rice and weeds could facilitate the development of improved rice varieties and effective weed management practices for direct-seeded rice.
Principal results
Rice genotypes with a greater ability to germinate and establish in flooded soils were identified, providing opportunities to develop varieties suitable for direct seeding in flooded soils. Tolerance of flooding in these genotypes was mostly attributed to traits associated with better ability to mobilize stored carbohydrates and anaerobic metabolism. Limited studies were undertaken in weeds associated with lowland rice systems. Remaining studies compared rice and weeds and related weed species such as Echinochloa crus-galli and E. colona or compared ecotypes of the same species of Cyperus rotundus adapted to either aerobic or flooded soils.
Conclusions
Tolerant weeds and rice genotypes mostly developed similar adaptive traits that allow them to establish in flooded fields, including the ability to germinate and elongate faster under hypoxia, mobilize stored starch reserves and generate energy through fermentation pathways. Remarkably, some weeds developed additional traits such as larger storage tubers that enlarge further in deeper flooded soils (C. rotundus). Unravelling the mechanisms involved in adaptation to flooding will help design management options that will allow tolerant rice genotypes to adequately establish in flooded soils while simultaneously suppressing weeds.
doi:10.1093/aobpla/pls019
PMCID: PMC3434364  PMID: 22957137
2.  Seed pre-treatment in rice reduces damage, enhances carbohydrate mobilization and improves emergence and seedling establishment under flooded conditions 
AoB Plants  2011;2011:plr007.
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.
Background and aims
Early flooding helps control weeds but reduces seedling establishment in direct-seeded rice (Oryza sativa). When combined with appropriate management practices, the use of genotypes that better tolerate flooding during emergence can enhance crop establishment in flood-prone areas. Management options include seed pre-treatment and we tested the influence of pre-soaking for 24 h prior to sowing or of priming (soaking for 24 or 48 h followed by drying).
Methodology
The effects on seedling establishment after 21-day flooding of pre-soaking seeds for 24 h before sowing and/or of priming seeds were examined together with physiological responses connected with reactive oxygen scavenging. Seeds of four lines with contrasting abilities to tolerate flooding at the germination stage were compared. Seeds were primed using KCl solutions (48 h) or water (24 h) and pre-soaked using water. Lipid peroxidation and activities of reactive oxygen-scavenging enzymes were measured in seeds before sowing. Carbohydrate mobilization in germinating seeds and seedling growth were also monitored at intervals.
Principal results
Seed pre-treatment by pre-soaking or by priming increased survival of flooding and accelerated and improved seedling establishment, especially in tolerant genotypes. Primed seeds had less lipid peroxidation and higher superoxide dismutase (SOD) and catalase (CAT) activities than non-primed seeds. Amylase activities and starch breakdown were also hastened in primed seeds. Survival after flooding was positively correlated with amylase activity but negatively correlated with the extent of lipid peroxidation.
Conclusions
Pre-soaking and priming improved seedling establishment in flooded soil, enhanced the capacity to scavenge reactive oxygen species in seeds by increasing SOD and CAT activities, and hastened carbohydrate mobilization. Tolerant genotypes responded better to these treatments, emphasizing the effectiveness of combining genetic tolerance with appropriate seed pre-treatment to improve seedling establishment of rice sown in flooded soils.
doi:10.1093/aobpla/plr007
PMCID: PMC3072768  PMID: 22476478
3.  Morphological and physiological responses of lowland purple nutsedge (Cyperus rotundus L.) to flooding 
AoB Plants  2010;2010:plq010.
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.
Background and aims
Purple nutsedge (Cyperus rotundus L.) is a major weed of upland crops and vegetables. Recently, a flood-tolerant ecotype evolved as a serious weed in lowland rice. This study attempted to establish the putative growth and physiological features that led to this shift in adaptation.
Methodology
Tubers of upland C. rotundus (ULCR) and lowland C. rotundus (LLCR) ecotypes were collected from their native habitats and maintained under the respective growth conditions in a greenhouse. Five experiments were conducted to assess the variation between the two ecotypes in germination, growth and tuber morphology when grown in their native or ‘switched’ conditions. Carbohydrate storage and mobilization, and variation in anaerobic respiration under hypoxia were compared.
Principal results
Tubers of LLCR were larger than those of ULCR, with higher carbohydrate content, and larger tubers developed with increasing floodwater depth. Stems of LLCR had larger diameter and proportionally larger air spaces than those of ULCR: a method of aerating submerged plant parts. The LLCR ecotype can also mobilize and use carbohydrate reserves under hypoxia, and it maintained relatively lower and steadier activity of alcohol dehydrogenase (ADH) as a measure of sustained anaerobic respiration. In contrast, ADH activity in ULCR increased faster upon a shift to hypoxia and then sharply decreased, suggesting depletion of available soluble sugar substrates. The LLCR ecotype also maintained lower lactate dehydrogenase activity under flooded conditions, which could reduce chances of cellular acidosis.
Conclusions
These adaptive traits in the LLCR ecotype were expressed constitutively, but some of them, such as tuber growth and aerenchyma development, are enhanced with stress severity. The LLCR ecotype attained numerous adaptive traits that could have evolved as a consequence of natural evolution or repeated management practices, and alternative strategies are necessary because flooding is no longer a feasible management option.
doi:10.1093/aobpla/plq010
PMCID: PMC3000701  PMID: 22476068

Results 1-3 (3)