Rolando O. Torres

Root attributes affecting water uptake of rice (Oryza sativa) under drought

Lowland rice roots have a unique physiological response to drought because of their adaptation to flooded soil. Rice root attributes that facilitate growth under flooded conditions may affect rice response to drought, but the relative roles of root structural and functional characteristics for water uptake under drought in rice are not known. Morphological, anatomical, biochemical, and molecular attributes of soil-grown rice roots were measured to investigate the genotypic variability and genotype×environment interactions of water uptake under variable soil water regimes. Drought-resistant genotypes had the lowest night-time bleeding rates of sap from the root system in the field. Diurnal fluctuation predominated as the strongest source of variation for bleeding rates in the field and root hydraulic conductivity (Lp r) in the greenhouse, and was related to expression trends of various PIP and TIP aquaporins. Root anatomy was generally more responsive to drought treatments in drought-resistant genotypes. Suberization and compaction of sclerenchyma layer cells decreased under drought, whereas suberization of the endodermis increased, suggesting differential roles of these two cell layers for the retention of oxygen under flooded conditions (sclerenchyma layer) and retention of water under drought (endodermis). The results of this study point to the genetic variability in responsiveness to drought of rice roots in terms of morphology, anatomy, and function.

Rice Root Architectural Plasticity Traits and Genetic Regions for Adaptability to Variable Cultivation and Stress Conditions.

Rice root genetic regions determining root architectural plasticity can be used for selection of improved adaptability to variable conditions. Future rice (Oryza sativa) crops will likely experience a range of growth conditions, and root architectural plasticity will be an important characteristic to confer adaptability across variable environments. In this study, the relationship between root architectural plasticity and adaptability (i.e. yield stability) was evaluated in two traditional × improved rice populations (Aus 276 × MTU1010 and Kali Aus × MTU1010). Forty contrasting genotypes were grown in direct-seeded upland and transplanted lowland conditions with drought and drought + rewatered stress treatments in lysimeter and field studies and a low-phosphorus stress treatment in a Rhizoscope study. Relationships among root architectural plasticity for root dry weight, root length density, and percentage lateral roots with yield stability were identified. Selected genotypes that showed high yield stability also showed a high degree of root plasticity in response to both drought and low phosphorus. The two populations varied in the soil depth effect on root architectural plasticity traits, none of which resulted in reduced grain yield. Root architectural plasticity traits were related to 13 (Aus 276 population) and 21 (Kali Aus population) genetic loci, which were contributed by both the traditional donor parents and MTU1010. Three genomic loci were identified as hot spots with multiple root architectural plasticity traits in both populations, and one locus for both root architectural plasticity and grain yield was detected. These results suggest an important role of root architectural plasticity across future rice crop conditions and provide a starting point for marker-assisted selection for plasticity.

Traits and QTLs for development of dry direct-seeded rainfed rice varieties

Highlight text Characterization and QTL identification of seedling-stage traits revealed relationships with nutrient uptake and grain yield; these traits may improve the adaptation and productivity of rice under direct-seeded conditions.

Grain yield and physiological traits of rice lines with the drought yield QTL qDTY12.1 showed different responses to drought and soil characteristics in upland environments

To improve yield in upland conditions, near-isogenic lines (NILs) of the major-effect drought yield quantitative trait locus qDTY12.1 in rice (Oryza sativa L.) were developed in the background of the upland variety Vandana. These NILs have shown greater water uptake a larger proportion of lateral roots, and higher transpiration efficiency under drought than Vandana, and one NIL (481-B) was selected as having the highest yield. In this study, the NILs were assessed in two greenhouse and 18 upland field trials for their response to drought and different soil textures. Performance of qDTY12.1 NILs was not affected by soil texture but showed a notable response to drought stress severity. The yield advantage of 481-B over Vandana was highest in field trials with intermittent drought stress, in which the mean trial yield was greater than 0.5tha-1, and in the least favourable well watered trial. The effects of qDTY12.1 on water uptake were most apparent under mild to moderate stress but not in very severe drought or well watered treatments, whereas the lateral root and transpiration efficiency responses were observed under a range of conditions. These results highlight the varying response of qDTY12.1 across upland environments and the complexity of multiple mechanisms acting together to confer an effect on rice yield under drought.

Yield stability of selected rice breeding lines and donors across conditions of mild to moderately severe drought stress

Although mild to moderately severe drought stress may have less of an effect on rice grain yield than severe drought stress during reproductive stage, its prevalence across rice farmers’ fields at the global level may be more economically significant. In this study, field experiments were conducted on selected genotypes with known tolerance to severe reproductive-stage drought in order to identify those that would produce high and stable grain yield across seasons and soil moisture conditions varying from well-watered to mild and moderately severe drought stress. Mild stress generally occurred during wet seasons and moderate stress happened during dry seasons. The drought stress was mild enough such that the time to flowering was similar under drought stress and well-watered conditions in either season. However, significant grain yield reductions were incurred even at mild drought levels. Using an AMMI1 biplot analysis, IR83142-B-7-B-B, Binuhangin, IR77298-14-1-2-13, IR70215-70-CPA-3-4-1-3 and IR77298-14-1-2 were identified as the genotypes with the highest and most stable grain yields in both well-watered and mild to moderately severe drought stress environments. In a characterization of traits conferring drought tolerance among the highest yielding genotypes under mild to moderate drought stress, genotypes Binuhangin and IR70215-70-CPA-3-4-1-3 stood out for multiple physiological traits under drought. However, no direct correlations among genotypes between stomatal conductance, normalized difference vegetation index (NDVI) or root dry weight with grain or total dry matter yield were observed under any soil moisture level. These results reflect the complex interaction of drought response traits contributing to grain yield. The genotypic variation and physiological responses observed in this study point to the potential of developing varieties targeted to mild and moderate drought stress using yield as the selection criterion.

Stem-nodulating legumes as relay-cropped or intercropped green manures for lowland rice

Abstract Green manures have consistently shown positive contributions to tropical rice (Oryza sativa) production, but will be produced on a substantial scale only when they fit into appropriate niches in the cropping system. We examined the effects of intercropped and relay-cropped Sesbania rostrata and Aeschynomene afraspera as green manures (GM) on the companion and succeeding rice crops for three consecutive seasons in an environment where two rice crops are grown each year. Pregerminated seeds of the legumes were sown between rice rows immediately after transplanting or at the ripening stage of the rice crops. They also were broadcast at the ripening stage of hill-planted rice. The effects of the GM on rice, with or without added urea, were compared with the effects of varying urea-N rates in the absence of the GM. Surface water was detrimental to pregerminated GM seeds. A. afraspera was more damaged by flooding because it emerged 4 d later than S. rostrata. Green manure effects on either the companion or succeeding rice crops varied as a function of GM crop stand and biomass production. Green manure biomass yield was greater with broadcasting than with row seeding. Green manure relayed into dry-season rice grew vigorously and increased the grain yield of the succeeding rice crop equivalent to the application of 30 kg urea-N/ha. The growth of GM relayed into wet-season rice was suppressed by the short photoperiod in that season. A urea-N topdress application did not improve the effect of GM on rice yield. The effect of relayed GM on the succeeding wet-season rice crop also was equivalent to or exceeded the effect of 60 kg urea-N/ha. The results indicated the potential feasibility of relaying GM in a rice cropping system when the turnaround time is too short to raise a preceding monocrop GM.