Genotypic variations in leaf gas exchange and yield were analysed in five upland-adapted and three lowland rice cultivars subjected to a differential soil moisture gradient, varying from well-watered ...to severely water-stressed conditions. A reduction in the amount of water applied resulted in a significant decrease in leaf gas exchange and, subsequently, in above-ground dry mass and grain yield, that varied among genotypes and distance from the line source. The comparison between the variable J and the Δ values in recently synthesized sugars methods, yielded congruent estimations of mesophyll conductance (gm), confirming the reliability of these two techniques. Our data demonstrate that gm is a major determinant of photosynthesis (A), because rice genotypes with inherently higher gm were capable of keeping higher A in stressed conditions. Furthermore, A, gs, and gm of water-stressed genotypes rapidly recovered to the well-watered values upon the relief of water stress, indicating that drought did not cause any lasting metabolic limitation to photosynthesis. The comparisons between the A/Ci and corresponding A/Cc curves, measured in the genotypes that showed intrinsically higher and lower instantaneous A, confirmed this finding. Moreover, the effect of drought stress on grain yield was correlated with the effects on both A and total diffusional limitations to photosynthesis. Overall, these data indicate that genotypes which showed higher photosynthesis and conductances were also generally more productive across the entire soil moisture gradient. The analysis of Δ revealed a substantial variation of water use efficiency among the genotypes, both on the long-term (leaf pellet analysis) and short-term scale (leaf soluble sugars analysis).
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 growth encompasses a remarkable range of genetic diversity. ► A detailed background of rice root growth at a range of scales is reviewed. ► Key root traits for drought response in rice ...are presented. ► Physiology and breeding perspectives for drought improvement in rice are considered.
Rice root growth encompasses a remarkable genetic diversity in terms of growth patterns, architecture, and environmental adaptations. In order to harness this valuable diversity for improving rice response to drought, an understanding of key root traits and effective drought response mechanisms is necessary. A trait-based approach with precise understanding of the target environment, including temporal and spatial heterogeneity, is a possible path toward the use of roots and dehydration avoidance traits for improved drought resistance in rice. The ability to grow deep roots is currently the most accepted target trait for improving drought resistance, but genetic variation has been reported for a number of traits that may affect drought response. Here, we review variation in rice root response to drought from a physiological perspective in terms of morphology and function with respect to the different growth environments (upland and lowland) commonly used by farmers. Recent advances in linking physiology and breeding are also presented.
Global production of rice (Oryza sativa) grain is limited by water availability and the low 'leaf-level' photosynthetic capacity of many cultivars. Oryza sativa is extremely susceptible to ...water-deficits; therefore, predicted increases in the frequency and duration of drought events, combined with future rises in global temperatures and food demand, necessitate the development of more productive and drought tolerant cultivars. We investigated the underlying physiological, isotopic and morphological responses to water-deficit in seven common varieties of O. sativa, subjected to prolonged drought of varying intensities, for phenotyping purposes in open field conditions. Significant variation was observed in leaf-level photosynthesis rates (A) under both water treatments. Yield and A were influenced by the conductance of the mesophyll layer to CO2 (g(m)) and not by stomatal conductance (g(s)). Mesophyll conductance declined during drought to differing extents among the cultivars; those varieties that maintained g(m) during water-deficit sustained A and yield to a greater extent. However, the variety with the highest g(m) and yield under well-watered conditions (IR55419-04) was distinct from the most effective cultivar under drought (Vandana). Mesophyll conductance most effectively characterises the photosynthetic capacity and yield of O. sativa cultivars under both well-watered and water-deficit conditions; however, the desired attributes of high g(m) during optimal growth conditions and the capacity for g(m) to remain constant during water-deficit may be mutually exclusive. Nonetheless, future genetic and physiological studies aimed at enhancing O. sativa yield and drought stress tolerance should investigate the biochemistry and morphology of the interface between the sub-stomatal pore and mesophyll layer.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This book features a comprehensive foresight assessment, exploring the pressures — threats as well as opportunities — on the global agriculture & food systems between now and 2050. The overarching ...aim is to help readers understand the context, by analyzing global trends and anticipating change for better planning and constructing pathways from the present to the future by focusing on the right questions and problems. The book contextualizes the role of international agricultural research in addressing the complex challenges posed by UN 2030 Agenda and beyond, and identifies the decisions that scientific leaders, donors and policy makers need to take today, and in the years ahead, to ensure that a global population rising to nine billion or more combined with rising incomes and changing diets can be fed sustainably and equitably, in the face of the growing climate threats.
This introductory paper provides an overview of the contributions to this special issue, addressing four key questions related to conservation agriculture (CA) in Sub-Saharan Africa and South Asia: ...What is the impact of CA on yields? What is the impact of adopting CA on farmers’ profits? What are the environmental impacts of adopting CA? How well does CA fit with wider agricultural, social, economic and political contexts for small-scale farmers? Papers in this special issue find that yield increases under CA are possible but uncertain given the low average yields that pertain in these regions, and yield gains are more likely to be observed after several years. CA is not widely adopted in Sub-Saharan Africa and South Asia owing to a lack of economic incentive for smallholder farmers—that the process of conversion to CA is not profitable over planning horizons of most farmers. There is no clear trend for greater carbon sequestration under CA, so the potential for subsidizing farmers to adopt CA using payments for ecosystem services/carbon credit schemes seems limited in scope. There is early evidence that farmers perceive a benefit from CA adoption in regions that are prone to erratic rainfall, suggesting a potential risk mitigation role. In addition, throughout this overview paper we offer a commentary on some of the scientific issues that constrain our ability to understand the performance of CA in these systems more comprehensively.
▶ Genetic variation for deep root growth in drought-stressed rice was observed. ▶ Differences in root length density at depth were related to soil moisture levels. ▶ Canopy temperature was lower in ...lines with greater deep root growth under drought.
Root growth at soil depths below 30
cm may provide access to critical soil water reserves during drought in rainfed lowland rice. In this study, the OryzaSNP panel, a set of 20 lines representing genetic diversity in rice used for the discovery of DNA sequence polymorphisms, was evaluated for root characteristics in the field over three seasons varying in drought severity. Root length density (RLD) at a depth of 30–45
cm varied up to 74–92% among genotypes under drought stress (2008–2009 dry seasons), ranging from 0.024 to 0.23
cm
cm
−3 in 2008 and from 0.19 to 0.81
cm
cm
−3 in 2009. Real-time monitoring of soil moisture profiles revealed significant differences among genotypes, and these differences were correlated with RLD at those soil depths. Among the lines evaluated, the Aus isozyme group, particularly the genotype Dular, showed greater drought resistance associated with deep root growth and the highest drought response index (less reduction in yield by drought stress). Since the set of genotypes used in this study has been completely sequenced for SNP markers, the phenotypic information on root growth and drought avoidance responses presented here could be used in initial analysis of the genetic basis of dehydration avoidance traits and in facilitating improvement in drought resistance in rice.
Drought is the major constraint to rice production in rainfed areas across Asia and sub-Saharan Africa. In the context of current and predicted water scarcity, increasing irrigation is generally not ...a viable option for alleviating drought problems in rainfed rice-growing systems. It is therefore critical that genetic management strategies for drought focus on maximum extraction of available soil moisture and its efficient use in crop establishment and growth to maximize biomass and yield. Extensive genetic variation for drought resistance exists in rice germplasm. However, the current challenge is to decipher the complexities of drought resistance in rice and exploit all available genetic resources to produce rice varieties combining drought adaptation with high yield potential, quality, and resistance to biotic stresses. The strategy described here aims at developing a pipeline for elite breeding lines and hybrids that can be integrated with efficient management practices and delivered to rice farmers. This involves the development of high-throughput, high-precision phenotyping systems to allow genes for yield components under stress to be efficiently mapped and their effects assessed on a range of drought-related traits, and then moving the most promising genes into widely grown rice mega-varieties, while scaling up gene detection and delivery for use in marker-aided breeding.
ABSTRACT
Rice is known to be sensitive to soil water deficit and evaporative demand, with a greatest sensitivity of lowland‐adapted genotypes. We have analysed the responses of plant water relations ...and of leaf elongation rate (LER) to soil water status and evaporative demand in seven rice genotypes belonging to different species, subspecies, either upland‐ or lowland‐adapted. In the considered range of soil water potential (0 to −0.6 MPa), stomatal conductance was controlled in such a way that the daytime leaf water potential was similar in well‐watered, droughted or flooded conditions (isohydric behaviour). A low sensitivity of LER to evaporative demand was observed in the same three conditions, with small differences between genotypes and lower sensitivity than in maize. The sensitivity of LER to soil water deficit was similar to that of maize. A tendency towards lower sensitivities was observed in upland than lowland genotypes but with smaller differences than expected. We conclude that leaf water status and leaf elongation of rice are not particularly sensitive to water deficit. The main origin of drought sensitivity in rice may be its poor root system, whose effect was alleviated in the study presented here by growing plants in pots whose soil was entirely colonized by roots of all genotypes.
Background Plant roots are important organs to uptake soil water and nutrients, perceiving and transducing of soil water deficit signals to shoot. The current knowledge of drought stress ...transcriptomes in rice are mostly relying on comparative studies of diverse genetic background under drought. A more reliable approach is to use near-isogenic lines (NILs) with a common genetic background but contrasting levels of resistance to drought stress under initial exposure to water deficit. Here, we examined two pairs of NILs in IR64 background with contrasting drought tolerance. We obtained gene expression profile in roots of rice NILs under different levels of drought stress help to identify genes and mechanisms involved in drought stress. Results Global gene expression analysis showed that about 55% of genes differentially expressed in roots of rice in response to drought stress treatments. The number of differentially expressed genes (DEGs) increased in NILs as the level of water deficits, increased from mild to severe condition, suggesting that more genes were affected by increasing drought stress. Gene onthology (GO) test and biological pathway analysis indicated that activated genes in the drought tolerant NILs IR77298-14-1-2-B-10 and IR77298-5-6-B-18 were mostly involved in secondary metabolism, amino acid metabolism, response to stimulus, defence response, transcription and signal transduction, and down-regulated genes were involved in photosynthesis and cell wall growth. We also observed gibberellic acid (GA) and auxin crosstalk modulating lateral root formation in the tolerant NILs. Conclusions Transcriptome analysis on two pairs of NILs with a common genetic background (approximately 97%) showed distinctive differences in gene expression profiles and could be effective to unravel genes involved in drought tolerance. In comparison with the moderately tolerant NIL IR77298-5-6-B-18 and other susceptible NILs, the tolerant NIL IR77298-14-1-2-B-10 showed a greater number of DEGs for cell growth, hormone biosynthesis, cellular transports, amino acid metabolism, signalling, transcription factors and carbohydrate metabolism in response to drought stress treatments. Thus, different mechanisms are achieving tolerance in the two tolerant lines.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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