A High-Yielding Rice Cultivar "Takanari" Shows No N Constraints on CO2 Fertilization.

Enhancing crop yield response to elevated CO2 concentrations (E-[CO2]) is an important adaptation measure to climate change. A high-yielding indica rice cultivar "Takanari" has recently been identified as a potential candidate for high productivity in E-[CO2] resulting from its large sink and source capacities. To fully utilize these traits, nitrogen should play a major role, but it is unknown how N levels influence the yield response of Takanari to E-[CO2]. We therefore compared grain yield and quality of Takanari with those of Koshihikari, a standard japonica cultivar, in response to Free-Air CO2 enrichment (FACE, +200 µmol mol-1) under three N levels (0, 8, and 12 g m-2) over three seasons. The biomass of both cultivars increased under E-[CO2] at all N levels; however, the harvest index decreased under E-[CO2] in the N-limited treatment for Koshihikari but not for Takanari. The decreased harvest index of Koshihikari resulted from limited enhancement of spikelet number under N-limitation. In contrast, spikelet number increased in E-[CO2] in Takanari even without N application, resulting in significant yield enhancement, averaging 18% over 3 years, whereas Koshihikari exhibited virtually no increase in yield in E-[CO2] under the N-limited condition. Grain appearance quality of Koshihikari was severely reduced by E-[CO2], most notably in N-limited and hot conditions, by a substantial increase in chalky grain, but chalky grain % did not increase in E-[CO2] even without N fertilizer. These results indicated that Takanari could retain its high yield advantage over Koshihikari with limited increase in chalkiness even under limited N conditions and that it could be a useful genetic resource for improving N use efficiency under E-[CO2].

Increasing canopy photosynthesis in rice can be achieved without a large increase in water use-A model based on free-air CO2 enrichment.

Achieving higher canopy photosynthesis rates is one of the keys to increasing future crop production; however, this typically requires additional water inputs because of increased water loss through the stomata. Lowland rice canopies presently consume a large amount of water, and any further increase in water usage may significantly impact local water resources. This situation is further complicated by changing the environmental conditions such as rising atmospheric CO2 concentration ([CO2 ]). Here, we modeled and compared evapotranspiration of fully developed rice canopies of a high-yielding rice cultivar (Oryza sativa L. cv. Takanari) with a common cultivar (cv. Koshihikari) under ambient and elevated [CO2 ] (A-CO2 and E-CO2 , respectively) via leaf ecophysiological parameters derived from a free-air CO2 enrichment (FACE) experiment. Takanari had 4%-5% higher evapotranspiration than Koshihikari under both A-CO2 and E-CO2 , and E-CO2 decreased evapotranspiration of both varieties by 4%-6%. Therefore, if Takanari was cultivated under future [CO2 ] conditions, the cost for water could be maintained at the same level as for cultivating Koshihikari at current [CO2 ] with an increase in canopy photosynthesis by 36%. Sensitivity analyses determined that stomatal conductance was a significant physiological factor responsible for the greater canopy photosynthesis in Takanari over Koshihikari. Takanari had 30%-40% higher stomatal conductance than Koshihikari; however, the presence of high aerodynamic resistance in the natural field and lower canopy temperature of Takanari than Koshihikari resulted in the small difference in evapotranspiration. Despite the small difference in evapotranspiration between varieties, the model simulations showed that Takanari clearly decreased canopy and air temperatures within the planetary boundary layer compared to Koshihikari. Our results indicate that lowland rice varieties characterized by high-stomatal conductance can play a key role in enhancing productivity and moderating heat-induced damage to grain quality in the coming decades, without significantly increasing crop water use.

Do the rich always become richer? Characterizing the leaf physiological response of the high-yielding rice cultivar Takanari to free-air CO2 enrichment.

The development of crops which are well suited to growth under future environmental conditions such as higher atmospheric CO2 concentrations ([CO2]) is essential to meeting the challenge of ensuring food security in the face of the growing human population and changing climate. A high-yielding indica rice variety (Oryza sativa L. cv. Takanari) has been recently identified as a potential candidate for such breeding, due to its high productivity in present [CO2]. To test if it could further increase its productivity under elevated [CO2] (eCO2), Takanari was grown in the paddy field under season-long free-air CO2 enrichment (FACE, approximately 200 µmol mol(-1) above ambient [CO2]) and its leaf physiology was compared with the representative japonica variety 'Koshihikari'. Takanari showed consistently higher midday photosynthesis and stomatal conductance than Koshihikari under both ambient and FACE growth conditions over 2 years. Maximum ribulose-1,5-bisphosphate carboxylation and electron transport rates were higher for Takanari at the mid-grain filling stage in both years. Mesophyll conductance was higher in Takanari than in Koshihikari at the late grain-filling stage. In contrast to Koshihikari, Takanari grown under FACE conditions showed no decrease in total leaf nitrogen on an area basis relative to ambient-grown plants. Chl content was higher in Takanari than in Koshihikari at the same leaf nitrogen level. These results indicate that Takanari maintains its superiority over Koshihikari in regards to its leaf-level productivity when grown in elevated [CO2] and it may be a valuable resource for rice breeding programs which seek to increase crop productivity under current and future [CO2].

Overcoming the difficulties in collecting apoplastic fluid from rice leaves by the infiltration-centrifugation method.

Physiological and biochemical studies on the leaf apoplast have been facilitated by the use of the infiltration-centrifugation technique to collect intercellular washing fluid (IWF). However, this technique has been difficult to implement in rice (Oryza sativa L.) for various reasons. We compared the collection efficiency of leaf IWF between two types of rice varieties (Indica and Japonica), as well as between rice and other species (spinach, snap bean and wheat). Although the extraction of IWF in most species took only 2-3 min, it took up to 35 min in rice. The difficulty in infiltration with rice was ascribed to the small stomatal aperture and hydrophobicity of the leaves. In this study, we have established an improved method for collecting IWF and determining the apoplastic air and water volumes in rice leaves. We have shortened the infiltration time to 8 min via the following improvements: (i) infiltration under outdoor shade in the daytime to prevent stomatal closure and a rise in temperature of the infiltration medium; (ii) soaking of leaves in a surfactant solution to decrease the leaf hydrophobicity; and (iii) continuous pressurization using a sealant injector to facilitate the infiltration. The rapid collection of IWF achieved using this technique will facilitate study of the leaf apoplast in rice.

Leaf ascorbic acid level--is it really important for ozone tolerance in rice?

Leaf ascorbic acid (ASA) level is thought to be an important trait conferring stress tolerance in plants, but definite evidence regarding its effectiveness in the breeding of stress tolerant crops is lacking. Therefore, the stress response of a rice TOS17 insertion mutant (ND6172) for a GDP-D-mannose-3',5'-epimerase gene, which is involved in ASA biosynthesis, was tested. Two fumigation experiments were conducted, in which rice plants (Oryza sativa L.) were exposed to (i) high ozone for ten days at the tillering stage (100 ppb, 7 h day⁻¹); and (ii) to four different ozone concentrations ranging from charcoal filtered air to 2.5 times the ambient concentration for the entire growth season. The mutant ND6172 had around 20-30% lower ASA level than the wild-type (Nipponbare), and exhibited a moderately higher level of visible leaf symptoms due to ozone exposure. Differences in ASA level between ND6172 and Nipponbare led to differential responses of the glutathione level, and the activities of glutathione reductase, ascorbate peroxidase, and dehydroascorbate reductase. With season-long ozone fumigation, yields and yield components were not negatively affected at ambient ozone level in both genotypes, but showed stronger decreases in ND6172 at higher ozone levels, especially at 2.5 times the ambient level. Similarly, the mature straw of ND6172 exhibited a higher degree of lignification at the 2.5 times ambient ozone level. In conclusion, a difference in leaf ASA level of around 20-30% is relevant for ozone tolerance in rice at levels exceeding the current ambient ozone concentrations.

The OzT8 locus in rice protects leaf carbon assimilation rate and photosynthetic capacity under ozone stress.

Tropospheric ozone (O3) is a phytotoxic air pollutant whose current background concentrations in parts of East Asia have caused estimated rice yield losses of up to 20%; currently, however, little is known about the mechanisms of O3 tolerance in rice. We previously identified a quantitative trait locus (QTL) in rice called OzT8, which was associated with relative dry weight under ozone stress. The photosynthetic response in SL46, a Nipponbare (NB)-Kasalath chromosome segment substitution line (SL) containing the OzT8 locus, was compared to the parent NB in multiple ozone fumigation experiments (100 ppb, 8 h d⁻¹, 23 d). By day 23, SL46 showed significantly less reduction of photosynthetic capacity compared to NB; the maximum carboxylation rate of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) decreased by 24% in SL46 compared to 49% in NB, and the maximum electron transport rate decreased by 16 and 39%, respectively. The midday carbon assimilation rates also showed a similar trend, but there was no genotypic difference in stomatal conductance. These results indicate that the OzT8 locus confers ozone tolerance via biochemical acclimation, not avoidance, making it a potentially valuable target for breeding of ozone tolerance into future rice lines. The sequence of photosynthetic response of rice to ozone stress and related tolerance factors are also discussed.

Mechanisms of ozone tolerance in rice: characterization of two QTLs affecting leaf bronzing by gene expression profiling and biochemical analyses.

High surface ozone concentration is increasingly being recognized as a factor that negatively affects crop yields in Asia. However, little progress has been made in developing ozone-tolerant genotypes of rice-Asia's major staple crop. This study aimed to identify possible tolerance mechanisms by characterizing two quantitative trait loci (QTLs) that were previously shown to influence visible leaf symptoms under ozone exposure (120 nl l(-1), 7 h d(-1), 13 d). Two chromosome segment substitution lines (SL15 and SL41) that carried introgressions of the QTLs OzT3 and OzT9, respectively, were exposed to ozone at 120 nl l(-1) along with their parent Nipponbare. In accordance with the expected QTL effect, SL15 showed stronger visible symptoms of ozone damage than Nipponbare, whereas SL41 had fewer symptoms. Gene expression profiling by microarray hybridization yielded 470 probes that were differentially expressed in SL15 and 314 in SL41. Potential tolerance mechanisms were evaluated by investigating changes in gene expression in three general categories. (i) Processes involved in programmed cell death, in which a number of genes related to ethylene or jasmonic acid metabolism or general disease resistance were identified that were differentially regulated in one of the substitution lines. (ii) Biosynthesis of antioxidants. Testing this hypothesis did not reveal any genes differentially regulated between genotypes, and it was thus rejected. (iii) Turnover of antioxidants and enzymatic detoxification of radical oxygen species (ROS), in which a number of differentially regulated genes were also identified. Genes encoding antioxidant enzymes (catalase and peroxidases) tended to be more strongly expressed in SL15. A potential tolerance gene which encodes a putative ascorbate oxidase was identified within the QTL introgression in SL41. This gene showed consistently lower expression in SL41 under ozone exposure across different points in time within independent experiments. Its expression may be involved in mechanisms leading to enhanced ascorbic acid status in SL41 under ozone exposure, and may be linked to a higher concentration of total apoplastic ascorbic acid in SL41 that was observed in an independent experiment.

Is a short, sharp shock equivalent to long-term punishment? Contrasting the spatial pattern of acute and chronic ozone damage to soybean leaves via chlorophyll fluorescence imaging.

Experimental investigations of ozone (O(3)) effects on plants have commonly used short, acute [O(3)] exposure (>100 ppb, on the order of hours), while in field crops damage is more likely caused by chronic exposure (<100 ppb, on the order of weeks). How different are the O(3) effects induced by these two fumigation regimes? The leaf-level photosynthetic response of soybean to acute [O(3)] (400 ppb, 6 h) and chronic [O(3)] (90 ppb, 8 h d(-1), 28 d) was contrasted via simultaneous in vivo measurements of chlorophyll a fluorescence imaging (CFI) and gas exchange. Both exposure regimes lowered leaf photosynthetic CO(2) uptake about 40% and photosystem II (PSII) efficiency (F(q)'/F(m)') by 20% compared with controls, but this decrease was far more spatially heterogeneous in the acute treatment. Decline in F(q)'/F(m)' in the acute treatment resulted equally from decreases in the maximum efficiency of PSII (F(v)'/F(m)') and the proportion of open PSII centres (F(q)'/F(v)'), but in the chronic treatment decline in F(q)'/F(m)' resulted only from decrease in F(q)'/F(v)'. Findings suggest that acute and chronic [O(3)] exposures do not induce identical mechanisms of O(3) damage within the leaf, and using one fumigation method alone is not sufficient for understanding the full range of mechanisms of O(3) damage to photosynthetic production in the field.

The effect of leaf-level spatial variability in photosynthetic capacity on biochemical parameter estimates using the Farquhar model: a theoretical analysis.

Application of the widely used Farquhar model of photosynthesis in interpretation of gas exchange data assumes that photosynthetic properties are homogeneous throughout the leaf. Previous studies showed that heterogeneity in stomatal conductance (g(s)) across a leaf could affect the shape of the measured leaf photosynthetic CO(2) uptake rate (A) versus intercellular CO(2) concentration (C(i)) response curve and, in turn, estimation of the critical biochemical parameters of this model. These are the maximum rates of carboxylation (V(c,max)), whole-chain electron transport (J(max)), and triose-P utilization (V(TPU)). The effects of spatial variation in V(c,max,) J(max), and V(TPU) on estimation of leaf averages of these parameters from A-C(i) curves measured on a whole leaf have not been investigated. A mathematical model incorporating defined degrees of spatial variability in V(c,max) and J(max) was constructed. One hundred and ten theoretical leaves were simulated, each with the same average V(c,max) and J(max), but different coefficients of variation of the mean (CV(VJ)) and varying correlation between V(c,max) and J(max) (Omega). Additionally, the interaction of variation in V(c,max) and J(max) with heterogeneity in V(TPU), g(s), and light gradients within the leaf was also investigated. Transition from V(c,max)- to J(max)-limited photosynthesis in the A-C(i) curve was smooth in the most heterogeneous leaves, in contrast to a distinct inflection in the absence of heterogeneity. Spatial variability had little effect on the accuracy of estimation of V(c,max) and J(max) from A-C(i) curves when the two varied in concert (Omega = 1.0), but resulted in underestimation of both parameters when they varied independently (up to 12.5% in V(c,max) and 17.7% in J(max) at CV(VJ) = 50%; Omega = 0.3). Heterogeneity in V(TPU) also significantly affected parameter estimates, but effects of heterogeneity in g(s) or light gradients were comparatively small. If V(c,max) and J(max) derived from such heterogeneous leaves are used in models to project leaf photosynthesis, actual A is overestimated by up to 12% at the transition between V(c,max)- and J(max)-limited photosynthesis. This could have implications for both crop production and Earth system models, including projections of the effects of atmospheric change.