Mayumi Yoshimoto

Rice cultivar responses to elevated CO2 at two free-air CO2 enrichment (FACE) sites in Japan

There is some evidence that rice cultivars respond differently to elevated CO2 concentrations ([CO2]), but [CO2]×cultivar interaction has never been tested under open-field conditions across different sites. Here, we report on trials conducted at free-air CO2 enrichment (FACE) facilities at two sites in Japan, Shizukuishi (2007 and 2008) and Tsukuba (2010). The average growing-season air temperature was more than 5°C warmer at Tsukuba than at Shizukuishi. For four cultivars tested at both sites, the [CO2]×cultivar interaction was significant for brown rice yield, but there was no significant interaction with site-year. Higher-yielding cultivars with a large sink size showed a greater [CO2] response. The Tsukuba FACE experiment, which included eight cultivars, revealed a wider range of yield enhancement (3-36%) than the multi-site experiment. All of the tested yield components contributed to this enhancement, but there was a highly significant [CO2]×cultivar interaction for percentage of ripened spikelets. These results suggest that a large sink is a prerequisite for higher productivity under elevated [CO2], but that improving carbon allocation by increasing grain setting may also be a practical way of increasing the yield response to elevated [CO2].

Stability of Rice Pollination in The Field Under Hot And Dry Conditions in The Riverina Region of New South Wales, Australia

Abstract Even Under Extremely Hot (40ºC) Conditions During Anthesis, Heat-induced Floret Sterility Does Not Appear To Be A Serious Issue For Australian Rice Growers. This Contradicts Previously Reported Temperature Thresholds For Floret Sterility. To Determine The Factors Associated With Stable Rice Production Under Hot and Dry Conditions in The Riverina Region of New South Wales (Australia), We Examined Rice (Cv. ‘Langi’) Pollination At Different Distances From The Windward Edge of A Paddy Field and Its Association With Canopy Microclimate. With An Air Temperature of 34.5ºC and A Relative Humidity of 20.7% During Anthesis, Poor Pollination of Florets Occurred At The Windward Edge, But Pollination Remained Stable Farther From The Edge. The Temperature Difference Between The Air and The Panicles in The Canopy Reached As High As 6.8ºC Under These Conditions Because of Low Humidity and Strong Transpirational Cooling. Moreover, The Length of The Dehiscence At The Base of The Thecae During Anthesis Was Long; This Is A Desirable Trait For Heat Tolerance. The Long Basal Dehiscence of The Thecae of This Cultivar and The Lower Panicle Temperatures Relative To The Ambient Temperature Caused By High Transpirational Cooling Appear To Be The Key Factors Responsible For Stable Pollination Under The Extremely High Temperatures of The Riverina Region.

Heat-induced floret sterility of hybrid rice (Oryza sativa L.) cultivars under humid and low wind conditions in the field of Jianghan basin, China.

Abstract Projected global warming is expected to increase the occurrence of heat-induced floret sterility (HIFS) in rice. However, there are few field-scale studies that could aid in predicting the potential risks to rice yield and developing countermeasures against yield losses. The aim of this study was to elucidate the factors that induce floret sterility under high temperature conditions during the flowering season in the field condition in China. Studies were conducted in irrigated paddy fields with the regional hybrid-rice cultivars grown in Jianghan Basin where air temperature is not so high during the flowering season but HIFS frequently occurs. The microclimate, panicle temperature, floret sterility, pollination, and size of dehiscence formed at the base of anthers were investigated. Significant losses in seed set were observed under the high temperature condition. Although the maximum atmospheric temperature was approximately 35ºC, the relative humidity was very high (around 70% at the time of maximum temperature), with low wind speeds, occasionally below 1 m s1. Under such conditions sunlit panicle temperature exceeded atmospheric temperature by as much as 4ºC. Moreover, the anthers of some cultivars exhibited short basal dehiscence, and the dehiscence length was positively correlated with the percentage of sufficiently pollinated florets (r=0.859, P<0.05, n=7) and with seed set (r=0.827, P<0.05, n=7) across the cultivars. The results suggest that the combination of hot, humid, and windless climatic conditions with short basal dehiscence of anthers induces HIFS in hybrid rice grown in this region.

An alternative approach to determining zero-plane displacement, and its application to a lotus paddy field

Abstract We propose an alternative approach to determining zero-plane displacement by using friction velocities obtained by measuring wind profile and eddy correlation. In this approach, zero-plane displacement is determined so that the slope of the regression line between the friction velocities obtained from the wind profile and eddy correlation is close to 1. The approach was applied to a lotus paddy field in the back marsh of Lake Kasumigaura, Japan, and yielded acceptable results. The values determined for the normalized displacement height (0.13–0.91) and the normalized roughness length (0.023–0.092) of the lotus canopy were close to those of hard-canopy stands, such as forests and cornfields. These results are attributed to the unique structure of the lotus canopy (large, round leaves on the top of hard, thick shoots). In addition, the normalized displacement height of the lotus canopy increased linearly with the leaf area index (LAI). Because the number of comprehensive observation sites for flux measurements and micrometeorology has increased in recent years, this approach could be applicable to other study sites where there are problems in determining zero-plane displacement by the conventional method, which uses wind profile data alone.

Effects of Temperature, Solar Radiation, and Vapor-Pressure Deficit on Flower Opening Time in Rice

Abstract Flower opening in the early morning helps to avoid sterility of rice (Oryza sativa L.) caused by heat stress at anthesis. Although flower opening time (FOT) is under genetic control, it is also affected by weather, particularly by air temperature (Ta). However, the effects of Ta, solar radiation (Rs), and vapor-pressure deficit (VPD) on rice FOT are unclear, making it difficult to predict FOT. Therefore, we investigated the correlation of FOT with Ta, Rs, and VPD during various periods before anthesis under field conditions. By photographing spikelets at 10-min intervals, we determined the FOT of five cultivars. To evaluate the individual effects of cultivar, Ta, Rs, and VPD on FOT, we constructed general linear models (GLMs) and calculated mean Ta, R s, and VPD every 3 hr from 0000 to 1200. The GLMs revealed that the average Ta, Rs, and VPD between 0600 and 0900 significantly affected FOT (adjusted R 2 =0.399; P <0.001). The standardized partial regression coefficients of Ta and Rs were negative and those of VPD were positive, indicating that higher Ta, higher Rs, and lower VPD in the early morning result in earlier FOT. Moreover, multiple-regression analysis showed that the period affecting FOT the most, and the relative contributions of Ta, Rs, and VPD to FOT differ with the cultivar.

The effects of free-air CO2 enrichment (FACE) on carbon and nitrogen accumulation in grains of rice (Oryza sativa L.)

Rising atmospheric CO2 concentrations will probably increase rice (Oryza sativa L.) yield but decrease grain nitrogen (GN) concentration. Grains attached to different positions in the panicles differ greatly in weight and quality, but their responses to elevated CO2 (e[CO2]) are poorly understood, which limits our understanding of the mechanisms of yield enhancement and quality degradation. Thus a free-air CO2 enrichment experiment was conducted to examine the effects of e[CO2] on grain mass (GM), grain carbon (GC), and GN accumulation in the spikelets attached to the upper primary rachis branch (superior spikelets; SS) and those attached to the lower secondary rachis (inferior spikelets; IS). e[CO2] stimulated the rice yield by 13% but decreased the N concentration in the panicle by 7% when averaged over two levels of N fertilizations (P < 0.01). The responses of SS and IS to e[CO2] were different particularly under higher N supply. For SS, e[CO2] decreased GN by 24% (P < 0.01) but did not affect GM. For IS, e[CO2] increased GM by 13% (P < 0.05) but GN was not affected. The reduction of GN due to e[CO2] started to appear at the beginning of grain filling. These results suggest that future [CO2] levels probably stimulate the grain growth of IS, most of which are not marketable due to limited size, at the expense of GN reduction in SS. Translocation of N from SS to IS may be a possible mechanism for reduction in GN of SS. This may degrade the grain quality of marketable rice under e[CO2].

Lower-than-expected floret sterility of rice under extremely hot conditions in a flood-irrigated field in New South Wales, Australia.

Abstract Rice florets are susceptible to high-temperature damage at anthesis, but rice production remains stable in the Riverina region of Australia even when the air temperature during flowering exceeds 40°C. To identify the mechanism that supports rice production under these conditions, we examined sterility and pollination in relation to microclimate and panicle temperature in an extremely hot paddy field in the Riverina region of New South Wales. In windy > 40°C weather, the panicle temperature was > 38°C at the windward edge of the crop but around 35°C inside the crop, probably because of strong transpirational cooling due to the extremely dry wind (15% RH). Pollen from the windward edge of the crop showed extremely poor germination, yet that from inside the crop showed sufficient germination for fertilization. Moreover, sterility inside the crop was significantly lower than that at windward edge. We concluded that the wind with large vapor pressure deficit enabled stable rice production under the extreme heat during flowering.

Varietal range in transpiration conductance of flowering rice panicle and its impact on panicle temperature.

Abstract Transpiration from rice (Oryza sativa L.) panicles can help lower the temperature of the panicle ( T p ), which is the susceptive organ for high temperature-induced spikelet sterility (HISS). By increasing the transpiration, the heat damage to the panicle predicted to occur due to global warming may be avoided. To examine the possibility of genetic improvement in transpiration conductance of intact rice panicles (gpI ), we measured gpI at the time of flowering in the open field in 21 rice varieties of widely different origins. We observed a difference in gpI among the varieties and three series of experiments, ranging from 0.15 to 0.67 cm s- 1 . We also estimated its impact on the difference between Tp and air temperature ( T a ) (Δtp, Tp - Ta) using a micrometeorology model, where T a was given as 28°C or 35°C. The varietal range in gpI was estimated to correspond to the range of 2.1°C in ΔTp under a humid atmospheric condition and the range of 3.5°C in Δ t p under a dry atmospheric condition. The estimated ΔTp ranges due to varieties may be useful for improving heat avoidance capacity under excessive heat at the critical stage. The sensitivity analysis of ΔTp to ranging gpI suggested that g pI higher than the highest gpI observed in this study may not be effective for additional cooling of T p . Thus, the target of improvement in gpI against HISS should be set at the level of the existing varieties with the highest gpI.

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.

Projection of Effects of Climate Change on Rice Yield and Keys to Reduce Its Uncertainties

The increase in atmospheric CO2 concentration and accompanying global warming should affect crop productivity. A number of experiments and simulations have been conducted to predict the impacts of climate change on rice yield. When conducting large-scale evaluation of rice yield, there are large uncertainties, which resulted from a number of sources, such as those in the greenhouse gas (GHG) emission scenarios, global climate models (GCMs) and its gaps between global and local climates. In addition, the rice development models themselves include uncertainties. In this paper, we present our recent studies on large-scale evaluation by crop models and trials to elucidate and reduce uncertainties accompanied with each aspect of evaluation. In modeling technique aspect, statistical approach for model parameters and the use of multi-scenarios and multi-GCMs are reviewed. In field experiment aspect, we present a field survey on spikelet sterility in the hot summer of 2007 and some insights from free-air CO2 enrichment (FACE) experiment. They strongly suggest the necessity for developing a process-based rice development model including heat balance. The synthesized process-based model study in tandem with FACE experiments contributes not only for reducing the evaluation uncertainties, but also for validating the adapting or avoiding studies of heat stress or negative influence on rice under projected climate change.