Bioethanol is gathering attention as a countermeasure to global warming and as an alternative energy for gasoline. Meanwhile, due to the synchronous increase in bioethanol production and grain prices, the food-fuel competition has become a public issue. It is necessary to see the issue objectively and to recognize that the real background is the change in allocation of limited resources such as farmland and water. In this review, we discuss which, where and how energy crops should be grown to establish a sustainable bioethanol production system. Several combinations of crops, areas and cultivation methods are recommended as a result of a survey of the bioethanol production system with various energy crops. In tropical and subtropical regions, sugarcane can be grown in agricultural and/or unused favorable lands. In other regions, cellulosic energy crops can be grown in abandoned and marginal lands, including lands contaminated with inorganic pollutant like heavy metals and some detrimental minerals. There also is the possibility that, for Japan and other Asian countries, rice can be grown as an energy crop in unused lowland paddy field. Regarding cultivation way, energy saving is beneficial for bioethanol production systems irrespective of energy efficiency. On the other hand, effective energy input should be considered for the systems with higher energy efficiency when available land area is limited. Exploring and developing new energy crops and varieties, which show higher biomass productivity and stress tolerances under marginal conditions, are necessary for sustainable bioethanol production because energy crop production would be restricted mostly to marginal areas in future.

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The objective of this study was to clarify the effects of soil temperature in the stage from late tillering to panicle initiation (SI) and during the grain-filling stage (SII) on grain setting, dry matter production, photosynthesis, non-structural carbohydrate (NSC), xylem exudation and abscisic acid (ABA) levels in rice (Oryza sativa L. cv. Koshihikari). Rice plants were exposed to four different soil temperatures during SI or SII: 17.5, 25, 31.5 and 36.5ºC (ST18, ST25, ST32 and ST37, respectively). The yield, yield components, grain filling and quality in SI were negatively influenced by high soil temperature of 37ºC. On the other hand, there was no significant difference in those characters among temperature treatments in SII. The root/shoot ratio was smallest in the ST37 plants in both SI and SII, mainly due to their lighter root weight. At 7 days after initiation of treatment (DAT) in both SI and SII, the photosynthetic and xylem exudation rate tended to increase slightly as soil temperature increased up to 32ºC. At 21 DAT, however, the photosynthetic rate was lowest in ST37, with concurrent decrease of diffusion conductance and SPAD value. In addition, decrease of NSC concentration in stem and xylem exudation rate, and increase of ABA level in leaves and xylem exudate were observed in ST37 plants at 21 DAT. These results suggested that high soil temperature before heading especially influenced yield, grain quality and plant growth. Possible mechanisms of the effect of soil temperature are discussed.

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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 s⁻¹. 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.

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We investigated the effect of drought stress on biomass productivity of newly established Erianthus ravennae (L.) Beauv. This species has recently drawn great attention as a novel cellulosic energy crop because of its excellent tolerance against various environmental stresses, but the shoot dry weight of the newly established E. ravennae was significantly decreased under drought compared to irrigated condition. A significant correlation between shoot dry weight and stem number suggested that the drought-induced decrease in stem number was ascribable to the reduced shoot dry weight in the drought condition. Decrease in soil water content was coincident with mid-day decrease in stomatal conductance, suggesting that limitation of CO₂ diffusion into leaf due to lower stomatal conductance in the drought condition caused decrease in photosynthesis followed by suppression of stem number. The present study suggested that E. ravennae was susceptible to drought, at least, in the first establishment year.

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We examined the function of CAM in reactive oxygen species (ROS) alleviation using a newly isolated CAM-deficient mutant of a facultative halophyte Mesembryanthemum crystallinum L. Salt-stress (0.4 M NaCl) induced nocturnal malate synthesis in the leaves of the wild-type plant, but not in the mutant. The content of hydrogen peroxide (H₂O₂) increased with elapse of time under salt-stress, but it dropped accompanied by the expression of CAM in the wild-type plants. The CAM-performing wild-type plant grown with 0.4 M NaCl for 12 d showed significantly larger diel fluctuation of malate and significantly lower levels of H₂O₂ than the mutant, particularly at the end of the light period. The transcript abundance of a gene encoding plastidic Cu/Zn-superoxide dismutase (SOD), a marker of ROS production, was higher in the leaves of mutant plants than in those of wild-type plants. These results indicated that the performance of CAM was accompanied by lower levels of ROS and that CAM may help to alleviate oxidative stress under conditions of environmental stress.

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In the field, plants show better root growth in drying soil than in wet soil. However, the root growth enhancement has not been demonstrated clearly in the laboratory. In this study, the root growth response of wheat seedlings to moderate soil water deficits was characterized quantitatively in an environment-controlled chamber. Germinated seeds of wheat were grown for 15 days in the soil with a water potential ranging from field capacity (FC) to approximately –0.08 MPa. The leaf area decreased with reduction in soil water potential. By contrast, the root surface area increased upon reduction of the soil water potential to –0.04 MPa while it decreased significantly in soil with a water potential of –0.08 MPa. The increase in surface area was obvious in the roots with a diameter of 0.2 to 0.4 mm and larger than 0.7 mm. Root weight increased with the reduction of soil water potential to –0.04 MPa. While specific root length decreased significantly with the reduction of water potential to –0.06 MPa, the specific root surface area did not. Assimilates transported from shoot might be used in roots to increae the surface area mainly by increasing the diameter rather than the length in response to a moderate soil water deficit in wheat seedlings. This might result from the drought tolerance mechanism of osmotic adjustment in roots.

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A greenhouse experiment was conducted to study the adaptive mechanism of cotton and peanut under water stress conditions. Five cultivars of cotton and six cultivars of peanut were grown in pots under two water levels; the control and water stress condition, where irrigation water equal to 100% and 50% of the daily transpiration, respectively, was daily applied. Peanut showed a greater increase than cotton in leaf temperature (T_L) and non-photochemical quenching (NPQ) and a greater decrease in water content per unit leaf area (WCLA), chlorophyll content and maximum quantum yield of photosystem II (PSII) (Fv/Fm) in the water stress condition. On the other hand, the water stress lowered the transpiration rate, actual quantum yield of PSII (ΔF/F’m) and leaf area (LA) more in cotton than in peanut. Cotton showed greater reduction in LA along with little reduction in the root dry weight (RDW) leading to high WCLA, while peanut showed increased RDW with little reduction in LA under the water stress condition. It was concluded that photodamage and down regulation in PSII were induced by water stress, coinciding with increases in leaf temperature regulated mainly by transpiration. Peanut showed more severe photodamage in PSII than cotton under the water stress condition.

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Spikelet number per panicle (SPP), differentiated spikelet number per panicle (D-SPP), and preflowering aborted spikelet number per panicle (A-SPP) were examined in five rice cultivars at three planting densities (HD; high, MD; medium, LD; low planting density) in the field condition. Rice plants at LD produced a higher panicle number per plant but lower panicle number per unit area, accompanied by higher D-SPP and SPP, on average. A-SPP and the ratio of A-SPP to D-SPP (%A) showed no consistent trends. There was a broader range of D-SPP values at LD than at HD because of larger D-SPP in higher order panicles (panicles with a higher spikelet number). D-SPP was smaller in lower order panicles in all cultivars and years, whereas %A increased. D-SPP and SPP of each panicle were positively correlated with tiller size (tiller height, leaf area, and neck internode diameter). Spikelet production efficiency for D-SPP or for SPP (spikelet number per leaf area) of each tiller was higher in IR65564-44-51 (NPT65) and Akihikari than in the other cultivars, indicating a greater capacity of tillers to produce spikelets or support spikelet growth. In each cultivar except NPT65, spikelet production efficiency for D-SPP increased as panicle order decreased, whereas spikelet production efficiency for SPP remained constant or decreased. This finding indicates that irrespective of planting density, lower order panicles produce more spikelets than they can afford physiologically, but they were regulated downward to a nearly constant value in four cultivars. In NPT65 different from other cultivars, spikelet production efficiency for D-SPP was lower in lower order panicles.

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Failure of fertilization in rice is a critical yield-determining factor in plants subjected to temperature or water stress at the early-reproductive stage and in high-yield cultivars bearing heavy spikelets. Although it is important to identify quickly the unfertilized spikelets for research and selection of stress-resistant or high-ripening cultivars from bulk samples, the identification takes time because unfertilized spikelets are usually determined by visual and manual procedures. Our objective was to develop a convenient method to identify unfertilized spikelets in rice. Takanari spikelets at maturity grown in the paddy field were separated into floating and sinking spikelets by different specific gravity solutions of ethanol/water mixture. The unfertilized spikelets were identified by checking the grains inside the spikelets by light penetration and examining the spikelets manually. The percentage of floating spikelets decreased with the increase in ethanol concentration, and that of floating spikelets approximately coincided with the percentage of unfertilized spikelets when the specific gravity was below 0.90×10³ kg m^-3, corresponding to over 70%-ethanol. In a practical range of temperature the specific gravity scarcely changed. In an 80%-ethanol solution, the percentages of floating spikelets in Takanari grown under different nitrogen applications and in rice cultivars having different spikelet size approximately coincided with percentages of unfertilized spikelets, though the percentages of floating spikelets was 5 to 7% higher than the unfertilized spikelets. The use of 70%-ethanol solution increased the difference in some rice cultivars. We concluded that the gravitation method would be convenient for identification of unfertilized spikelets in bulk samples of rice.

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We analyzed the relationship between growth-related traits at anthesis and protein content of grain at maturity to develop a method of estimating the nitrogen (N) application rates at anthesis required to obtain the target protein content of grain in the bread wheat (Triticum aestivum L.) cultivar Minaminokaori in three crop seasons in southwestern Japan. The protein content of grain had a higher positive correlation with leaf blade dry weight (DW) and the leaf area index (LAI)×SPAD value than with the SPAD value. Moreover, the relationships between leaf blade DW and protein content of grain and between LAI×SPAD value and protein content of grain could be approximated well using quadratic functions for each N application rate at anthesis. Thus, N application rate at anthesis required to obtain the target protein content of grain can be estimated by using these quadratic functions.

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Plant-induced solubilization of soil nitrogen (N) is a key process for plants to utilize the recalcitrant form of N. To evaluate its contribution to plant uptake, the factors affecting the contribution and the forms of N solubilized by plants, we analyzed the results of a pot experiment in which komatsuna (Brassica rapa L. var. peruviridis) was grown in 3 different soils applied with 5 types of sewage sludge as a source of N for plants. The amount of N solubilized by plants, which was defined experimentally as the difference in the amount of solubilized N between the planted and unplanted treatments, varied with the soil types more than the types of sewage sludge. This accounted for 30% (Arenosol), 15% (Fluvisol) and 1.6% (Andosol) of the amount of N uptake on average. These percentages were high when the level of soil soluble N after the experiment was below approximately 30 mg kg^-1. Sequential analyses of insoluble N in soil after the experiment indicated the occurrence of plant-induced solubilization of both bio-soluble and acid-soluble N in many of the Arenosol and Fluvisol treatments and that of acid-soluble N in the Andosol treatments. The plant-induced solubilization in the Andosol resulted in the accumulation of more labile bio-soluble N rather than enhanced plant uptake. For komatsuna grown in fertilized soil, the depletion of soluble N in the root zone seems to be important for the increase in the contribution of the plant-induced solubilization to uptake but not for the occurrence of the solubilization.

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The effects of addition of rice straw to submerged soil on the emergence and growth of rice (Oryza sativa L.) and two paddy weeds (Echinochloa oryzicola Vasing. and Echinochloa crus-galli (L.) Beauv. var. crus-galli) were investigated. Rice straw suppressed both the emergence and growth of transplanted plants depending on the amount of rice straw added (0%, 0.3%, 0.6% and 0.9% (w/w)) in the order of E. crus-galli > E. oryzicola > rice. The severe suppression of emergence and growth of E. crus-galli in the presence of 0.9% rice straw in hydroponic culture was thought to be due to high Fe content of the shoots. Since the difference in tolerance for the toxicity of rice straw is an important factor, the addition of organic materials into soil may help to suppress Echinochloa weeds selectively.

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