Kazuhiro Kobayasi

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.

Correlation between viability of pollination and length of basal dehiscence of the theca in rice under a hot-and-humid condition

Abstract Anticipated global warming may increase the floret sterility of rice (Oryza sativa L.). For selection of genotypes tolerant to high temperatures during the flowering period, it is important to identify morphological traits associated with tolerance to temperature stress. This study investigated the relationship between the length of dehiscence at the basal part of thecae and the viability of pollination in 18 cultivars of rice subjected to a hot-and-humid condition (37/25 ˚C, day/night, >90% relative humidity) for three days at flowering. Control plants were left under the ambient conditions in a semi-cylindrical house covered with cheesecloth (30% shading; temperature range: 24-35 ˚C). The length of basal dehiscence of thecae and the number of pollen grains on the stigmata were examined with a light microscope after flowering. The length strongly correlated with the percentage of florets having more than 80 pollen grains on the stigmata under the ambient condition (r = 0.72, P < 0.001), and with the percentage of florets having more than 20 pollen grains on the stigmata under the hot condition (r = 0.93, P < 0.001). In other words, the length correlated with pollination viability or reliability under both conditions. In addition, basal dehiscence was shorter in the non-japonica-type cultivars than in many of the japonica-type cultivars under both conditions. We concluded that the low pollination viability in the non-japonica-type cultivars is associated with their small basal dehiscence on the thecae, and the length of basal dehiscence can be used as a selection marker of high temperature tolerance.

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.

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.

Relationship between Apical Dome Diameter at Panicle Initiation and the Size of Panicle Components in Rice Grown under Different Nitrogen Conditions during the Vegetative Stage

Summary A pot experiment was conducted to analyze the relationship between the size of each apical dome (AD) and the numbers of differentiated primary rachis-branches (PBs) and spikelets. Two rice cultivars were used : one was a heavy-panicle type ‘Akenohoshi’ and the other was a many-tillering type ‘Nipponbare’. Rice plants were applied nitrogen (N) at various rates (75-600mg N pot–1 week–1) during the vegetative stage. The base diameter and the height of ADs were measured at the panicle initiation (PI) stage. At heading, the numbers of differentiated PBs, secondary rachis-branches (SBs), and spikelets were counted. The N treatment increased shoot N concentration in both cultivars. The N treatment increased the base diameter of AD in Akenohoshi at the PI stage, but not in Nipponbare. The ADs in Akenohoshi had a base diameter about 6.2% larger on average than that in Nipponbare. The N treatment did not affect AD height. In Akenohoshi, the plants with a larger AD base diameter differentiated more PBs per panicle and then differentiated more SBs and spikelets than did those plants with a smaller AD diameter. However, in Nipponbare, the N treatments did not affect the number of differentiated PBs per panicle. These results suggest that the AD size at the PI stage, which is enlarged by higher N nutrition in the vegetative stage, is a determinant of the number of differentiated PBs and spikelets and that a larger AD size is responsible for a higher number of PBs and spikelets in Akenohoshi.

Effects of Non-Structural Carbohydrates on Spikelet Differentiation in Rice

Summary Spikelet number per unit area is a strong determinant of rice yield, and spikelet differentiation must be promoted to increase spikelet number. Nitrogen has been considered to be the most critical factor in promoting spikelet differentiation and the role of non-structural carbohydrates (NSCs) in spikelet differentiation is unclear. To reveal the relation between NSCs and the number of differentiated spikelets, we conducted a field experiment in Matsue, Shimane, Japan, using two japonica cultivars Koshihikari and Nipponbare. The NSC content was changed by shading and thinning during the early reproductive stage. These treatments did not change the amount of nitrogen per hill in the leaves and stems. The number of differentiated spikelets, which was defined as the sum of the numbers of surviving spikelets and vestiges of degenerated spikelets, was not influenced by NSC content in rice plants; neither was the number of primary rachis-branches, which is considered to determine spikelet number per panicle. The ratio of dry matter allocation to leaves and the rest of the plants was changed, but the morphological characteristics such as tiller number and plant length were not influenced by the treatments. It is concluded that the differentiation of spikelets was not influenced by the NSC content within the range examined in the present experiment. The role of NSC in the mechanism of spikelet differentiation is discussed.

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.

Sterility and Poor Pollination Due to Early Flower Opening Induced by Methyl Jasmonate

Abstract Flower opening in the early morning helps avoid sterility induced by heat stress at anthesis in rice (Oryza sativa L.). A pot experiment was conducted to reveal the effects of methyl jasmonate (MeJA) on flower opening time (FOT), sterility, pollination, and anther dehiscence. Four mmol L−1 MeJA solution (3 mL per panicle) and water, as a control, was applied to the panicles of the japonica type rice cultivar‘Hinohikari’ at 0900, 1000, or 1100. By photographing flowers at 4-min intervals, we determined the FOT. Flower sterility, pollination, and anther dehiscence were also examined. Application of MeJA solution at 0900 advanced FOT about 2 hr compared with that without application, and 1.5 hr compared with that after application of water. MeJA application at 1000 also advanced FOT, but that at 1100 did not. Application of MeJA solution significantly increased the numbers offlowers opening on the day of treatmentand decreased that on the day after treatment. Application of MeJA solution at 0900 significantly increased flower sterility compared with that without treatment. More than 40% of the flowers that were treated with MeJA and opened on the day oftreatment were those expected to open on the next day. These results suggest that sterility induced by application of MeJAis caused by the early opening of flowers expected to open the day after treatment.

Relationship between the size of the apical dome at the panicle initiation and the panicle components in rice

Abstract Breeding for high-yielding rice has been directed toward those types that have a larger number of spikelets per panicle. The objective of this study was to examine the relationship within a cultivar between the size of the apical dome (AD) and the morphological characters of the panicle components such as the number of primary rachis-branches (PBs), secondary rachis-branches (SBs), and spikelets per panicle. Rice plants (cv. Akenohoshi) were subjected to four short-day (lOh photoperiod) treatments at various developmental stages to change the duration of the vegetative stage and the AD size at panicle initiation (PI). Shoot apices having ADs were sampled at two-day intervals from the late vegetative stage to the early reproductive stage. The sections of the shoot apices were embedded in paraffin and stained with toluidine blue O. The base diameter and height of the ADs were measured with an ocular micrometer, and at heading, the numbers of PBs, SBs, and spikelets were counted. A delay in the start of the short-day treatment increased the total number of leaves on the main culm, extending the vegetative stage. Extension of the vegetative stage increased the AD diameter at PI. However, the effect of the short-day treatments on the AD height was unclear. Enlargement of ADs was accompanied by the increase in the number of differentiated PBs, but not always by that in the number of SBs and spikelets. The results suggested that spikelet number is influenced by plant nutritional conditions as well as by the AD size at PI.

Molecular Cloning, Functional Expression, and Tissue Distribution of a Potato Sprout Allene Oxide Synthase Involved in a 9-Lipoxygenase Pathway

Potato (Solanum tuberosum) plants are rich in 9-lipoxygenase, which converts linoleic acid and α-linolenic acid to 9S-hydroperoxy-10E,12Z-octadecadienoic acid (9-HPOD) and 9S-hydroperoxy-10E,12Z,15Z-octadecatrienoic acid (9-HPOT) respectively. The allene oxide synthase (AOS) involved in 9-HPOD/9-HPOT metabolism in potato, however, has not been characterized in detail. We cloned a cDNA encoding a novel AOS from potato sprouts by reverse transcriptase-PCR based on a partial sequence in the EST database. This AOS was successfully expressed in the yeast Pichia pastoris, and purified using Ni-NTA resin. The recombinant enzyme metabolized 9-HPOD, 9-HPOT, 13-HPOD, and 13-HPOT with reaction efficiencies of 2.5×107, 1.0×107, 2.5×106, and 7.6×106 M−1 s−1 respectively. The α-ketol formed from 9-HPOD was composed mainly of the 9R-enatimomer (90%). Besides sprouts, the mRNA of this AOS was detected in buds, flowers, and stems, but not in leaves, tubers, or roots of mature plants, suggesting that this enzyme has a tissue-specific function.