Panicle Temperature Research Articles

Nitrogen regulated reactive oxygen species metabolism of leaf and grain under elevated temperature during the grain-filling stage to stabilize rice substance accumulation

Rational additional nitrogen is an important agronomic measure to cope with the adverse effects of warming on rice production. However, the mechanism by which nitrogen mitigated the adverse impact of substance accumulation due to elevated temperature is poorly clarified. Therefore, in this study, a field warming experiment during grain filling and 60kg·ha-1 of additional nitrogen was established. Under elevated temperature, panicle temperature was higher and increased more substantially than leaf temperature. However, nitrogen application did not significantly reduce the leaf, panicle, and canopy temperatures. Additional nitrogen under elevated temperature delayed the decline of chlorophyll, maintained leaf photosynthesis, and prolonged grain-filling period to alleviate the decrease of starch due to warming. Hydrogen peroxide (H2O2) was sensitive to elevated temperature in leaves and grains. However, application of nitrogen under elevated temperature improved the activity of antioxidant enzymes to mitigate the increase of H2O2, resulting in a 30.31% and 45.33% decrease of H2O2 in leaves and grains compared to elevated temperature, respectively. Elevated temperature promoted the expression of heat-responsive genes, especially HSP16.9 and HSP26.7, which were consistently increased in response to warming at 5-30d after flowering. In addition, the expression of HSP16.9 at 5d and 10d after flowering and HSP26.7 at 10d after flowering was further increased with nitrogen application under elevated temperature. Therefore, HSP may be the key regulator of grain response to warming with additional nitrogen under elevated temperature. In conclusion, the relevant results revealed the physiological mechanism of nitrogen to guarantee substance accumulation and provided new ideas for cultivation measures to protect against the likely scenario of global warming.

Increased panicle nitrogen application improves rice yield by alleviating high-temperature damage during panicle initiation to anther development.

The grain yield is closely associated with spikelet fertility in rice (Oryza sativa L.) under high temperatures, and nitrogen (N) plays a crucial role in yield formation. To investigate the effect of panicle N application on yield formation under high temperatures at the panicle initiation stage, two rice varieties [Liangyoupeijiu (LYPJ, heat susceptible) and Shanyou63 (SY63, heat tolerant)] were grown and exposed to high daytime temperature (HT) and control temperature (Control) during the panicle initiation stage. Low (LPN) and high (HPN) panicle N applications were conducted. HT markedly decreased the yields by 87% at LPN and 48% at HPN in LYPJ and 31% at LPN and 36% at HPN in SY63. The decrease in grain yield under HT was primarily attributed to the decline in spikelet fertility, HPN increased spikelet fertility. HT resulted in the abnormal development of anthers, which included disordered, enlarged, and broken anther wall layers, degraded and irregularly shaped microspores, delayed tapetum degradation, less vacuolated microspores per locule, abnormal and aborted pollen grains; however, HPN improved the development of anthers under HT, particularly in LYPJ. A high rate of evapotranspiration resulted in an approximately 1°C decrease in panicle temperatures at HPN compared with that at LPN in both varieties under HT. Overall, these results demonstrate that the increased panicle N application favors normal anther development in LYPJ by decreasing the panicle temperature, which results in high pollen viability and spikelet fertility, and consequently less yield loss under HT.

Micrometeorological monitoring reveals that canopy temperature is a reliable trait for the screening of heat tolerance in rice.

Micrometeorological monitoring is not just an effective method of determining the impact of heat stress on rice, but also a reliable way of understanding how to screen for heat tolerance in rice. The aim of this study was to use micrometeorological monitoring to determine varietal differences in rice plants grown under two weather scenarios-Long-term Heat Scenario (LHS) and Normal Weather Scenario (NWS)- so as to establish reliable methods for heat tolerance screening. Experiments were conducted with two heat susceptible varieties-Mianhui 101 and IR64-and two heat tolerant varieties, Quanliangyou 681 and SDWG005. We used staggered sowing method to ensure that all varieties flower at the same time. Our results showed that heat tolerant varieties maintained lower canopy temperature compared to heat susceptible varieties, not just during the crucial flowering period of 10 am to 12 pm, but throughout the entire day and night. The higher stomatal conductance rate observed in heat tolerant varieties possibly decreased their canopy temperatures through the process of evaporative cooling during transpiration. Conversely, we found that panicle temperature cannot be used to screen for heat tolerance at night, as we observed no significant difference in the panicle temperature of heat tolerant and heat susceptible varieties at night. However, we also reported that higher panicle temperature in heat susceptible varieties decreased spikelet fertility rate, while low panicle temperature in heat tolerant varieties increased spikelet fertility rate. In conclusion, the results of this study showed that canopy temperature is probably the most reliable trait to screen for heat tolerance in rice.

Revealing the spatial characteristics of rice heat exposure in Japan through panicle temperature analysis

Revealing the spatial characteristics of rice heat exposure in Japan through panicle temperature analysis

Elevated CO2 Could Reduce Spikelet Fertility and Grain Appearance Quality of Rice(Oryza sativa L.) Grown under High-temperature Conditions

Two Japanese rice cultivars with different heat-tolerance, Hinohikari (sensitive) and Nikomaru (tolerant), were grown in pots inside open-top chambers and exposed to ambient CO2 (400 µmol mol−1) or elevated CO2 (550 µmol mol−1) from the beginning of the tillering stage to maturity. The study was conducted in Nagasaki, in the Kyushu region of Japan, where heat stress on rice has become increasingly evident. Although elevated CO2 significantly improved the net photosynthesis and whole-plant growth of the cultivars, there were no significant effects on grain yield, which in turn reduced harvest index. In both cultivars, adverse effects occurred with elevated CO2, such as reductions in spikelet fertility and grain appearance quality, which are typical manifestations of heat stress in rice. During the flowering period, the air temperature was high that spikelet fertility was reduced even under ambient CO2 conditions for both cultivars. These results suggest that, under high-temperature conditions, elevated CO2 could induce or exacerbate the manifestations of heat stress in rice. Because transpiration rate in the flag leaf was significantly reduced by the exposure to elevated CO2, it is possible that elevated CO2 increased plant temperature via a reduction in transpiration during flowering period, although we did not detect significance of the increase in leaf and panicle temperature. To ensure a more confident conclusion, further studies focusing on the effects of elevated CO2 on the determinants of spikelet fertility and grain appearance quality with other cultivars in different year are required.

Effect of drought stress on flowering characteristics in rice (Oryza sativa L.): a study using genotypes contrasting in drought tolerance and flower opening time

ABSTRACT Rice is most susceptible to heat and drought stress at flowering stage, but flowering characteristics under drought stress are not well characterized. This study investigated flowering characteristics of rice genotypes contrasting in their flower opening time (FOT) and level of drought tolerance. Near-isogenic lines for the early-morning flowering trait (IR64+ qEMF3) and for drought tolerance (IR87707-445-B-B-B), and their recurrent parent cultivar (IR64) were used. IR64+ qEMF3 had stable earlier FOT than IR64 and IR87707-445-B-B-B under drought stress conditions. Drought stress occasionally affects FOT depending on genotype. The number of open spikelets was higher in IR87707-445-B-B-B than in IR64 and IR64+ qEMF3, and the difference among genotypes increased as the rice plants were subjected to more severe stress levels. Panicle temperature increased under drought stress conditions and was similar among genotypes when it was measured at the same time of day, demonstrating that earlier FOT in IR64+ qEMF3 must be beneficial to avoid heat stress at flowering under drought stress conditions. However, IR64+ qEMF3 did not exhibit drought avoidance, as evidenced by the root mass at depth. To assess the potential for the EMF trait to complement ongoing drought breeding efforts, heat tolerance among 13 advanced drought breeding lines and released cultivars was tested. Wide variation in heat tolerance at flowering was observed and, notably, none of the 13 lines possessed the EMF trait. This study therefore proposes that a breeding strategy that transfers the EMF trait into drought tolerant lines could enhance the resilience of rice spikelets to the combined stresses of heat and drought at flowering. Abbreviations: Analysis of variance (ANOVA), dry season (DS), early-morning flowering (EMF), flower opening time (FOT), near-isogenic line (NIL), panicle water potential (PWP), quantitative trait locus (QTL), wet season (WS)

Quantitative assessment on the grain appearance of a new Japanese rice cultivar ‘Niji-no-kirameki’ with a novel heat-avoidance mechanism during ripening

There is an increased demand for developing a heat-resilient cultivar with high palatability and acceptable grain yield to cope with the impacts of progressive changes in global climate conditions. This study attempted to quantify the heat resilience level and explore the heat resilient mechanism of ‘Niji-no-kirameki’, a new Japanese cultivar with heat resilience during ripening, and compare it with the leading Japanese cultivar ‘Koshihikari’. Multi-environmental testing on grain appearance under diverse high temperature field conditions across three prefectures revealed superior performance of ‘Niji-no-kirameki’ compared to ‘Koshihikari’, especially when the average of daily mean temperatures during the first 20 days after heading was over 27 °C, a general threshold temperature previously reported to decrease grain appearance in ‘Koshihikari’. Starch properties such as amylose content and amylopectin chain length distribution were similar between cultivars across seasons. Severely chalked grains by high temperature stress had lower protein content and 13 kDa prolamin content in the grains, suggesting that inter-relationships between starch and storage protein biosynthesis affect the formation of the chalky phenotype irrespective of cultivars with different heat resilience levels. As a possible heat resilience mechanism, a novel biological response of ‘Niji-no-kirameki’ for preventing an increase in panicle temperature to rising temperatures during ripening was found. It is assumed that the erect plant phenotype until maturity concealing panicles inside the canopy could create a cooler environment for panicles of ‘Niji-no-kirameki’ during ripening. We conclude that ‘Niji-no-kirameki’ is a promising cultivar for the improvement of grain appearance in the heat-vulnerable regions of Japan.

Monitoring canopy micrometeorology in diverse climates to improve the prediction of heat-induced spikelet sterility in rice under climate change

Rice is well adapted to a wide range of climates, but is highly susceptible to heat during flowering. However, there are uncertainties in assessing the occurrence of heat-induced spikelet sterility (HISS) and the impact of climate change. One reason is the gap between the ambient air temperature and the panicle temperature, which determines the magnitude of HISS in field studies. To improve our understanding of this gap, we established a multi-site monitoring network (MINCERnet) to measure canopy micrometeorology and heat stress in the major rice growing regions (Sub-Saharan Africa; South, Southeast, and East Asia; and the USA). MINCERnet assessed the processes that determine panicle temperature and the resulting HISS in open fields using the same cultivars (‘IR64’, ‘N22’, and ‘IR52’) and a standard system (MINCER) for micrometeorological monitoring under diverse climates. By using the MINCERnet data in the canopy heat-balance model (IM2PACT), we confirmed that the canopy and panicle transpiration and the resulting evaporative cooling strongly affected the gap between the ambient air temperature and the panicle temperature, and that the HISS rate in open fields could be predicted accurately in diverse climates by using the mean panicle temperature during the flowering hours. The “oasis effect” in the broad sense, that is, evaporative cooling and the increase of relative humidity, which is nested at the various levels along the continuum from the landscape to the panicle, formed temperature and relative humidity gradients along the continuum in response to different climatic conditions.The heat-balance characteristics (i.e., a stronger evaporative cooling under drier climate conditions) suggested that the risk of HISS caused by global warming will increase more in wetter climates, where panicle temperatures tended to increase. Thus, accurate relative humidity data as well as air temperature will be required, along with spatial downscaling, to permit accurate prediction of rice heat stress and yield. HISS prediction using an approach based on the panicle temperature as input for models and monitoring of canopy micrometeorology will reduce uncertainties in rice yield prediction and the response of yield to various climate change adaptation measures.

Use of infrared thermography imaging for assessing heat tolerance in high and low iron pearl millet lines

In the arid regions of Asia and Africa, pearl millet serves as a staple source of dietary energy and mineral micronutrients for millions of people. These regions are more vulnerable to increased temperature. The availability of rapid and efficient screening tools based on the relevant non-destructive quantifiable traits would facilitate pearl millet improvement for heat tolerance. The objective of this study was to evaluate pearl millet lines with contrast micronutrients for heat tolerance using infrared thermal imaging, a rapid proxy-canopy (panicle and flag leaf) temperature measurement. Results showed the highly significant genotypic differences between high-Fe and low-Fe genotypes for grain Fe and Zn densities and agronomic traits. Both high-Fe and low-Fe group genotypes differed significantly for panicle temperature depression (PTD) during high- vapor deficit (VPD) at stigma stage (3.0 to 6.73°C). PTD values were positive across all genotypes during stigma stage and were very low or negative during the low-VPD. Cooler canopy temperature (high-PTD) was observed during stigma stage rather than seed-set stage at higher-VPD in both high-Fe and low-Fe genotypes. The cooler temperature achieved by panicle might be helpful in maintaining stigma receptivity for longer periods in the female parents, whereas in male parents it might be helpful in maintaining pollen viability for longer periods. Flag leaf temperature (FTD) was cooler than PTD at both high-VPD and low-VPD as well in both stigma (less by 2.1°C) and grain-filling stage (less by 2.7°C), again signifying that the reproductive parts are more prone to heat stress as compared to vegetative parts. Since, thermal imaging discriminates the heat stress and non-stress canopies, this can serve as a proxy canopy temperature tool for heat stress tolerance screening in pearl millet.

Estimating Rice Panicle Temperature with Three-Layer Model

Rice panicle temperature (Tp) is a key factor for studying high temperature impacts on spikelet sterility. Comparing with measuring Tp by hand, a Tp simulation model could obtain Tp data readily. The two-layer energy budget model which divides the soil layer and canopy layer was widely used to predict rice canopy temperature (Tc), but panicle existed mostly in the upper layer canopy, and we have proved that Tc was different from the upper layer canopy temperature (Tc1), and the upper layer must be separated from the whole canopy for the purpose of estimating Tp. Thus, we developed the three-layer model, contained upper canopy layer with panicle (50–100 cm), lower rice canopy layer (10–40 cm), and water surface layer (≤10 cm) to estimate Tp with general meteorological and vegetation growth data. There were two steps to estimate Tp. The first step was calculating Tc1 and lower layer canopy temperature (Tc2) by solving heat balance equations with canopy resistances. And the second step was estimating Tp with following parameters: (a) the inclination factors of leaves and panicles (F1, F2, and Fp) which were decided by fitting the calculated transmissivity of downward solar radiation (TDSR) to the measured TDSR, (b) the aerodynamic resistance between the panicle and atmosphere (rap) denoted by wind speed, (c) the panicle resistance for transpiration (rp) denoted by days after heading, and (d) air temperature and humidity at the panicle’s height (Tac1 and eac1) calculated from the resistances of the pathways of sensible and latent heat fluxes in accordance with Ohm’s law. The model simulated fairly well the Tc1, Tc2, and Tp with root mean square errors (RMSEs) of 0.76°C, 0.75°C, and 0.81°C, respectively, where RMSE of measured Tp and predicted Tp by integrated micrometeorology model for panicle and canopy temperature (IM2PACT) including two-layer model was 1.27°C. This model was validated well by two other rice cultivars, and thus, it demonstrated the three-layer model was a new feasible way to estimate Tp.

Physiological Characterization of Diverse Landraces of Rice to Identify Donors for High Temperature Stress Tolerance Based on First Spikelet Opening Time

Rice is susceptible to heat stress at flowering, where temperatures above 33 °C during anthesis causes spikelet sterility. Heat-induced spikelet sterility at flowering is associated with reduction in grain yield. So far only N22 has been used as universal donor for heat stress. One of the major traits employed in N22 in mitigating the impact of high temperature is the early morning flowering (EMF). Hence, the present study was designed to identify novel donors from the indigenous landraces of rice available at TNAU. The physiological basis of flower opening time (FOT) showed early flowering lines had higher transpiration rate coupled with low photosynthetic rate and stomatal conductance during anthesis. The transpiration rate also led to a reduction in panicle temperatures in these lines. It was concluded that at the time of anthesis, the panicle temperatures were 24 °C and less which led to advanced FOT. The results revealed that among the 21 lines, seven landraces, namely Thatan samba, Panamara samba, Thooyamali, Norungan, Mattikar, Sornavari and Seakar, flowering (9.30 to 10.00 IST). Hence, the lines Thatan samba, Panamara samba which flowered before 9.40 IST can be used as potential donors of EMF trait to develop heat-tolerant varieties.

Effects of free-air CO2 enrichment on flower opening time in rice

ABSTRACTFlower opening time (FOT) is important for reproductive success in higher plants. Rice plants exhibit phenotypic plasticity in FOT in response to environmental factors such as temperature. However, the effect of the concentration of atmospheric CO2 ([CO2]) on FOT has not yet been reported. Elevated [CO2] (E-[CO2]) increases the temperature of panicles, which may in turn advance FOT. We investigated the effect of E-[CO2] on FOT in rice using a free-air CO2 enrichment facility where we increased [CO2] by about 200 μmol mol−1 above the ambient level (A-[CO2]). By photographing panicles at 10-min intervals, we determined 10%FOT and 50%FOT (the time of the day in Japan Standard Time when 10% and 50% of the flowers had opened, respectively). E-[CO2] advanced 10%FOT and 50%FOT by 4 and 5 min at the 10% and 5% levels of significance, respectively. Daily mean air temperature (Ta), solar radiation, and vapor-pressure deficit were negatively correlated with 50%FOT. Regression line slopes for Ta versus 10%FOT and 50%FOT were slightly steeper for A-[CO2] than those for E-[CO2]. Our results suggest that the most probable reason why E-[CO2] advanced FOT is an increase in panicle temperature arising from a reduction in leaf stomatal conductance.Abbreviations: 10%FOT: the time of the day in Japan Standard Time when 10% of the flowers had opened; 50%FOT: the time of the day in Japan Standard Time when 50% of the flowers had opened; A-[CO2]: ambient CO2; [CO2]: the concentration of atmospheric carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; FOT: flower opening time; JST: Japan Standard Time; RH: relative humidity; Rs: solar radiation; Ta: air temperature; Tp: panicle temperature; VPD: vapor-pressure deficit

Effects of free-air CO2 enrichment on heat-induced sterility and pollination in rice

ABSTRACTGlobal climate changes may cause heat-induced sterility in rice, threatening the global production of this important crop. Although little is currently known about the combined effects of the concentration of atmospheric CO2 ([CO2]) and temperature on heat-induced sterility, elevated [CO2] (E-[CO2]) will likely increase the panicle temperature and thereby exacerbate heat-induced sterility, but this was not tested in open fields. Therefore, we investigated the effect of E-[CO2] on heat-induced sterility and sterility-related traits in rice by increasing E-[CO2] by approximately 200 μmol mol−1 above ambient levels using a free-air CO2 enrichment (FACE) facility for six growing seasons with variable growing season temperatures. The percentage fertility was not significantly correlated with the air temperature (Ta) between 09:00 and 12:00 on each flowering day, but it did significantly vary among the years, with 2011 experiencing cool temperatures resulting in chilling-induced mild sterility. When data from 2011 were removed, there was a significant negative correlation between fertility and Ta between 09:00 and 12:00 on each flowering day under E-[CO2], whereas no such effect was seen under ambient [CO2]. E-[CO2] also significantly reduced the number of pollen grains deposited on the stigma by 10%, but it slightly increased the anther length by 1.3%, indicating that it had both negative and positive effects on heat-induced sterility. These findings suggest that E-[CO2] affects many traits related to heat-induced sterility and may sometimes exacerbate sterility by reducing pollen grain deposition.Abbreviations: A-[CO2]: ambient CO2; [CO2]: the concentration of carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; Ta: air temperature

数種のインド型イネにおける開花期高温障害抵抗性に関する葯裂開，受粉，花粉発芽の寄与評価

数種のインド型イネにおける開花期高温障害抵抗性に関する葯裂開，受粉，花粉発芽の寄与評価

Development of a new heat tolerance assay system for rice spikelet sterility

Background Reduction in rice yield caused by high temperature-induced spikelet sterility has been a serious concern in rice production. To date, several screening methods have been used, although their reproducibility is sometimes poor due to artifacts mainly caused by varietal differences in heading dates and panicle heights (i.e., the distance from the lamps).MethodsWe have developed a novel assay system for heat-induced spikelet sterility by using artificial rice paddies in phytotrons to conduct a highly reproducible assay throughout a year. Plants restricted to the main culm were treated under a series of heat conditions, and height uniformity of each plant was ensured by using height-adjustable pots.ResultsResults suggested that a 3-day heat treatment of 35 °C-day/29 °C-night cycles was the most suitable condition. Under the treatment, two distinct groups were identified among nine heat tolerant cultivars, with no varietal difference in panicle temperature, indicating that the system is capable of eliminating the varietal difference in panicle temperature.ConclusionsIt is concluded that the assay system would be a powerful tool for selecting heat tolerant varieties, as well as the analysis of genetic factors from various cultivars, eliminating potential artifacts.

Responses of spikelet fertility to air, spikelet, and panicle temperatures and vapor pressure deficit in rice

High temperature-induced spikelet sterility is expected to become a major factor reducing rice yield under the future climate conditions. To examine the responses of spikelet sterility to air temperature, humidity, and temperatures of panicle and spikelet, two japonica rice cultivars in different maturity groups were exposed to four sets of different temperature conditions from initial heading stage in 2013 and 2014. The temperature conditions included ambient temperature (AT), AT + 1.5°C, AT + 3.0°C, and AT + 5.0°C. Spikelet fertility showed a wide range of variation from 100 to 4.6% depending on temperature treatments. The ridge regression revealed that not only air temperature but also vapor pressure deficit (VPD) was negatively associated with spikelet fertility. The spikelet fertility was well fitted to logistic equations not only of air temperature, spikelet internal temperature, and panicle surface temperature but also of VPD. No clear difference in the accuracy of sterility estimation was observed for models that use air temperature, panicle or spikelet temperature as inputs. In contrast, the logistic equation model that uses both air temperature and VPD as independent variables had better accuracy in predicting spikelet sterility. These results suggested that spikelet or panicle temperature would be no better predictor for high temperature-induced spikelet sterility than air temperature. Therefore, further study is merited to verify the VPD effects on spikelet sterility under high temperature conditions.

Capturing heat stress induced variability in spikelet sterility using panicle, leaf and air temperature under field conditions

Heat stress induced increase in spikelet sterility is often correlated with prevailing air temperature during anthesis. However, plant tissue temperature can be different from air temperature, depending on transpiration cooling which is a function of relative humidity (RH). Controlled environment studies involving heat stress are generally conducted at targeted RH, hence excluding the dynamics associated with this factor. Using 81 diverse indica cultivars over three years (wet season-2012, dry season-2013 and 2014), flag leaf temperature (FLT), panicle temperature (PT) and air temperature (AT) were measured during flowering under field conditions. On average the FLT and PT were 10.4 and 8.7°C lower than AT during the wet season and were 9.9 and 8.0°C lower when averaged across all three seasons. Spikelet sterility in dry seasons ranged between 1.6 and 67% compared to <1 and 35.5% during the wet season. Spikelet sterility was significantly correlated with PT (r=0.50; p<0.01) and FLT (r=0.55; p<0.01), compared with a weaker relationship with AT (r=0.23; p<0.05) among the tested 81 cultivars, pooled across both 2013 and 2014 dry seasons. By employing an unbiased Z score approach, cultivars with (i) consistent and contrasting for sterility percentage and (ii) phenology (duration from seeding till 50% flowering) based on flowering date confirmed the strong relationship between FLT or PT with spikelet sterility compared to AT. In conclusion, we recommend using either PT or FLT (tissue temperatures) instead of AT for future field based heat stress phenotyping. Where direct PT measurements are not available, mechanistic and empirical models can be used to derive PT or FLT from weather station data, or remotely sensed canopy temperature might be used as a proxy in large scale phenotyping experiments.

EFFECTS OF WATER PONDING ON DECREASING LEAF AND PANICLE TEMPERATURE IN RICE PADDY FIELDS

Several studies have suggested the spikelet fertility would be significantly damaged if the air temperature (Ta) was high at heading and flowering stage. In this study, we evaluated the effect of water ponding in two paddy fields to decrease leaf temperature (Tl) and panicle temperature (Tp) during the 2014 growing season. Within the first conventionally water managed paddy field (cultivar Akitakomachi), we set 1 m × 1 m experiment plot (Plot A1) from July 8th to August 24th, and water was put in 15 cm depth in the morning at 8:30. For expecting larger difference of leaf and panicle temperature between in and outside the plot, the plot was expended to 2 m × 2 m (Plot A2) from August 25th to September 8th, 2014, and water was put in 15 cm depth at noon. This method was also used in the plot B (2 m ×2 m) which was installed in another conventionally water managed field (cultivar Nikomaru) from September 9th to 30th, 2014. Tl and Tp were measured every two or three hours during daytime in every 10 cm canopy layer in and outside plots. In the first experimental paddy field, at largest, Tl and Tp in the plot were 4.3 ℃, 5.5 ℃ lower than Tl and Tp outside the plot, respectively. Tp was 6.6 ℃ lower than Ta under low relative humidity condition. In the second experimental paddy field, Tl and Tp in the plot were 3.6 ℃, 3.4 ℃ lower than Tl and Tp outside the plot, respectively. It revealed water ponding was a useful method to decrease leaf and panicle temperature under larger solar radiation, higher air temperature and lower relative humidity conditions at heading and flowering stage.

Quantifying rice spikelet sterility in potential heat-vulnerable regions: Field surveys in Laos and southern India

Global warming is projected to cause yield losses due to heat-induced spikelet sterility (HISS) in hot, vulnerable rice-growing regions, but reports documenting HISS at a regional level in the tropics are very limited. As a case study, HISS at flowering stage was surveyed under field conditions using local popular varieties in Savannakhet, Laos, and Tamil Nadu, India, during the dry season. At the field site in Savannakhet, sterility in variety Thadokkham1 (TDK1) was 10.8% when the maximum average temperature at heading was 32.9°C, which increased to 23.3% at 37.3°C. When plots were covered with a shading net, solar radiation was reduced by 60% at heading, which significantly reduced sterility to 10.0%. Panicle temperature estimates produced with a micrometeorology model revealed that shading during the hours when flowering occurs decreased panicle temperature by 2.2–2.6°C. At the field sites in Tamil Nadu, sterility of variety Coimbatore 51 (CO51) was highly correlated with the maximum temperature at heading; sterility was around 5–10% when maximum average temperature was below 36.5°C, which increased to 33.4% at 38.0°C. Marking of opened spikelets at hourly intervals allowed us to detect increases in sterility as temperature increased from early morning until noon. We conclude that HISS at flowering stage can occur in local popular varieties grown in Savannakhet and Tamil Nadu during the hot, dry season. Effective protocols such as using a shading net and hourly marking of open spikelets are useful approaches to quantifying HISS during flowering stage across hot and potential heat-vulnerable rice-growing regions.

Lower-Than-Expected Floret Sterility of Rice under Extremely Hot Conditions in a Flood-Irrigated Field in New South Wales, Australia

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.