Rising air temperatures have the capacity to impact rice yields in future climates. Studies in large temperaturecontrolled field chambers were established to examine the responses of four contrasting rice genotypes to elevated daytime temperatures (ET) during reproductive development under paddy conditions. Field chambers were effective in raising mean above-canopy maximum daytime temperatures from 29.9 to 41.1 degrees C during 12 d of ET treatment (68-80 d after emergence, DAE), while increased transpiration under ET resulted in lowering of mean lower-canopy maximum temperature to 33.2 degrees C. Nevertheless, the earliest genotype Vandana encountered a hot spell of 37.0 degrees C at 68-74 DAE in the lower canopy at its late reproductive stage, which exceeded the spikelet sterility threshold of 33.7 degrees C, so its spikelet fertility, grain number and grain yield were reduced under ET. Genotypes differed in the extent of canopy cooling, with less reduction in Vandana and IR64 than in N22 and Takanari. For canopy cooling to be effective, stratification of air layers must occur within the canopy, which was more effective under the shorter and denser canopy of N22 and Takanari (plant height of 70-80 cm) than under IR64 (90-110 cm) and Vandana (115-130 cm). Genotypes with appropriate canopy structures should be chosen for high vapour pressure deficit (VPD) conditions. Both maximum canopy temperature and VPD need to be specified to define the critical threshold for heat tolerance. Takanari was notable for greater leaf area retention and greater leaf photosynthetic capacity due to the maintenance of a higher internal leaf CO2 concentration, which led to higher spikelet and grain numbers and higher yield potential under ET conditions.

In Asia, direct sowing and water savings are major trends in previously transplanted and flooded irrigated rice systems because of the higher cost of wages and increasing water shortage. Previous experiments showed that the leaf appearance rate varies between aerobic and flooded cropping systems. This study aimed to further understand how the planting method affects the development rate, flowering time, and yield of lowland irrigated rice crops. A two-year experiment was undertaken at the International Rice Research Institute, Philippines, using three contrasting rice varieties and three planting methods (transplanted, wet direct-seeded, and dry direct-seeded; at a density of 25 plants m-2) in a field submerged in 3–5 cm water from two weeks after the transplanting date. The effect of the planting method was similar in the three varieties, mostly without interaction between the two factors. In 2013, the leaf number of seedlings grown in seedling trays was two leaves behind that of direct-seeded plants at the time of transplanting. However, the young transplanted plants recovered quickly; produced new leaves at a faster rate (with a shorter phyllochron); reached panicle initiation, flag-leaf emergence and flowering time 1 week later; and developed more leaves compared to the direct-seeded plants. In 2014, growing in the nursery induced no delay in leaf appearance due to temperatures lower than those in 2013; therefore, the planting method did not affect the leaf appearance rate. Thus, plant development was primarily delayed by the density stress in the seedling trays under warm temperatures; however, the transplanted rice plants had lower plastochron duration than the direct-seeded plants, which made up for the initial delay in leaf appearance. In both years and at similar plant density, the transplanted plants produced more tillers bearing larger upper leaves that led to a higher leaf area index; however, grain yields were similar for the transplanted and direct-seeded crops. Highlights - In seedling trays, leaf appearance stopped at the appearance of the fourth leaf. - After transplanting, leaf appearance resumed at a faster rate than in direct-seeded plants. - Transplanted plants had delayed panicle initiation and flowering time, more tillers, and more and larger leaves per tiller, but similar grain yield compared to direct-seeded plants at similar planting density.

Quantifying and comparing sustainability indicators are essential to improve the sustainability of smallholder rice cropping systems. The sustainability of rice production systems can be measured based on economic, environmental, social, and institutional indicators. In this paper, we restrict our assessment to economic and environmental indicators. During 2012-2015, farmers were interviewed from 847 households from intensively irrigated rice production regions in Vietnam, Thailand, Indonesia, Myanmar, Sri Lanka, and China. We assessed the sustainability of their farming practices using economic and environmental indicators, i.e., eight of the 12 performance indicators (PIs), as defined by the Sustainable Rice Platform (SRP). Across the six sites, there was a yield gap of 24-42% and a profit gap of 36-82% between the 10% highest-performing farms (mean of top decile) and the mean-performing farms. In addition, there was a labor productivity gap of 12-32%, a nitrogen use efficiency (NUE) gap of 11-20%, a phosphorus use efficiency (PUE) gap of 1-29%, and a water productivity gap of 12-42%. Deliberate modification of conventional practices, including not flooding the field for > 30 days before rice planting, incorporating pre-rice crop residue > 30 days before planting, and adoption of mid-season drainage or alternate wetting and drying irrigation rather than continuous flood irrigation during the rice growing period, could substantially reduce the greenhouse gas (GHG) emission in irrigated rice fields without yield penalty. There is an urgent need to adopt improved management strategies for nitrogen (N), phosphorus (P), potassium (K) fertilizer, irrigation water-use efficiency, as well as for decreasing pesticide use frequency, without sacrificing profitability and yield. We identified the following priority interventions for each site: a) increasing fertilizer use and adopting higher-yielding varieties in Bago, Myanmar; b) reducing pesticide application rates in Can Tho, Vietnam; c) reducing fertilizer use in Guangdong, China; d) reducing nitrogen and labor use in Yogyakarta, Indonesia; e) reducing fertilizer and water use in Polonnaruwa, Sri Lanka and; f) reducing fertilizer use in Nakhon Sawan, Thailand. Additional uses of the PI analysis clearly demonstrated that rice yield and profit gaps can sustainably be closed by increasing efficiencies that will also lead to reduced environmental footprint.

Genetic variation in the growth response to temperature is a basis for developing adaptation measures to global warming, but evaluation of cultivars for the temperature responses may depend on other environmental factors such as light. In this study, we tested the growth responses of 18 diverse rice cultivars to constant day/night temperature of 25, 28, 31 and 34ºC in artificially-lit growth chambers (ALC) in Wagga Wagga (7.8 MJ m-2 d-1), and in naturally-lit chambers (NLC) in Yanco (25 and 28ºC and 13.4 MJ m-2 d-1; 31 and 34ºC and 11.5 MJ m-2 d-1), both in NSW, Australia. There was a significant interaction between temperature and chamber type for total shoot and panicle biomass; total shoot biomass was largest at 31ºC in ALC, and at 25 and 28ºC in NLC. From the average of all temperatures, the total shoot biomass declined by 29.5% in plants grown in ALC compared with those grown in NLC. Importantly, cultivar performance in ALC was similar to that in NLC at these temperatures, as evidenced by the highly significant correlation in total shoot biomass between ALC and NLC. Among 18 cultivars, IR64, IR72, N22, Vandana, Takanari and Koshihikari commonly produced a larger total shoot biomass under higher temperature conditions. Leaf area at earlier measurement date was highly correlated with the final total shoot biomass at the higher temperature more than specific leaf area.

Yield advantage of hybrid rice in the tropics has been reported recently as the result of higher biomass accumulation and better biomass partitioning over the whole crop growth. Considering that increasing biomass accumulation is the main target for higher yield potential in sub-tropical and temperate conditions, it is relevant to investigate in a wide range of growing conditions in the tropics if improved biomass partitioning plays a significant and consistent role in higher yield of hybrids. The growth pattern of two high-yielding and popular hybrid (H1) and inbred (I1) of the same maturity group was compared under six contrasted growing conditions to evaluate traits related to sink regulation. Grain yield of H1 was consistently higher than that of I1 by 16–32% with respect to the situation. Higher partitioning coefficients of the hybrid to key organs were confirmed over the whole crop growth for this set of environments whereas crop growth rates of hybrid were not consistently higher than that of inbreds. Sink strength index, as a way to express sink regulation at maturity more efficiently than harvest index, was higher with hybrids in five out of six environments. In search for promising traits related to sink regulation, higher specific leaf area of hybrids at very early stage was associated with higher leaf area, and earlier cessation of tiller production with hybrids coincided with higher partitioning of biomass to early culm growth: yet, maximum tiller number ranged from 548 to 962tillerm−2 with H1 and from 629 to 1427tillerm−2 with I1 while culm dry weight at 55 days after sowing ranged from 65 to 81gm−2 with H1 and from 46 to 53gm−2 with I1. This analysis strongly reinforced the pertinence of improving sink regulation for increasing yield potential in the tropics.

Crop establishment techniques need to be improved, integrated and adapted to local environments in order to approach yield potential of irrigated rice. Although several options are available (distinct types of direct-seeding, transplanting with distinct nursery management, different plant density), no extensive and comparative analysis of these techniques has been conducted to provide strategies to reduce yield gaps. Broadcasting and row seeding are attractive for their ease of operation, with low labor cost. In direct seeding, however, land leveling and water control are critical to ensure success of crop establishment, weeds are strong competitors and seeding rate is high. To overcome these issues, transplanting rice seedlings from a nursery to the main field has been the common practice in irrigated rice production areas where flooded water of 2-3 cm depth can be maintained from transplanting onwards. In Asia, this practice concerns about 90% of cultivated rice area in countries like Japan, Indonesia, South Korea and Myanmar (Pandey and Velasco, 2002). In farmers' fields, seedlings are commonly grown in nurseries at about 3000 to 4000 seeds m-2 and transplanted when 20 to 30 days old. Transplanting at this age gives vigorous seedlings that are more resistant to pests like snails, easier to handle by transplanters, and have reduced tiller production, which lowers physiological costs of tiller abortion (Schnier et al, 1990). The practice of transplanting relatively old seedlings, however, delays onset of tiller emergence which has been correlated with reductions in grain yield of 1 t ha-1 (Pasuquin et al, 2008). This suggests that improvements in nursery management can reduce yield gaps so long as improvements can be adapted to farmers' conditions. This study investigated interactive effects of nursery management techniques on grain yield and underlying processes controlling yield, and to formalize integrated strategies, including direct-seeding, for reducing yield gaps. (Resume d'auteur)

Rice is one of the most important crops that provide food for most of the poor in the world. Rice production is highly vulnerable to increase in growing temperature and one strategy to cope is to use adapted genotypes. This study aims to determine rice physiological and growth responses to temperature at similar vapour pressure deficit to understand mechanisms to select for genotypes or traits adapted to high temperature. Two experiments in the Climatron of NIAES, Tsukuba, Japan: 35°C/22°C day/night temperature and 70% RH as high temperature compared with 32°C/22°C and 68% RH as control, and 32°C/22°C and 70% RH as control compared with 28°C/22°C and 69% RH as low temperature. The experiments used Akihikari, IR64, N22 and Takanari. Maximum photosynthetic rate, stomatal conductance and transpiration significantly increased while intercellular CO 2 concentration remained similar in N22 after 30 days of high temperature treatment. This translated to higher biomass in N22 during the early growth. Stem sucrose content in N22 was also highest in both temperatures. In the low temperature experiment, transpiration, intercellular CO 2 concentration and stomatal conductance in all the genotypes increased in the 32°C/22°C compared to those in the 28°C/22°C. However, there was no difference in the photosynthetic rate and total biomass between the temperatures. Total biomass followed similar trend as the leaf biomass. Takanari had higher total biomass in both temperatures. Stem sucrose content in Akihikari was higher in both temperatures. (Texte integral)

Changes in climatic conditions are expected to directly affect agricultural production systems including rice farming. However, maintaining and increasing productivity in the face of climate change will require improved understanding of the effects of climatic factors on rice physiological traits. Increasing temperatures will lead to changes in atmospheric vapor pressure deficit (VPD). VPD can affect rice growth by affecting the water status of the plants, the thermal conditions of leaves or the canopy thereby regulating plant functions. This experiment aimed to determine the effects of VPD on growth and physiological responses of a range of rice genotypes in order to better understand these effects and their implications for adaptation to climate change.

The predicted increases in global atmospheric temperatures are likely threats to rice productivity. In order to help develop better genotypes and crop management strategies to sustain the continuously increasing demand for rice, deeper understanding of the growth and physiological responses under increased temperature and other related climatic conditions is needed. This study aims to evaluate the response of 18 rice genotypes from Japonica, Tropical Japonica, Indica, Aus/Japonica and Glaberima/Japonica germplasm group with a wide range of high temperature response. Seeds were direct-sown and grown in pots under continuous day/night temperatures of 25°C, 28°C, 31°C and 34°C under naturally-lit chambers in Yanco (11.5 MJ m-2 dI) and in artificially-lit chambers in Wagga Wagga (7.8 MJ m2-ld- New South Wales, Australia .. Growth was poor (6.9 g/pl) at Wagga Wagga under artificial light at all temperatures, so those results were not considered further. In contrast, individual plant dry weight at 72 DAS at Yanco decreased from 27.1 g/pl at 25°C to 14.66 g/pl at 34°C under natural light (11.5 MJ m2-l dI). Vandana had high, Takanari had intermediate and Akihikari had low dry weight at all temperatures. Contrary to expectation, N22 had reduced dry weight at high temperature while IR 72 and especially IR64 retained or even increased dry weight at high temperature. The response of the individual genotypes to climatic conditions inside the growth rooms was quite wide, but discrimination between the climatic factors and their effect on plant growth could not be achieved in the facilities exploited here. A more intensive characterization of the responses in well controlled environments is needed to better identify and discriminate the mechanisms for adaptation to elevated temperature. (Texte integral)

The vapor pressure deficit (VPD) can affect rice growth by affecting water status of the plants and/or thermal conditions of leaves or the canopy. Rice responses to increasing temperatures may also be altered by changes in VPD. We are currently conducting a series of experiments to determine the interactive effects of temperature and VPD on rice growth, but we need to examine genotypic variation in growth responses to VPD to better understand the interaction and it implication for adaptation to climate change. This study aims to determine the genotypic variability in responses to VPD using four varieties that showed different temperature responses in our previous study. We used two naturally-sunlit growth chambers (Climatron) of NIAES, Tsukuba, Japan, setting day/night temperature at 32/22DC for both chambers but different relative humidity (RH) levels; one at 80% (Low VPD9l.74 KPa) and the other at 50% (High VPIY=1.84 KPa). Seedlings of Akihikari, IR64, N22 and Takanari were grown outside until 21 days after sowing (DAS), then moved to the chambers and allowed to grow for 30 days. Growth and physiological responses were determined during the treatment period. Leaf temperature (Tl) measured under the mid-day cloudless conditions at 45 DAS with infra-red thermometers (Konica-Minolta, TA-0510) was significantly higher in low VPD than in high VPD (P

A significant proportion of arable land in south-western Australia is highly susceptible to subsoil compaction, which limits access of roots of wheat to water and nutrients at depth. Genotypic variation in the ability of roots to penetrate a hardpan has been reported for other cereals, using a pot technique, where a thin wax-layer of paraffin wax and petroleum jelly is placed in a soil column to simulate a hardpan. Previously we have modified and validated this technique for measuring root penetration ability of wheat seedlings under contrasting water regimes. Here we report on a series of five experiments (runs), two in well-watered and three in drought stress conditions, which evaluated seminal and nodal root penetration ability through thin wax layers among 24 Australian wheat cultivars and breeding lines (entries). These results were compared with observations on their rooting depths in two contrasting soil types in field trials, including a sandy duplex that contained a hardpan and a red clay that increased in soil strength with depth. Nodal roots ceased growth early under soil water deficit, and water uptake was instead dependant on seminal roots under conditions imposed in the pots. Plants were then reliant on the ability of seminal roots to penetrate the wax layer. Eight entries had superior root penetration ability in both well-watered and drought stressed conditions. Roots of three other entries, which failed to penetrate the wax layers, died under drought stress conditions. In field trials, there was a significant interaction between site and entry for maximum root depth. Our results from the pot studies and field trials indicate that there exists genotypic variation in root traits that are required to penetrate uniformly hard soil, dry soil or soil containing a hardpan. As four of the eight superior entries also showed superior root penetration ability at both sites in the field, there was an overall consistency, but there were exceptions at individual field sites. Factors likely to result in such exceptions were discussed, and topics for further research identified.

Transplanting rice seedlings 20 days old or older has been commonly reported to generate an increase in grain yield as a result of higher tiller production. A series of experiments was conducted at the IRRI farm during the dry and wet seasons to quantify, in a range of plant types, the impact of even younger seedlings and contrasting nursery management on grain yield and to identify plant traits supporting high performance under a given establishment technique. Seedling age at transplanting, ranging from 7 to 21 days, and contrasting nursery types (seedling tray, dapog, mat nursery, and traditional wet-bed seeding) were evaluated for an elite line, a new plant type and hybrid rice. To avoid any confounding effect, sowing date in the nursery, seed rate and crop management in the main field were all the same. In the two seasons, and for all genotypes and nursery types, transplanting older seedlings induced a delay in the onset of linear dry matter accumulation and tiller emergence, while the rate of dry matter accumulation and tiller emergence was unchanged. This delay reduced nitrogen content in the seedlings. Plants recovered quickly, however, after transplanting. The delay also reduced maximum tiller number, and extended crop duration with delayed maximum tillering, flowering and maturity. Grain yield was consistently higher for younger seedlings, with, in some cases, a difference as large as 1tha−1 between 7- and 21-day transplanting. This result was valid for the four genotypes evaluated, with a higher impact during the dry season. In contrast, no significant difference was observed for the influence of nursery type on the timing of tiller emergence and on grain yield. Some differences in seedling vigor (plant dry weight, specific leaf area, N content), higher in the case of dapog and wet bed, and in maximum tillering, higher in the case of the seedling tray, however, were observed. But these differences did not have a significant impact on the late increase in crop dry matter and on panicle number at maturity. No significant interaction between seedling age and nursery management for all genotypes and for all the parameters measured was found. Promoting early tiller emergence as a response to transplanting young seedlings increased grain yield in all cases despite the associated decrease in tillering efficiency. Extended growth inside the nursery, rather than transplanting shock per se, appeared to be the main reason for delayed tiller emergence in late transplanting.

Improving yield potential in rice implies the characterization of particular crop traits that may be used by breeders in their breeding programs. The rice crop is known to initiate and develop many tillers; a significant part of this may be higher than 50%, do not produce any grain. Recent breeding programs for yield potential have selected genotypes with high tillering efficiency (low tiller mortality) such as new plant types to reduce dry matter loss (Schnier et al 1990) and respiration cost (Dingkuhn et al 1990), but they did not produce the expected high yield in experimental fields. A positive correlation of grain yield with tillering efficiency would also imply a detrimental effect of non-productive tillers. We reviewed here a number of our recent field works to characterize the correlation between grain yield and tillering efficiency across genotypes of the same crop duration grown in similar favorable conditions and across crop managements for the same genotype. (Resume d'auteur)

This project, funded through the new GRDC initiative ‘Root Systems for Australian Soils’, builds on current and past research undertaken in WA that has described the pattern of root growth of annual crops in a range of field soils with chemical and/or physical barriers to growth, including hard soils and drought. It is not known whether genetic diversity exists for root growth in soils containing a hardpan among the currently-available wheat cultivars and breeding lines. Genotypic variation in root penetration ability has been reported in other cereals (Yu et al . 1995; Kubo et al . 2004), and validated in our own research, using a pot technique where a thin disc of wax and petroleum jelly is placed in a soil column to simulate a hardpan (Botwright Acuna and Wade 2005). Partitioning of the soil column by the wax layer makes it possible to examine the interaction between hardpan strength and soil moisture stress. Our pot experiments have revealed differences in root penetration ability under drought among 24 wheat cultivars and breeding lines. These results are compared with observations on their rooting depths in two contrasting soil types in field experiments undertaken in Merredin. This technique will have application in identifying promising lines for wheat breeding programs and in the interpretation of field performance of wheat grown in soils containing a hardpan.