A Preliminary Study of the Impacts of Duckweed Coverage during Rice Growth on Grain Yield and Quality.

The overuse and misuse of fertilizers have been causing duckweed outbreaks in irrigation ditches and paddy fields in many rice-growing areas. However, how duckweed coverage in a paddy field affects the rice yield and grain quality is under debate because duckweed may act as either a weed, competing with rice for mineral nutrients, or a "nutrient buffer", providing significant ecological and economic benefits. To understand the effects of duckweed coverage throughout rice growth on the yield and quality of rice grains, an experiment with three Japonica rice cultivars was conducted with fertile lotus-pond bottom soil as a growth medium to provide sufficient mineral nutrients for both the duckweed and rice. Averaged across three rice cultivars, duckweed coverage decreased the panicle density but increased the spikelet density and grain weight, resulting in no significant change in the rice yield. Duckweed coverage had no impact on the processing and appearance quality in general, but significant duckweed-by-cultivar interactions were detected in the head rice percentage and grain chalkiness, indicating different sensitivities of different cultivars in response to the duckweed treatment. The decrease in breakdown and increase in setback values in the rapid visco analyzer (RVA) profile of rice flour suggested that duckweed coverage during rice growth worsened the cooking quality of the rice. However, no significant change in the palatability of the cooked rice was found. The most profound change induced by the duckweed was the nutritional quality; duckweed coverage increased the protein concentration but decreased the concentrations of Mg, Mn, Cu, and Zn in rice grains. This preliminary study suggested that duckweed coverage during rice growth has profound effects on the rice nutrient uptake and grain nutritional quality under the circumstances, and further research on the responses of the rice quality to the duckweed coverage in paddy fields in multiple locations and years is needed.

Response of rice growth and leaf physiology to elevated CO2 concentrations: A meta-analysis of 20-year FACE studies.

The Free Air CO2 Enrichment (FACE) facility enables the study of plant responses to climate change under open field conditions. This meta-analysis was conducted to quantitatively assess the effects of elevated CO2 concentration ([CO2]) on 47 variables describing rice growth physiology and whether CO2 effects were influenced by cultivar, plant growth stage, nitrogen application rate or temperature. On average, elevated [CO2] increased root and shoot biomass by 28% and 19%, respectively. Among shoot organs, the [CO2]-induced increase in leaf biomass was only 9%, significantly smaller than a 24% increase in stems or a 25% increase in panicles. The higher biomass for FACE rice was consistent with the stimulation in plant height (4%), maximum tiller number (11%), leaf area index (9%) and light-saturated photosynthetic rate (Asat, 22%). When compared within rice groups, hybrid rice showed the greatest CO2 response in growth and leaf physiological variables. Elevated [CO2] increased plant biomass and Asat at each rice growth stage, but the increment tended to decline with the advancement of rice growth and development. The increase in aboveground biomass at elevated [CO2] was enhanced by a higher nitrogen supply but reduced with a temperature elevation of 1-2 °C. Rice growth benefited more from elevated [CO2] in Chinese FACE studies than in Japanese FACE studies, which may result from the different cultivars and nitrogen application rates used in the two countries. Combined with a previous meta-analysis of the rice yield response to FACE, the [CO2] level predicted in the middle of this century will improve rice productivity by stimulating leaf photosynthesis. However, the effects of CO2 on the photosynthetic rate and rice growth tend to shrink over the plant life cycle. Selecting heat-resistant, high-yield hybrid rice cultivars with large sink capacity, supplemented with appropriate nitrogen input, will maximize the CO2 fertilizer effect in the future.

Alterations in Source-Sink Relations Affect Rice Yield Response to Elevated CO2: A Free-Air CO2 Enrichment Study.

To understand the effects of source-sink relationships on rice yield response to elevated CO2 levels (eCO2), we conducted a field study using a popular japonica cultivar grown in a free-air CO2 enrichment environment in 2017-2018. The source-sink ratio of rice was set artificially via source-sink treatments (SSTs) at the heading stage. Five SSTs were performed in 2017 (EXP1): cutting off the flag leaf (LC1) and the top three functional leaves (LC3), removing one branch in every three branches of a panicle (SR1/3) and one branch in every two branches of a panicle (SR1/2), and the control (CK) without any leaf cutting or spikelet removal. The eCO2 significantly increased grain yield by 15.7% on average over all treatments; it significantly increased grain yield of CK, LC1, LC3, SR1/3, and SR1/2 crops by 13.9, 18.1, 25.3, 12.0, and 10.9%, respectively. The yield response to eCO2 was associated with a significant increase of panicle number and fully-filled grain percentage (FGP), and the response of crops under different SSTs was significantly positively correlated with FGP and the average grain weight of the seeds. Two SSTs (CK and LC3) were performed in 2018 (EXP2), which confirmed that the yield response of LC3 crops (25.1%) to eCO2 was significantly higher than that of CK (15.9%). Among the different grain positions, yield response to eCO2 of grains attached to the lower secondary rachis was greater than that of grains attached to the upper primary rachis. Reducing the source-sink ratio via leaf-cutting enhanced the net photosynthetic rate response of the remaining leaves to eCO2 and increased the grain filling ability. Conversely, spikelet removal increased the non-structural carbohydrate (NSC) content of the stem, causing feedback inhibition and photosynthetic down-regulation. This study suggests that reducing the source-sink ratio by adopting appropriate management measures can increase the response of rice to eCO2.

Impact of Elevated CO2 and Reducing the Source-Sink Ratio by Partial Defoliation on Rice Grain Quality - A 3-Year Free-Air CO2 Enrichment Study.

Evaluating the impact of increasing CO2 on rice quality is becoming a global concern. However, whether adjusting the source-sink ratio will affect the response of rice grain quality to elevated CO2 concentrations remains unknown. In 2016-2018, we conducted a free-air CO2 enrichment experiment using a popular japonica cultivar grown at ambient and elevated CO2 levels (eCO2, increased by 200 ppm), reducing the source-sink ratio via cutting leaves (LC) at the heading stage, to investigate the effects of eCO2 and LC and their interactions on rice processing, appearance, nutrition, and eating quality. Averaged across 3 years, eCO2 significantly decreased brown rice percentage (-0.5%), milled rice percentage (-2.1%), and head rice percentage (-4.2%) but increased chalky grain percentage (+ 22.3%) and chalkiness degree (+ 26.3%). Markedly, eCO2 increased peak viscosity (+ 2.9%) and minimum viscosity (+ 3.8%) but decreased setback (-96.1%) of powder rice and increased the appearance (+ 4.5%), stickiness (+ 3.5%) and balance degree (+ 4.8%) of cooked rice, while decreasing the hardness (-6.7%), resulting in better palatability (+ 4.0%). Further, eCO2 significantly decreased the concentrations of protein, Ca, S, and Cu by 5.3, 4.7, 2.2, and 9.6%, respectively, but increased K concentration by 3.9%. Responses of nutritional quality in different grain positions (brown and milled rice) to eCO2 showed the same trend. Compared with control treatment, LC significantly increased chalky grain percentage, chalkiness degree, protein concentration, mineral element levels (except for B and Mn), and phytic acid concentration. Our results indicate that eCO2 reduced rice processing suitability, appearance, and nutritional quality but improved the eating quality. Rice quality varied significantly among years; however, few CO2 by year, CO2 by LC, or CO2 by grain position interactions were detected, indicating that the effects of eCO2 on rice quality varied little with the growing seasons, the decrease in the source-sink ratios or the different grain positions.

Response of rice yield traits to elevated atmospheric CO2 concentration and its interaction with cultivar, nitrogen application rate and temperature: A meta-analysis of 20 years FACE studies.

The Free Air CO2 Enrichment (FACE) facility simulates future high CO2 environment in an open field, and is considered the best approach to assess the actual response of crop production to climate change. This meta-analysis synthesizes all studies conducted under FACE conditions on rice yield response to elevated atmospheric CO2 concentration ([CO2]) and its interaction with cultivar, nitrogen application rate and temperature. On average, elevated [CO2] enhanced rice yield by 16.2%, which resulted from positive response of each yield component. The yield enhancement by elevated [CO2] of hybrid rice (24.7%) was significantly greater than conventional rice (14.2%), and among conventional rice cultivars, indica rice had a larger yield response (20.4%) than japonica rice (12.7%). The superior performance of hybrid and indica rice under FACE conditions was mainly attributed to the larger increase in spikelet density. The response of rice yield to elevated [CO2] varied with nitrogen supply. The maximum increase of 21.1% occurred at the nitrogen application rate of 21-30 g m-2. Both insufficient and excess nitrogen supply negate yield increase by FACE but through different approaches. Elevated [CO2] increased rice yield by 16.7% at ambient temperature but only 10.1% at elevated temperature (1-2 °C); The smaller yield increase at elevated temperature was due to the negative response of filled grain percentage and grain mass. In conclusion, atmospheric CO2 concentration projected in the middle of this century will enhance rice yield mainly through the increase of spikelet density, whereas the magnitude of CO2 fertilizer effect will be affected by the cultivar, nitrogen application rate and temperature.

[Effects of elevated CO2 concentration on grain filling capacity and quality of rice grains located at different positions on a panicle.] / 高浓度CO2对稻穗不同位置籽粒结实和米质性状的影响.

The rising atmospheric CO2 concentration affects spikelets development, grain filling process, and rice quality. However, it is unclear that whether such effects are related to grain positions on rice panicle. By using a rice FACE (Free-Air CO2 Enrichment) platform, we grew a japoni-ca rice cultivar Wuyunjing 23, characterized with high yield and good quality, under ambient (Ambient) and elevated CO2 concentrations (+200 µmol·mol-1, FACE). The effects of increased CO2 concentration on spikelet density, grain filling capacity, the appearance and eating quality of rice grains were examined and the association of such effects with grain positions on rice panicle were investigated. The results showed that CO2 enrichment increased grain yield of Wuyunjing 23 by 18.3%. The panicle number per unit land area and filled-grain weight increased by 21.4% and 9.4%, respectively; whereas the number of spikelets per panicle and filled-grain percentage decreased by 9.0% and 2.2%, respectively. The decreased filled-grain percentage of rice grown under FACE treatment was mainly related to the increases of empty-grain percentage in all parts of rice panicle. The decrease of rice spikelets number per panicle by FACE treatment was mainly due to the substantial decrease of surviving spikelets of secondary branches in upper and middle parts of rice panicles instead of other positions. The CO2-induced changes of filled-grain weight and filled-grain percentage were similar among grains located at different positions on rice panicle. FACE treatment reduced the green grain rate and increased the grain length and width, with the grains at different positions on rice papnicle showing similar responses. FACE significantly increased chalky grain percentage by 59% and chalkiness degree by 55%, with the increases for both parameters following the order of primary branches＞secondary branches and upper part＞middle part＞lower part. FACE treatment slightly increased amylose content while decreased peak viscosity, hot viscosity, breakdown, final viscosity and setback, but most of these effects were nonsignificant. The gelatinization temperature of rice also reduced by 5% under FACE, and the decrease of inferior spikelets was greater than that of superior spikelets. In summary, the yield increase of Wuyunjing 23 under high CO2 concentration was mainly related to the increases of panicle number and individual grain weight, while the panicle size was reduced. Elevated CO2 concentration reduced green grain percentage but increased grain chalkiness, and had little effect on cooking and eating quality. The grain positions on rice panicle affected the responses of spikelets development, grain filling capacity and grain quality of rice to elevated CO2 concentration, but the effects varied across different indices.