Browsing by Person "Vu, Duy Hoang"

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Effects of temperature and vapor pressure deficit on genotypic responses to nitrogen nutrition and weed competition in lowland rice
(2021) Vu, Duy Hoang; Asch, Folkard
Since rice is the major food for more than half of the world’s population, rice production and productivity have significant implications for food security. In adaptation to increasing water scarcity, as well as to reduce greenhouse gas emissions, water-saving irrigation measures (e.g., alternate wetting and drying – AWD) have been introduced in many rice growing regions. Previous studies have shown that AWD increases water use efficiency and reduces methane (CH4) emissions, while grain yield remains equal or is slightly increased compared to continuous flooding. However, the absence of a ponded water layer in formerly flooded rice fields creates new challenges, such as altered root zone temperature (RZT), enhanced nitrification leading to higher nitrate (NO3-) concentrations in the soil, or stimulated weed germination leading to changes in weed flora. All these factors may affect nutrient uptake and assimilation of rice plants and thus plant growth. Further, vapor pressure deficit (VPD) drives transpiration and water flux through plants, so nutrient uptake and assimilation by plants may be subject to adjustment under varying VPD conditions. As VPD varies largely between rice growing regions and seasons, and is also predicted to continuously increase under global warming, it was included as a factor in this study. The overall objective of the study was to evaluate the response of different rice varieties to arising challenges under water-saving irrigation. Experiments were conducted in the greenhouse and VPD chambers at the University of Hohenheim, where plants were grown in hydroponics. Both during day and night, nutrient uptake rates of rice increased linearly with RZT in the observed temperature range up to 29°C, implying that the optimum temperature for nutrient uptake of rice must be above 29°C. However, the uptake rates of different nutrient elements responded differently to RZT, with the increase in nitrogen (N) uptake per °C being greater than that of phosphorus (PO43-) and potassium (K+), which can potentially lead to an imbalance in plant nutrition. Therefore, the increase in RZT either due to climate change or water management may call for an adjusted fertilizer management. In general, the increase in nutrient uptake per °C was more pronounced during the day than during the night, while the amino acid concentration in the leaves both during the day and night was positively correlated with N uptake during the day, suggesting that plants may benefit more from increased temperature during the day. When both ammonium (NH4+) and NO3- were supplied, rice plants took up a higher share of NH4+. However, after depletion of NH4+ in the nutrient solution, plants took up NO3- without decreasing the total N uptake. The N form taken up by the rice plant had no effect on leaf gas exchange at low VPD, whereas NO3- uptake and assimilation increased stomatal conductance in some rice varieties at high VPD, resulting in a significantly higher photosynthetic rate. However, the increase in photosynthesis did not always result in an increase in dry matter, probably due to a higher energy requirement for NO3- assimilation than for NH4+. The effect of N form on leaf gas exchange of some rice varieties was only found at high VPD, indicating genotype-specific adaptation strategies to high VPD. However, maintenance of high stomatal conductance at high VPD will only be beneficial at sufficient levels of water supply. Therefore, we hypothesize that with increasing VPD, intensified nitrification under water-saving irrigation may improve leaf gas exchange of rice plants, provided a careful choice of variety and good water management. Furthermore, N form had an effect on the competition between rice and weeds. In mixed culture with rice, a large share of NO3- increased the growth and competitiveness of upland weeds but reduced the growth and competitiveness of lowland weeds. Consequently, enhanced nitrification under AWD may reduce the competitive pressure of lowland weeds, but increase the competition of upland weeds. In contrast to rice, growth of the upland weed was not reduced by high VPD, while its nutrient uptake was correlated with water uptake, suggesting that upland weeds will more successfully compete with rice for nutrients as VPD increases. Selection of rice varieties better adapted to NO3- uptake will improve rice growth and its competitiveness against weeds under AWD. The cumulative effects of RZT and soil nitrification on rice growth should be considered when evaluating the effects of climate change on rice growth.
Leaf gas exchange of lowland rice in response to nitrogen source and vapor pressure deficit
(2021) Vu, Duy Hoang; Stürz, Sabine; Pieters, Alejandro; Asch, Folkard
Background: In anaerobic lowland fields, ammonium (NH4+) is the dominant form of nitrogen (N) taken up by rice plants, however, with the large expansion of water-saving irrigation practices, nitrification is favored during drained periods, leading to an increased availability of nitrate (NO3−). Aim: Since the uptake and assimilation of the two N-sources differ in their demand of pho- tosynthates, leaf gas exchange may be subject to adjustments in response to N-sources, particularly at high evaporative demand, when stomatal conductance (gs ) is very sensitive. Methods: Three experiments were carried out to study leaf gas exchange of various low- land rice varieties in response to N-source at low and high vapor pressure deficit (VPD). In the first experiment, seedlings of 12 rice varieties were grown at high VPD for 3 weeks. From this, four rice varieties differing in gs and CO2 assimilation rate (A) were selected and grown for 2 weeks at low VPD, and after that, they were shifted to high VPD for 1 week, whereas in the third experiment, the same varieties were grown separately at low and high VPD conditions for 2 weeks. In all three experiments, plants were grown hydroponi- cally in nutrient solution with N-sources as sole NH4+ or NO3−. Results: At high VPD, NO3− nutrition led to a higher gs and A in four out of 12 vari- eties (IR64, BT7, NU838, and Nipponbare) relative to NH4+ nutrition, while no effect was observed at low VPD or after a short-term exposure to high VPD. Further, varieties with a high intrinsic water-use efficiency (WUEi; IR64 and BT7) showed the strongest response to N-source. Higher gs was partially supported by increased root/shoot ratio, but could not be fully explained by the measured parameters. However, higher A in NO3−-fed plants did not always result in increased plant dry matter, which is probably related to the higher energy demand for NO3− assimilation. Our results suggest that at high VPD, NO3− nutri- tion can improve leaf gas exchange in varieties having a high WUEi, provided a sufficient water supply. Conclusion: Therefore, intensified nitrification under water-saving irrigation measures may improve leaf gas exchange and the growth of rice plants under high transpirational demand. However, choice of variety seems crucial since large varietal differences were observed in response to N-source. Further, breeding strategies for genotypes adapted to aerobic soil conditions should consider responses to NO3−, potentially using gas exchange measurements as a screening tool.
Rice–weed competition in response to nitrogen form under high and low transpirational demand
(2021) Vu, Duy Hoang; Stürz, Sabine; Asch, Folkard
Implementation of water‐saving irrigation practices in lowland rice results in increased availability of nitrate (NO3−) in the soil and favours germination of upland weeds. Since plant species show a specific preference for either ammonium (NH4+) or NO3− as nitrogen (N) source, changes in both soil NO3− concentration and weed flora may affect the competition between rice and weeds. Further, the transpirational demand of the atmosphere might affect growth and competitiveness of lowland (wetland) and upland (dryland) weeds differently due to their adaptation to different ecological environments. Therefore, the study aimed to evaluate the effects of N source on growth, N uptake and competition between rice and common upland and lowland weeds under high and low vapour pressure deficit (VPD). Two rice (Oryza sativa) varieties (NU838 and KD18) differing in growth characteristics and two weed species (Echinochloa crus‐galli and Solanum nigrum) differing in their natural habitat were selected and grown hydroponically as monoculture or mixed culture at low or high VPD. N was supplied as 75%/25% or 25%/75% NH4+/NO3−. N uptake rates were measured in the first week, whereas dry matter (DM), N concentration in the plant, total N uptake and the activities of nitrate reductase and glutamine synthetase in the fresh leaves were determined two weeks after the onset of treatments. Independent of N source, both rice varieties and E. crus‐galli took up a larger share of NH4+, whereas S. nigrum took up a larger share of NO3−. N uptake of rice and E. crus‐galli was hardly affected by N source, whereas high NO3− led to significantly higher N uptake rates and total N uptake of S. nigrum. NU838 showed a higher competitiveness against weeds than KD18. In competition, high NO3− decreased the competitiveness of E. crus‐galli against NU838 but increased the competitiveness of S. nigrum against NU838. High VPD did not affect DM but increased N uptake of S. nigrum, leading to increased competitiveness of the weed at high transpirational demand. Competitiveness for N uptake appears to be an important trait as the relative N concentration in mixed plant communities was correlated with the activity of N‐assimilating enzymes and leaf growth, with a stronger response in rice than in weeds. Our results support the hypothesis that increased availability of NO3− in aerobic rice soils may be advantageous for the competitiveness of upland weeds, especially at high VPD, whereas it may be disadvantageous for common lowland weeds.
Traits contributing to salinity tolerance in rice genotypes from the Mekong Delta
(2023) Johnson, Kristian; Vu, Duy Hoang; Asch, Folkard
Increasing sea level rise and subsequent salinization in mega deltas, such as the Vietnamese Mekong Delta (VMD), pose a risk to rice (Oryza sativa L.) production during the dry season. This study investigated the salinity resistance of a selection of common rice genotypes from the VMD along with an international check, IR64. The 20 rice varieties were grown hydroponically for 5 weeks in a greenhouse and then exposed to three levels of NaCl concentration (0 mM, 50 mM and 100 mM) over a period of 2 weeks to determine their susceptibility to salinity. Rice plants were scored and SPAD (leaf greenness) and PRI (photochemical reflectance index) were measured on the youngest fully developed leaf on the main tiller. After harvesting the 7‐week‐old plants, biomass and ion (K+, Cl−, Na+) content were determined by organ across all tillers. Averaged over all varieties, both at 50 mM and 100 mM NaCl, there was a significant reduction in plant biomass, 39% and 52% respectively. However, the effect of the NaCl treatments and the uptake of Cl− and Na+ were significantly different between varieties (p < .0001). Using biomass and ion content as part of a multivariate analysis, varieties were classified according to their susceptibility to salinity and their predominant strategy towards managing ion accumulation. The grouped varieties were further characterized by patterns in Cl− and Na+ partitioning and nondestructive parameters such as SPAD and PRI.