Browsing by Subject "Weed competition"

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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.
Unkrautbekämpfung in Zuckerrüben - Ermittlung der Kritischen Periode
(2003) Kobusch, Henner; Hurle, Karl
Early leaf stages of sugar beet are very sensitive to weed competition, which is a major reason for the absence of thresholds for weed control in sugar beet. In combination with non-selective herbicides, the use of herbicide resistant sugar beets appears to allow the control of weeds at a later date than usual applications of common selective herbicides. Therefore, it is necessary to know the critical period, in which the crop should be weed free in order not to loose yield. The influencing factors of the critical period are the moment until weed can be tolerated (beginning of the critical period) and the moment after weed can be tolerated (end of the critical period). The primary objective of the present work was the establishment of a parameter, which would allow a determination of the critical period independent of location and season. Therefore, triannual field trials were carried out at three different sites in the Ukraine and in Stuttgart-Hohenheim in order to evaluate the suitability of different parameters. In addition, by use of a glufosinat resistant sugar beet transformant, the practicability of the critical period was investigated. Application of the critical period and moreover the definition of a general period threshold requires a reference value defining the beginning and end of the critical period which is both independent of location and season. The primary aim of this work was to establish a parameter, which fulfills this condition. All parameters relate to the growth of sugar beet or of the weed quantify their interaction. The following parameters were investigated: the leaf stage of the sugar beet, the weed and sugar beet coverage level, the relative weed coverage, the temperature sum and the intensity of weed shading of the beets. The investigation took place at three separate sites in the Ukraine and in Stuttgart-Hohenheim enabling the effect of different sites to be taken into account. A uniform sugar beet leaf stage until and after weeds could be tolerated was not found. The leaf stage until weeds could be tolerated varied between the 2 and 10 leaf stage. Similarly the leaf stage after which the weeds could be tolerated varied between the 2 and 12 leaf stage of the beet. A uniform and therefore location and year-independent degree of sugar beet coverage and weed coverage relating to leaf stage was not found at the beginning of the critical period at the Hohenheim site (1999 and 2000) and Poltava (1999) in the Ukraine. The degree of weed cover varied at the beginning of the critical period between 96.7% and 66.5% in Hohenheim. The same applies to the degree of sugar beet coverage which varies between 5.3% and 15%. The difference between the two levels of coverage is almost completely compensated by the parameter relative weed coverage. At the Hohenheim site it only varied between 94.8% and 84.5%. The minimum value was found at Poltava with 83.8%. On this basis, a maximum relative weed coverage of 83 % can be tolerated without significant yield loss. Herewith, a decisive parameter is defined as a measure for timing weed control in sugar beets. However, an important requirement is the availability of efficient control methods at this certain point of time. In a further step an attempt was made to apply the critical period in relation to the leaf stage of the beet by using a glufosinate resistant sugar beet transformant. In no trail it could be waited with glufosinate applications until the beginning of the critical period. The latest leaf stage, when glufosinate application had to start in Poltava and Vinnitsa was the 6-leaf stage, whereas the critical period began at the 10- or 12-leaf stage. A limiting factor for the definitive application of the beginning of the critical period was shown in the field trials by a decreasing tolerance of the glufosinate resistant transformant at ever later leaf stages of beet development. Prediction model investigations confirmed this correlation. In addition to the effect of the leaf stage the effect of weather conditions was also apparent. The increase in air humidity from 50 % to 80 % led to an increase in NH3 concentration in the resistant transformant, regardless of its leaf stage. NH3 is found in non-resistant plants due to the inhibition of glutamine-synthetase by glufosinate, which leads to cell death. The largest increase in NH3 when the air humidity was increased from 50 % to 80 % occurred at the youngest leaf at the 6-leaf stage. In addition to the dependency of NH3 concentration on leaf stage the effect of leaf age was also apparent. Concluding, the control of weeds, related to the leaf stage of glufosinate resistant sugar beet, has to be done before the critical period begins. Unfortunately, technologies, which offer the possibility to control weeds by an integration of the critical period, are so far not available.