Institut für Pflanzenproduktion und Agrarökologie in den Tropen und Subtropen

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/15

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Environmental effects on physical properties of Geohumus and effects of its application on drought responses in maize
(2013) Duong, Van Nha; Asch, Folkard
Geohumus belongs to a new generation of soil melioration/hydrophilic polymers; however, evidence is limited with regard to both, the ability of Geohumus to store water in variable abiotic environments and the effects of Geohumus or other hydrophilic polymers on plant genotypes in response to drought condition. Therefore, this study aims at providing necessary and complementary information for improving Geohumus usage under field condition, and to improve our ecophysiological understanding of the interactions between Geohumus, plant genotype and the growing environment. Three series of experiments were conducted to investigate (1) how abiotic factors affect the water holding capacity and restorability of Geohumus, (2) how the application of Geohumus affects plant morphological and physiological traits in response to different irrigation scenarios such as full irrigation, water deficit, and re-watering and (3) how the application of Geohumus in different soil types affects drought induced plant root-shoot communication. Water holding capacity (WHC) and restorability of Geohumus in mL water g-1 was determined by immersing teabags with fresh and used Geohumus in prepared media under laboratory conditions. A greenhouse experiment was carried out in order to analyze morphological and physiological responses of the two maize cultivars Mikado and Companero to progressive drought or full irrigation (field capacity) as affected by Geohumus. To obtain in depth information on Geohumus-plant interactions, a split root system experiment was conducted as a tool to investigate hydraulic and bio-chemical root-shoot communication of Mikado and Companero under full irrigation, partial rootzone drying, and deficit irrigation. Our results showed a negative correlation between salt concentration and water holding capacity (WHC) of Geohumus due to replacement of water molecules by ions at the polarized sites within the polymer chain (James and Richards 1986). Furthermore, salt types affected the WHC of Geohumus differently; in particular, multivalent ions were stronger impeding Geohumus compared to monovalent ions. Consequently, Geohumus application to sandy soil with base fertilizer application or to compost could not improve soil water content. However, split fertilizer application to sandy soil containing Geohumus led to a significantly improved soil moisture content indicating that timing and amount of fertilizer should be carefully considered under Geohumus application. Furthermore, for field applications the effect of climate needs to be considered, since the WHC of Geohumus increased with increasing temperature. The preferential ion uptake of Geohumus could translate into competition with plant roots for nutrient uptake from soil solution. On the other hand, Geohumus can capture nutrients which might have been lost for plants due to drainage. We found indications of these positive effects since biomass and leaf area of Mikado and Companero maize genotypes were increased compared to soils without Geohumus. Theoretically, polymers could release stored water to plants under drought stress; which in turn could inhibit or delay chemical signaling. However, our results showed increased concentrations of [ABA]leaf and [ABA]xylem of both Mikado and Companero grown in sandy soil with Geohumus in response to drought compared to treatments without Geohumus. This hormonal response was associated with larger leaf area and greater biomass resulting in a higher plant water demand due to its increased transpiration area while Geohumus did not improve soil water content significantly. On the other, hand root/shoot ratio, absolute root length and root biomass were decreased in plants grown with Geohumus. This suggests that plants grown with Geohumus under drought conditions could not extract water from deeper soil layers. The split root experiments showed that the larger leaf area of plants grown with Geohumus in combination with limited moisture content of sandy soil resulted in a stronger chemical root-shoot signal related to water stress. Regardless the increased [ABA]xylem which is associated with a reduction of stomatal conductance, Geohumus application could result in a decreased leaf water potential under partial root zone drying. Mikado grown with and without Geohumus, as a genotype potentially adapted to drought conditions, was able (1) to maintain its water potential under water limited conditions by penetrating roots into deeper soil layers (2) to delay the expression of physiological traits associated with drought, and (3) to maintain its shoot weight in contrast to Companero, a drought sensitive cultivar. The presented results are of relevance for the improvement of our understanding of the impact of abiotic factors such as temperature, salt concentration, and salt types on the WHC of Geohumus and therefore will help to optimize the application of hydro-gels under field conditions. Beneficial traits of plant genotypes grown under Geohumus application were identified, which will be valuable for breeding and applied programs targeting at crop improvement in arid and sub-arid regions and areas vulnerable to climate change.
Physiological and growth responses of Jatropha curca L. to water, nitrogen and salt stresses
(2012) Rajaona, Arisoa Mampionona; Asch, Folkard
This thesis provides necessary and complementary information for an improved understanding of jatropha growth to guide further research to evaluate the response of jatropha to abiotic stressors and for designing plantations adapted to the plants? requirements. Given the fact that jatropha is claimed to grow on marginal lands, we studied effects of water supply, salt stress, nitrogen and air humidity as major abiotic stressors on gas exchange parameters and biomass production followed by management options for pruning the trees to positively influence biomass productivity and to contribute to optimize resource use. The effects of water availability (rainfed versus irrigated) on growth and gas exchange parameters were investigated for 4-year old jatropha grown in a semi-arid environment at a plantation site in Madagascar in 2010. The results confirmed that 1250 mm water in addition to a 500 mm rainfall did not affect biomass production and instantaneous gas exchange. Nevertheless, leaf light responses of irrigated plants were higher than that of rainfed plants. The study showed to what extent salt stress affected water use, canopy water vapour conductance, leaf growth and Na and K concentrations of leaves of 3-year old and young jatropha plants. 3-year old plants were exposed to seven salt levels (0-300 mmol NaCl L-1) during 20 days and young plants to five salt levels (0-200 mmol NaCl L-1) during 6 days. In both experiments, plants responded rapidly to salt stress by reducing water loss. The threshold value of responses was between 0 and 5 dS m-1. Leaf area increment of young jatropha had a threshold value of 5 dS m-1 implying that jatropha is sensitive to external salt application in terms of canopy development, conductance and CO2 assimilation rate. Transpiration of plants in both experiments was reduced to 55% at EC values between 11 and 12 dS m-1 as compared to non-stressed plants. These findings indicate that jatropha responds sensitive to salt stress in terms of leaf elongation rate and consequently canopy development, and to immediate physiological responses. Leaf gas exchange characteristics of jatropha as affected by nitrogen supply and leaf age were intensively studied, as carbon assimilation is one of the central processes of plant growth and consequently a key process embedded in modelling approaches of plant productivity. This study showed that N supply effects on leaf gas exchange of jatropha leaves were small with only the treatment without nitrogen resulting in lower rates of CO2 assimilation rate and light saturated CO2 assimilation rate, nevertheless, effects of N supply on biomass formation were pronounced. Instantaneous rates of leaf gas exchange of different leaves subject to variable air humidity (atmospheric vapour pressure deficit (VPD)) were investigated. This study showed that CO2 assimilation rate (A) and stomatal conductance (gs) were correlated in a hyperbolic fashion, and that gs declined with increasing VPD. Maximal stomatal conductance of jatropha was in the range of 382 mmol m-2 s-1 and gs is predicted to be close to zero at 6 kPa. Effects of VPD, via stomatal conductance, by preventing high transpiration rates, have been demonstrated to be decisive on water use efficiency. Our findings are in this regard relevant for the estimation of water use efficiency of jatropha. The outcome further indicates favourable conditions at which stomatal opening is high and thereby allowing for biomass formation. This information should be considered in approaches which aim at quantifying leaf activity of field-grown bushes which are characterized by spatially highly diverse conditions in terms of microclimatic parameters. Microclimatic parameters can be modified by the tree structure. The reported field experiment on 4-year old jatropha indicated that the biomass production and canopy size depended mainly on primary branch length. A comparison of plants of different pruning types with regard to trunk height (43 versus 29 cm) and total length of primary branches (171 versus 310 cm), suggest that higher biomass production and greater leaf area projection was realized by trees with short trunks and long primary branches. Growth of twigs and leaves was positively correlated with total length of branches. Relative dry mass allocation to branches, twigs and leaves, length of twigs per cm of branches and specific leaf area were not affected by pruning and water supply. Trees with shorter branches had a higher leaf area density. As opposed to an allometric relationship between the average diameter of primary branches and total above ground biomass, our data suggest that these traits were not constantly correlated. Our data indicate that the length of newly formed twigs, where the leaves are attached to, can be related to the total length of already established branches. Leaf area density and relative dry mass allocation to leaves were not affected by the two pruning techniques, indicating that pruning differences in leaf area size were proportionally converted to corresponding pruning differences in the canopy volume exploited by plants. The results reported in this study are relevant for understanding jatropha growth. It helps farmers first for a better plantation management and researchers as well as contribution to future modelling purpose concerning jatropha growth under variable climatic conditions. Additionally, it should complement information for a better set of priorities in research, contribute indirectly to breeding programs and adjust agricultural policies in terms of encountering global change.
Rice genotypic variation on phenological development and yield performance in cold prone high altitude cropping systems
(2021) Abera, Bayuh Belay; Asch, Folkard
Despite a huge potential for rice intensification, several constraints have been reported as bottlenecks for rice production in the East African high-altitude cropping system. In this system, yield reductions are mainly caused by moisture deficit, which dictates the sowing date of the crops, and cold stress, which can occur either during specific crop growth phases or during the entire cropping period. In order to minimize yield losses, cultivation of suitable genotypes and timely implementation of proved crop management options are implicitly needed. Therefore, the objectives of this study were to investigate the effects of weather during specific development stages on yield and yield components of a large number of rice genotypes contrasting in crop duration; to explore the effects of crop establishment method on the performance of a set of rice genotypes in high altitude; and to identify key data sets required for the adaptation of agricultural decision support tools to new environments: the case of RiceAdvice being introduced to the highlands of East Africa. Field trials were conducted during the cropping seasons of 2016 and 2017 at the Fogera rice research station in Ethiopia. Further, to generate data to be used for the advancement of RiceAdvice, trials were implemented in Madagascar (Ambohibary and Ivory) and Rwanda (Bugarama and Rwasave) at different altitudes. Thirty contrasting genotypes were included in the study to investigate the effects early and late sowing and the related weather variation experienced by the crop. The crop establishment methods (direct seeding and transplanting) were evaluated using nine contrasting genotypes. Daily mean, minimum, and maximum temperature, rainfall, radiation, and relative air humidity were recorded during the experimental period; and the phenological development of each genotype was closely monitored in all trials. Data on grain yield and yield components were recorded and finally subjected to analysis of variance. Results showed that yield was positively correlated with the percentage of filled spikelets and the number of productive tillers, and negatively correlated with the number of tillers per hill. Genotypes differed in duration, yield, and yield components between the two years, which was related to both, differences in sowing date as well as differences in weather conditions. Early sowing in 2017 led to an extended duration until maturity of short-duration genotypes, which was related to low radiation levels as the vegetative phase of short duration genotypes entirely took place during the cloudy rainy season. Contrastingly, the duration to maturity of medium- and long-duration genotypes was shortened after early sowing in 2017, probably related to higher relative air humidity. In 2016, late sowing in combination with the early onset of the cool period led to high spikelet sterility in medium- and long-duration genotypes, as the cold-sensitive booting phase took place during the cold spell. Therefore, effects of sowing date on yield differed between genotype groups with short-duration genotypes suffering and medium- and long-duration genotypes profiting from early sowing and vice versa for late sowing. Similar results were obtained in the experiment conducted in Madagascar and Rwanda. At high altitude in Madagascar, short-duration genotypes performed well after late sowing, whereas medium-duration genotypes performed better after sowing one to two months earlier. Also, in Rwanda, delayed sowing compromised yield because of spikelet sterility related to low-temperature during the reproductive stage. Therefore, it was concluded that the choice of variety should depend on the sowing date, which is dictated by the onset of rains. Further, decisions on management intervention have to consider season-specific constraints. Comparison of transplanted and direct seeded rice showed that, in general, transplanting had a strong advantage over direct seeding. While at high-altitudes, growing medium- and long-duration genotypes with a high yield potential bears the strong risk of yield loss due to cold sterility, transplanting, which resulted in significantly higher yields than direct seeding, can mitigate this risk. As after transplanting, physiological maturity was observed earlier in the season than after direct seeding, rice plants, including medium- and long-duration genotypes, escaped the low temperature stress at the critical reproductive stage, and thus, low spikelet fertility. Thus, with a relatively cold tolerant genotype such as Yun-Keng, sowing a few weeks earlier within an irrigated nursery can make use of the full potential and increase yields. Comparison of the experimental sites in Ethiopia, Madagascar and Rwanda, showed that the mean temperature between sowing and flowering of the four tested genotypes was negatively correlated with altitude. In general, precise knowledge of the duration of the potentially suitable genotypes is required and a crop model that is well-calibrated for the genotypes as well as for the environment, in combination with a smartphone application such as RiceAdvice, would be of great help to support farmers’ decision-making. The data recorded from the three countries field trials can be used as data source to validate RiceAdvice, and thus, increase its applicability.

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