Institut für Pflanzenproduktion und Agrarökologie in den Tropen und Subtropen

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Effects of seed coating on germination and early seedling growth in cereals
(2014) Gorim, Linda Yuya; Asch, Folkard
Grain cereals such as barley, rye, wheat or sorghum, constitute a major share in human diet worldwide. Climate change threatens cereal production systems due to emerging unreliable rainfall patterns and thus, renders crop production vulnerable to early season failure of crop establishment. Breeding drought tolerant genotypes is a long and complicated process, thus not suited to respond to environmental changes quickly but rather to address the problem in a longer time frame. Seed coats increasing water availability to the seed and enhancing early vigor of the seedling may be a better short term solution. Seed coats containing hydro-absorbers such as Stockosorb® or Geohumus® can improve water availability and in combination with other substances for example humic acids or plant fortifiers such as Biplantol® may improve early vigor. However, very low germination rates have been observed in seeds coated in this way resulting in slow adoption of seed coating technology in agriculture. The present study analyzed the effects of seed coating on germination, which seed coat composition can enhance germination rate and early vigor and why, the effects of seed coat on germination and seed physiology, and which influence seed coats containing different kinds of hydro-absorbers have on the drought resistance of seed during germination. Studies were conducted on barley, rye, and wheat seeds. In these species the mode of action of differently composed seed coats and the effects of differences in seed coat strength were studied whereas the studies on drought resistance were performed with sorghum seeds. The following results were obtained and submitted for international publication: 1) Seed coating in general reduced germination rates as compared to uncoated seeds. Seed coating thickness was the determining factor. With a share of the seed coat of 75% of the total grain (seed + seed coat) germination was very little affected by coating. However, smaller seed coat shares and particularly shares smaller than 50% of the total grain severely reduced germination rate. This effect was especially pronounced in wheat. 2) With seed coat shares larger than 75% and the coats dotted with different substances generally resulted in high germination rates. However, strong genotypic effects were observed in responses of seed physiology and dry matter partitioning to the different substances included in the coat, with barley generally responding positively, rye intermediate and in wheat generally the weakest effects were observed. Across genotypes Biplantol included in the coat mainly promoted shoot growth, humic acid increased root growth and hydro-absorbers mainly the rate of germination and early vigour. Due to those results only seed coated with hydro-absorber containing coat and with coat not smaller than 75% were used for all following studies. 3) Seedlings growing from coated seeds with coat shares > 75% showed accelerated early seedling growth with strongly reduced respiratory losses during the mobilization of endosperm reserves, combined with significantly increased mobilisation efficiency in all three cereals. Analyses showed that the sucrose metabolism and thus the availability of glucose as energy provider for growth differed strongly between coated and uncoated seeds as well as among the cereal species. Embryos from coated seed (particularly in barley and wheat) seemed to grow better with significantly less glucose indicating a chance in the enzymatic cleavage of sucrose that could only be due to the higher energy efficiency of the enzyme sucrose synthase. 4) Studies during germination in sensu stricto (the first 48h hours after soaking) showed that in the embryos of coated seeds conditions were hypoxic with oxygen concentrations of less than 5% of atmospheric oxygen as compared to 60-80% oxygen of atmosphere in embryos of uncoated seeds. From this it was deduced that the lower respiratory losses during germination of coated seeds are due to a switch in sugar metabolism from invertase based cleavage of sucrose to sucrose synthase based cleavage of sucrose which is the less energy demanding pathway in the near absence of oxygen. 5) A last study on drought resistance of coated seed whose coats comprised two different hydro-absorbers (Stockosorb or Geohumus) showed that the drought responses of coated seed differed little form uncoated seeds. However, seedling growth under both drought and fully watered conditions was affected by the type of hydro-absorber in the coat. Whereas Stockosorb promoted rather root growth,Geohumus enhanced shoot growth. In conclusion, it can be stated that seed coats investigated in this study in general promoted germination rate and success in cereals if the seed coat has the appropriate strength i.e. coat shares not below 75% of total grain. Additives such as Biplantol or humic acid promote vigour and influence dry matter partitioning in favour of specific organs which could be employed as management options during germination. It was shown that seed coats influence the germination and seedling metabolism und induce hypoxic conditions in embryonic tissue which shift the sugar metabolism to a more energy efficient pathway. Oxygen dynamics in the different seed tissues require further studies and need to be better understood in order to employ the positive effects of seed coating in a targeted and species-specific approach to improve and enhance crop establishment particularly in drought prone cereal production systems. Another future pathway could be including nutrients in the coat that promote early seedling growth and for systems threatened by early drought spells or unreliable rainfall a seed coat that would conduct water to the seed only after soil moisture has surpassed a given threshold and thus induces germination only when water availability is optimal.
Effects of water management on microclimate and yield physiology in irrigated rice in semi-arid environments
(2014) Stürz, Sabine; Asch, Folkard
Growth and grain yield reductions have been widely observed when traditionally flooded rice fields were subjected to water-saving irrigation measures, where a continuous floodwater layer is avoided. These observations led to the perception of rice being a plant extremely sensitive to soil water deficits even when grown in soils close to their water holding capacity. Since the rice plant’s meristem is below the water surface until the early reproductive stage in flooded fields, the difference in heat capacity between water and air can lead to changes in meristem temperature, when a ponded water layer is omitted. Therefore, the objectives of this study were to quantify the effects of water-saving irrigation on the field’s microclimate and its influence on gas-exchange parameters and to investigate growth and yield parameters under flooded and non-flooded conditions in response to microclimate and varying climatic conditions. On two sites in Senegal, field experiments were conducted, where rice was sown on bi-monthly staggered dates and grown under flooded and non-flooded conditions. In the flooded treatment, a ponded water layer was maintained in the field throughout the growing season, whereas in the non-flooded treatment, irrigation water was applied until soil saturation on a frequent basis, in order to avoid standing water and soil water deficits at the same time. Microclimatic parameters and phenology were observed and leaf gas-exchange and plant growth parameters, yield and yield components were determined. Minimum soil temperature and temperature at meristem level were usually lower without standing water, whereupon temperature differences between irrigation treatments increased with decreasing air temperature. Stomatal conductance depended mainly on minimum soil and meristem temperature and minimum relative humidity inside the canopy. Assimilation rate was positively correlated with solar radiation and soil and meristem temperature, but depended mainly on stomatal conductance. Without standing water, stomatal conductance and assimilation rate were significantly lower, but the results could be explained with differences in microclimate. In most cases, leaf area was reduced under non-flooded conditions. Leaf area expansion rate was correlated with meristem temperature during the night. With minimum meristem temperature being lower under non-flooded conditions, lower leaf area expansion rates under non-flooded conditions could be attributed to lower meristem temperature. Yield reductions under non-flooded conditions were mainly observed in the cold-dry-season, whereas slight yield increases were found in the hot-wet-season. Among the yield components, reduced number of spikelets per panicle and decreased spikelet fertility accounted for the largest share of the yield gap. Leaf area per tiller was positively correlated with meristem temperature in the observed temperature range, and a positive relationship was found between leaf area per tiller and the number of spikelets per panicle. Furthermore, spikelet fertility increased with meristem temperature between panicle initiation and booting stage. Therefore, lower meristem temperature led to smaller leaf area per tiller, less spikelets per panicle and decreased spikelet fertility under non-flooded conditions. We concluded that water-saving irrigation in lowland rice production can lead to growth and yield reductions in comparison to traditional lowland irrigation even in the absence of soil water deficits, due to changes in soil and meristem temperature when a ponded water layer is omitted. Differences in assimilation rate, leaf growth and yield between irrigation treatments increased with decreasing air temperature and a clear seasonal pattern was observed, with large growth and yield reductions in the cold-dry-season, whereas in the hot-wet-season, growth and yield were less affected by irrigation treatment. When water-saving irrigation measures are applied in areas where night temperatures below 20°C occur, the effect of changes in meristem temperature should be considered. To mitigate impairment of growth under water-saving irrigation, a floodwater layer could be used to bridge cool periods, or a less temperature-responsive variety should be chosen. Nevertheless, the physiological mechanisms of the differential effects of day and night temperature remain unknown und need further investigation. Possibly, there is a combined effect of low night temperature and high evaporative demand during the day, which could lead to growth limitations due to restrictions of the plant’s water status. Furthermore, we want to highlight the need for a robust model of water temperature in paddy fields, which should be incorporated in rice growth models, since even tough existing models simulate growth and grain yield under upland and lowland conditions, the effects of changes in microclimate due to irrigation method are inadequately considered so far.
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.
Genotypic responses of rainfed sorghum to a latitude gradient
(2016) Abdulai, Alhassan Lansah; Asch, Folkard
Climate change poses various challenges to crop production systems. Coping with the changing climate requires adaptation strategies that will enhance the resilience of crop production systems to the resultant aberrant weather. However, the impacts of the changing climate are extremely difficult to predict because the associated extreme events result in a complex of abiotic stresses. These stresses act singly or in synergy with others to affect physiological processes at the different growth and development stages of crop plants. Currently, the physiological and phenological (developmental) response mechanisms of crops, as well as adaptation of cultivars to these stresses are not very clear and well understood. The complex interactions between crops and abiotic stresses make it difficult to accurately predict crop responses to climate change using the available crop growth models that have been parameterized and validated using some climate scenarios. While prediction of the complex ideotype-trait combinations may benefit breeders, physiological models that are well validated for target environments are equally important. Therefore, this study investigated elite grain sorghum genotypes from three races (Caudatum, Durra, and Guinea) and a Guinea-Caudatum composite, with different degrees of sensitivity to photoperiod and adaptation to a wide range of latitude locations, for their grain yield and yield stability responses to different environments. The aim was to calibrate growth models in for use in quantifying climate change effects on rainfed sorghum production systems. Field experiments were established to investigate the yield performance and yield stability of ten genotypes in eighteen environments created from a factorial combination of three locations (along a latitudinal gradient) and three monthly-staggered dates of sowing within years in 2008 and 2009. Field trials to study the phenology of seven of the ten genotypes were also established in a similar fashion in 2009 and 2010. Data were also collected on yield and other traits for the first two dates of sowing on six of the genotypes used for the yield performance trial to analyze the relations between grain yield and the selected traits and also evaluate the potential of path analysis in improving understanding of trait yield relations of grain sorghum. Mean grain yields of 0 to 248 g m were recorded across environments and from 74 to 208 g m-2 across the 10 genotypes and generally reduced with delayed sowing. Grain yield was significantly influenced by the main and interactive effects of location, year, sowing date, and genotype, necessitating the assessment of yield superiority and stability for each of the ten cultivars. The only two Caudatum cultivars (Grinkan and IRAT 204) were ranked among the top three by six of the indices. The study also brought to the fore that some yield stability indices correlate perfectly or very highly and could be substituted one for the other when assessing yield stability of sorghum. Very strong correlations were found between grain yield and each of shoot biomass, panicle weight, the number of grains per panicle, and threshing ability across environments, but path coefficient analysis confirmed that these traits are auto-correlated, with grains per panicle being the major mediating trait in all the relationships. Relationships between grain yield and the remaining traits were weak to medium and very inconsistent across the environments. This study brings to the fore, the location- and / or environment-specific adaptation of existing genotypes which should be exploited for tactical adaptation to changed climates, whiles genotypes with general or wider adaptations to environments are being sought. The phenology study showed that for photoperiod sensitive (PPS) genotypes, the number of days from emergence to panicle initiation and the number of leaves increased with latitude and decreased with sowing date, a day-length difference between locations of < 8 minutes increasing crop duration of some varieties by up to 3 weeks and decreasing number of leaves by up to 11 for the same sowing date. Some varieties exhibited photoperiod-insensitivity at one location and photoperiod-sensitivity at another location, indicating the complex nature of photoperiod responses. The study also showed that existing models do not accurately simulate the effect of latitude on the phenology of PPS sorghum, and latitude has to be taken into account in adjusting coefficients to improve the accuracy of such simulations. We conclude that genotypic response of rainfed sorghum is influenced by latitude, sowing date, and their interactions, but very little by years. Some existing cultivars could be deployed as tactical adaptive measures, while efforts are intensified to develop strategic adaptive measures. If changes in rainfall and temperature reduce the length of growing seasons, genotypes which are currently adapted to higher latitudes could easily be shifted southwards to lower latitudes, while those at lower latitudes may fit poorly into the new environments. A large potential for contributing to food security exist for the low latitudes if climates change in the direction predicted in future. It is absolutely necessary to develop new models that will be able to accurately simulate effects of sowing date and latitude on phenology. More research is needed to understand physiological response mechanisms of the pronounced latitude effects on sorghum phenology.
Genotypic responses of upland rice to an altitudinal gradient
(2012) Shrestha, Suchit Prasad; Asch, Folkard
Adaptation strategies are required for crops to cope with changing climate. The impact of climate change on crop production is not straight forward to predict as extreme events comprise multiple combination of abiotic stresses and their impact differs in crop physiological growth stages. The mechanism on how new abiotic stress combinations translate into phenology and yield, and which cultivars are better adapted is yet unclear. Crop growth models are available that have been parameterized and validated for some aspects of possible climate change scenarios but in view of complex interactions crop responses to climate change are difficult to predict. On the other hand, prediction of the complex ideotype trait combinations may be interesting for breeders but physiological models are required that are well validated for the target environments. In upland rice grown under rainfed conditions without surface water accumulation methane emission is negligible and therefore greenhouse gas emission much lower compared to irrigated paddy rice systems. In addition, growing demand for rice and the increasing pressure on irrigated land leads to development of upland rice areas to supplement irrigated rice. Therefore, this study investigates genetically diverse upland rice genotypes from a wide range of origins across altitudinal gradient locations. The main objective of this study is to investigate genotypic responses of upland rice to different environments in order to calibrate crop growth models, which allow the evaluation of effects of climate change on upland rice systems. Multi-locational field (three locations: 1625, 965 and 25 m asl) trials comprising non-replicated phenological plots with five sowing dates (monthly staggered) in two consecutive years creating thirty different environments, and replicated physiological yield trials with two sowing dates (monthly staggered; early and late sowing) in two consecutive years creating twelve different environments were established in Madagascar. Ten contrasting upland rice genotypes were included in both field trials. Meteorological data were recorded on a daily basis during trial periods. Developmental stages were observed in the phenological plots; in the physiological plots yield and yield components were recorded. In addition, greenhouse trials were conducted with one upland rice genotype subjected to seven N-supply levels in a hydroponic system at the University of Hohenheim in order to understand the relationship between chlorophyll index, photochemical reflectance index and chlorophyll fluorescence parameters. Various statistical tools were applied to analyse field and greenhouse data sets. The phenological trial showed that duration to flowering was 117, 81 and 67 d in high (HA), mid (MA) and low (LA) altitudinal locations respectively. 90% of the total variance was explained by location when pooled over genotype, location, sowing dates and year. In HA, factors such as genotype, sowing date and year equally contributed to the observed variability whereas in MA year was the most determining factor and genotype had no significant contribution. Similarly, in LA sowing date was the main influencing factor and year had no significant effect. Aggregated data over locations, sowing dates and years indicated that each degree Celsius rise in mean air temperature decreased crop duration by 5 to 9 days depending upon genotype. Basic genotypic thermal constants Tbase ranged from 9.8 to 13.9 °C and Tsum from 816 to 1220 °C d within the selected genotypes. Cold tolerant genotypes were less affected by lower Tmin (14 °C) at booting to heading stage regarding spikelet sterility in HA, whereas others were highly affected at 15 °C (cold stress). Similarly, both cold sensitive and tolerant genotypes were affected by Tmax (above 30 °C) at flowering in MA and LA locations (heat stress). Grain yield and yield components were highly affected by location, year, sowing date, and genotypes and the interactions between these yield-determining factors were obvious. In HA, early sown cold tolerant genotypes had more than 5 t ha-1 grain yield and one month delay in sowing led to highly reduced yield whereas other genotypes had very poor yield on both sowing dates due to cold stress. In MA, yield difference between sowing date and genotypes was small (4.3 - 4.9 t ha-1). Grain yield in LA was vulnerable due to frequent tropical storms. Yield stability analysis showed that cold tolerant genotypes had above average stability. AMMI model for grain yield showed that environment and genotype by environment interactions were highly significant. Yield components determined during specific development stages of the genotype such as tillers per hill and percentage of filled spikelets were mainly influenced by environment, spikelets per panicle and thousand grain weight were influenced by genotype, and percentage of productive tillers was equally influenced by both genotype and environment. PCA biplots showed that all HA environments were equally influenced by all weather parameters with minimum air temperature having the strongest positive influence on genotypic performance. In all MA environments genotypic performance in all phenophases was strongly and positively influenced by rainfall, and strongly and negatively influenced by vapour pressure deficit, solar radiation and potential evapotranspiration. In the LA environments, main weather parameters influencing genotypic performance were maximum temperature and high rainfall accompanied by strong winds. The field measured SPAD values of the upper canopy leaves reflected the location specific N-remobilization and leaf senescence levels after flowering. Similarly, PRI values showed the abiotic stress responses among development stages and locations along the altitudinal gradient. These readings showed that genotypes were efficient in radiation use and N-remobilization after flowering in MA. The unsynchronized relationship between source (leaf) and sink (grain) explained the yield penalty. Emphasis on identification of morpho-physiological traits contributing to cold tolerance should be placed for further breeding. We conclude that genotypic responses of upland rice cultivars differed across altitudinal gradients. Genotypes that are well adapted in HA can easily be adapted in MA without yield decrease. But genotypes well adapted in MA may show a huge yield penalty in HA due to lower temperature during reproductive phase and consequently reduced sink formation. Frequent tropical storms and high temperature reduced yield potential in LA. Therefore, HA has a large potential for the future food security considering climate change scenarios. At present, MA is favorable for upland rice production systems, whereas LA is highly vulnerable and is expected to be even more vulnerable in future. Those results on genotype-specific responses to environmental conditions allow further improvement of crop models such as RIDEV and SAMARA (synthesis of SARRAH and EcoMeristem), which can be used to test a number of phenotypic traits x environments combinations to define ideotypes of upland rice varieties adapted to changing climate and cropping calendars. Genotypic responses of phyllochron, biomass production and crop growth rate, and radiation use efficiency across altitudinal gradients will be included to parameterize these models. In this regard, collaborations with AfricaRice, CIRAD and IRRI are ongoing.
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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