T.J. Stomph

Physiological basis of genetic variation in leaf photosynthesis among rice (Oryza sativa L.) introgression lines under drought and well-watered conditions.

To understand the physiological basis of genetic variation and resulting quantitative trait loci (QTLs) for photosynthesis in a rice (Oryza sativa L.) introgression line population, 13 lines were studied under drought and well-watered conditions, at flowering and grain filling. Simultaneous gas exchange and chlorophyll fluorescence measurements were conducted at various levels of incident irradiance and ambient CO2 to estimate parameters of a model that dissects photosynthesis into stomatal conductance (g s), mesophyll conductance (g m), electron transport capacity (J max), and Rubisco carboxylation capacity (V cmax). Significant genetic variation in these parameters was found, although drought and leaf age accounted for larger proportions of the total variation. Genetic variation in light-saturated photosynthesis and transpiration efficiency (TE) were mainly associated with variation in g s and g m. One previously mapped major QTL of photosynthesis was associated with variation in g s and g m, but also in J max and V cmax at flowering. Thus, g s and g m, which were demonstrated in the literature to be responsible for environmental variation in photosynthesis, were found also to be associated with genetic variation in photosynthesis. Furthermore, relationships between these parameters and leaf nitrogen or dry matter per unit area, which were previously found across environmental treatments, were shown to be valid for variation across genotypes. Finally, the extent to which photosynthesis rate and TE can be improved was evaluated. Virtual ideotypes were estimated to have 17.0% higher photosynthesis and 25.1% higher TE compared with the best genotype investigated. This analysis using introgression lines highlights possibilities of improving both photosynthesis and TE within the same genetic background.

Temporal dynamics in wheat grain zinc distribution: is sink limitation the key?

BACKGROUND AND AIMS Enhancing the zinc (Zn) concentration in wheat (Triticum aestivum) grain is a breeding objective in order to improve human Zn nutrition. At enhanced plant Zn uptake, grain Zn levels do not increase proportionally and within the grain the endosperm Zn levels remain below grain Zn levels. This study analysed the temporal dynamics of Zn concentrations in grain tissues during grain filling to find major bottlenecks. METHODS Plants of two cultivars were grown at 1 and 5 mg Zn kg(-1) soil. Individual panicles were harvested 7, 14, 24 or 34 d after their flowering or at maturity and seeds were dissected into constituting tissues, which were analysed for Zn and other minerals. KEY RESULTS The Zn concentration of the crease was found to increase five- to nine-fold between 7 and 34 d after anthesis, while that of the endosperm decreased by 7 and 45 % when grown at 1 or 5 mg Zn kg(-1), respectively. The Zn turnover rate (d(-1)) in the crease tissues was either independent of the Zn application level or higher at the lower Zn application level, and the Zn concentration increased in the crease tissues with time during grain filling while the turnover rate gradually decreased. CONCLUSIONS There is significant within-seed control over Zn entering the seed endosperm. While the seed crease Zn concentration can be raised to very high levels by increasing external Zn supply, the endosperm Zn concentrations will not increase correspondingly. The limited transfer of Zn beyond the crease requires more research to provide further insight into the rate-determining processes and their location along the pathway from crease to the deeper endosperm.

Improving zinc bioavailability in transition from flooded to aerobic rice. A review

Zinc (Zn) deficiency is a widely occurring constraint for rice production and for human nutrition. Scarcity of water is leading to a shift from flooded to aerobic rice production, which can have an impact on Zn deficiency in rice. Zinc bioavailability is a function of both soil and plant factors that can be altered by water management, particularly in relation to conditions in the rhizosphere. Biogeochemical modeling based on bulk soil conditions failed to predict the effect of water management on Zn bioavailability, but revealed that dissolved organic anions, pH, and redox conditions were major determinants. Rhizosphere sampling is needed to understand the difference in Zn mobilization and uptake between flooded and aerobic cultivation systems. Zinc bioavailability is not only affected by changes in the chemical properties of the soils, but also by biological processes such as mycorrhizal inoculation and root release of organic compounds into rhizosphere. Phytosiderophores and organic acids are two classes of Zn chelators secreted from roots that have been linked to the release of Zn from soil-bound forms and its subsequent uptake by plants. A shift to aerobic condition provides a favorable environment for activity of mycorrhizal fungi and enhanced mycorrhizal inoculation under aerobic conditions has been shown to increase plant Zn uptake. Aerobic rice genotypes with varying tolerance to Zn deficiency display a trade-off between mycorrhizal Zn responsiveness and root exudation of Zn chelator in the rhizosphere, which is probably due to a competition for carbon. Potential agronomic management practices in aerobic rice production systems are discussed, with an emphasis on their roles in improving bioavailability of Zn. Addition of Zn fertilizers by soil or foliar application have been shown to increase Zn concentration in cereal grains but the extent of the increase differs among crop species. The shift from flooded to aerobic condition can cause significant N transformations, which may consequently affect Zn mobilization and uptake. An appropriate N management strategy, including an effective combination of source, rate, application method, and timing, should consider the effects on soil pH. Application of P fertilizer should be done with careful consideration to the effect on Zn uptake. A reasonable cropping system (intercropping and crop rotation) could prevent Zn deficiency and offer an effective and sustainable pathway to Zn biofortification. Keeping these points in mind, this review describes our current knowledge of Zn bioavailability as affected by changes in soil–plant interactions caused by the transition from flooded to aerobic rice cultivation.

Zinc allocation and re-allocation in rice

Aims: Agronomy and breeding actively search for options to enhance cereal grain Zn density. Quantifying internal (re-)allocation of Zn as affected by soil and crop management or genotype is crucial. We present experiments supporting the development of a conceptual model of whole plant Zn allocation and re-allocation in rice. Methods: Two solution culture experiments using 70Zn applications at different times during crop development and an experiment on within-grain distribution of Zn are reported. In addition, results from two earlier published experiments are re-analyzed and re-interpreted. Results: A budget analysis showed that plant zinc accumulation during grain filling was larger than zinc allocation to the grains. Isotope data showed that zinc taken up during grain filling was only partly transported directly to the grains and partly allocated to the leaves. Zinc taken up during grain filling and allocated to the leaves replaced zinc re-allocated from leaves to grains. Within the grains, no major transport barrier was observed between vascular tissue and endosperm. At low tissue Zn concentrations, rice plants maintained concentrations of about 20 mg Zn kg−1 dry matter in leaf blades and reproductive tissues, but let Zn concentrations in stems, sheath, and roots drop below this level. When plant zinc concentrations increased, Zn levels in leaf blades and reproductive tissues only showed a moderate increase while Zn levels in stems, roots, and sheaths increased much more and in that order. Conclusions: In rice, the major barrier to enhanced zinc allocation towards grains is between stem and reproductive tissues. Enhancing root to shoot transfer will not contribute proportionally to grain zinc enhancement.

Analysing lodging of the panicle bearing cereal teff (Eragrostis tef)

Lodging, the permanent displacement of crop plants from their vertical because of root or shoot failure, is a major yield constraint of the gluten free, panicle bearing cereal teff. The objective of this paper was to analyse the causes of lodging of teff by using, modifying and validating conventional biomechanical models. The model parameters were obtained from a field trial with two contrasting teff cultivars, using novel in situ and laboratory measurements under wet and dry conditions. Cross-species model validation was done with rice (Oryza sativa). Teff is more susceptible to root lodging than to shoot lodging, although the data indicated that shoot strength is also insufficient. Hence, simultaneously breeding for both improved root anchorage and shoot strength is advocated. The study showed that the lodging model, derived for the spike-bearing cereal wheat, needed modifications in order to be able to deal with panicle-bearing plants such as teff and rice. Water adhering to plants owing to rain or dew increased calculated lodging susceptibility. To prevent underestimation of lodging susceptibility, future lodging research should be done under completely wet conditions (water saturated soil and wetted shoots).

Indices to screen for grain yield and grain- zinc mass concentrations in aerobic rice at different soil-Zn levels

Zinc is an important micronutrient for both crop growth and human nutrition. In rice production, yields are often reduced and Zn mass concentrations in the grains are often low when Zn is in short supply to the crop. This may result in malnutrition of people dependent on a rice-based diet. Plant breeding to enhance low-Zn tolerance might result in higher yields and nutritional quality but requires effective selection criteria embedded in physiological insight into the Zn husbandry of the crop and applicable in field evaluation of advanced breeding material or in screening of existing varieties. Using existing and newly developed low-Zn tolerance indices, this study presents the results of screening experiments carried out in high- and low-Zn soils. Sixteen accessions of aerobic rice were grown under greenhouse conditions to conceptualize the indices and 14 under field conditions to validate the indices. As the differences in soil-Zn levels in these experiments did not result in differences in grain yield, literature data were used from experiments where the soil-Zn level did have an effect on grain yield, to further check the validity of the indices. Several indices were applied to evaluate the genotypic low-Zn tolerance performance in attaining (relatively) high grain yield, high grain-Zn mass concentration, or both. The results indicate that the grain-Zn mass concentration efficiency index is different from the grain yield efficiency index and that the low-Zn tolerance indices identified superior genotypes best. Amongst the indices tested, the low-Zn tolerance index for grain yield and the low-Zn tolerance index for grain-Zn mass concentration were closely correlated with grain yield and grain-Zn mass concentration, respectively. Therefore, the low-Zn tolerance index for grain yield was effective in screening for high stability and high potential of grain yield, and the low-Zn tolerance index for grain-Zn mass concentration was effective for grain-Zn mass concentration under low and high soil-Zn conditions. Genotypic differences in yield and grain-Zn mass concentration were shown to be unrelated and therefore deserve separate attention in breeding programmes. Combining the low-Zn tolerance index for grain yield and the low-Zn tolerance index for grain-Zn mass concentration in a single low-Zn tolerance index was considered but did not appear to be superior to using the two indices separately.

Institutional change and the quality of palm oil: an analysis of the artisanal processing sector in Ghana

In Ghana, most oil palm fruits are produced by smallholders and processed by female artisanal processors. However, the ensuing crude palm oil (CPO) is high in free fatty acids and therefore cannot be sold in remunerative local or export markets. An earlier diagnostic study indicated that two main factors cause the poor quality: the processing practice of leaving harvested fruits unprocessed for up to 21 days and the use of lorry tyres as fuel to cook the fruits. Furthermore, the tyre-burning practice affects the health of people working and living around the processing facilities. This study describes the effect of action research undertaken with processors and the creation of a stakeholder platform in which Chiefs, the District Assembly, and a Concertation and Innovation Group collaborated to address the issues. The emerging institutional changes are assessed against baseline information. Awareness was raised about the dangers of tyre-burning, and CPO quality was improved by establishing the optimal time to leave fruits before processing. However, the prevailing market circumstances led producers to opt to produce greater quantities of oil rather than better-quality oil.

Influence of transplant size on the above- and below-ground performance of four contrasting field-grown lettuce cultivars

Background and aims: Modern lettuce cultivars underperform under conditions of variable temporal and spatial resource availability, common in organic or low-input production systems. Information is scarce on the impact of below-ground traits on such resource acquisition and performance of field-grown lettuce; exploring genetic variation in such traits might contribute to strategies to select for robust cultivars, i.e., cultivars that perform well in the field, even under stress. Methods: To investigate the impact of below-ground (root development and resource capture) on above-ground (shoot weight, leaf area) traits, different combinations of shoot and root growth were created using transplants of different sizes in three field experiments. Genetic variation in morphological and physiological below- and above-ground responses to different types of transplant shocks was assessed using four cultivars. Results: Transplanting over-developed seedlings did not affect final yield of any of the four cultivars. Small transplant size persistently impacted growth and delayed maturity. The cultivars with overall larger root weights and rooting depth, “Matilda” and “Pronto,” displayed a slightly higher growth rate in the linear phase leading to better yields than “Mariska” which had a smaller root system and a slower linear growth despite a higher maximal exponential growth rate. “Nadine,” which had the highest physiological nitrogen-use efficiency (g dry matter produced per g N accumulated in the head) among the four cultivars used in these trials, gave most stable yields over seasons and trial locations. Conclusions: Robustness was conferred by a large root system exploring deep soil layers. Additional root proliferation generally correlates with improved nitrate capture in a soil layer and cultivars with a larger root system may therefore perform better in harsh environmental conditions; increased nitrogen use efficiency can also confer robustness at low cost for the plant, and secure stable yields under a wide range of growing conditions.

Teff (Eragrostis tef) production constraints on Vertisols in Ethiopia: farmers' perceptions and evaluation of low soil zinc as yield-limiting factor

Teff (Eragrostis tef (Zucc.) Trotter) is a major food crop in Ethiopia and Eritrea. It is well adapted to Vertisols. Yields are low (around 1000 kg ha−1) despite fertilization with urea and diammonium phosphate. The objectives of this study were to understand farmers’ perception on teff production constraints and to evaluate on-farm yield response of teff to zinc (Zn) fertilization. We conducted a farm survey and a participatory fertilization experiment in three teff-based sites (peasant associations) on Vertisols in the mid highland and lowland agroecological zones in Ethiopia. Per site 10 farmers participated in the survey and on-farm experiment. Poor soil fertility in the mid highland and moisture deficit in the lowland agroecological zones were mentioned by farmers as major teff production constraints, respectively. On-farm application of Zn fertilizer at a rate of 8 kg Zn ha−1 increased teff grain and straw yields by 14% and 15% on average, respectively, which could be economically profitable. Not all plots showed a positive response, however, indicating the necessity for enhanced insight in indicators for soil Zn bioavailability as a yield-limiting factor. Our study indicates the importance of Zn in teff production on Vertisols. We propose further research on management options to prepare for effective interventions based on the farm survey and on-farm experiment.

Effects of land use changes on water and nitrogen flows at the scale of West African inland valleys: an explorative model.

Land use and cover, as influenced by agricultural practices, and the changes in these with increasing pressure on land, are among the factors determining water flows in inland valleys. Changing water flows affect nitrogen flows both at the plot level and at levels higher than plots. We present a conceptual model to show the potential influence of changes in land use on water and nitrogen flows. The model first calculates the three-dimensional redistribution of water at the inland valley level using topographic characteristics as input. For that purpose, the inland valley is divided in columns. Subsequently, the annual one-dimensional water balance is computed for each column using transfer functions. These functions use soil characteristics as input variables. Finally, the annual nitrogen balance is calculated with an equation which uses the calculated infiltration as input variable and differentiates between nitrogen uptake efficiency of annual and perennial vegetation. Both this equation and the transfer functions were tested for West Africa in an earlier study. The model-runs simulating different land-use scenarios illustrate that effects of changes in land use on water and nitrogen flows are not additive, when spatial effects of water redistribution are explicitly modelled. The addition of an explicit time dimension will make the model more dynamic in order to analyse the effects of patterns of land use throughout the year on water and nitrogen flows. The model can be used to target further research on the processes underlying water and nitrogen flows in inland valleys: in particular, the scale dependency of run-off in relation to agricultural practices merits attention in field experiments.