Institut für Tropische Agrarwissenschaften (Hans-Ruthenberg-Institut)

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/21

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Atmospheric and soil water deficit induced changes in chemical and hydraulic signals in wheat (Triticum aestivum L.)
(2022) Tatar, Özgür; Brück, Holger; Asch, Folkard
Plant responses to soil drying and the metabolic basis of drought‐induced limitations in stomatal opening are still being discussed. In this study, we investigate the roles of root‐born chemical and hydraulic signals on stomatal regulation in wheat genotypes as affected by soil drought and vapour pressure deficit. Twelve consecutive pot experiments were carried out in a glasshouse. Two bread wheat cultivars (Gönen and Basribey) were subjected to drought under high and low vapour pressure deficit (VPD) in a growth chamber. Total dry matter, specific leaf area, xylem ABA content, xylem osmotic potential, xylem pH, root water potential (RWP), stomatal conductance, leaf ABA content and photosynthetic activity were determined daily during 6 days after the onset of treatments (DAT). In the first phase of drought stress, soil drying induced an increase in the xylem ABA with a peak 3 DAT while RWP drastically decreased during the same period. Then the osmotic potential of leaves decreased and leaf ABA content increased 4 DAT. A similar peak was observed for stomatal conductance during the early stress phase, and it became stable and significantly higher than in well‐watered conditions especially in high vapour deficit conditions (H‐VPD). Furthermore, xylem pH and xylem osmotic potential appeared to be mostly associated with atmospheric moisture content than soil water availability. The results are discussed regarding possible drought adaptation of wheat under different atmospheric humidity.
Drought affects the synchrony of aboveground and belowground phenology in tropical potato
(2023) Hoelle, Julia; Khan, Awais; Asch, Folkard
The literature describes the belowground and aboveground phenology of potato to be linearly related. Bud formation is synchronous with tuber initiation and flowering with tuber filling. Many agronomic and breeding studies on potato use non‐destructive aboveground phenology to assess belowground development. No information is currently available on the influence of water deficit on the synchrony of above‐ and belowground development in potato. Five contrasting potato genotypes were subjected to four irrigation treatments on two different soil types. The irrigation treatments were as follows: fully watered, early drought, intermediate drought, and late drought. In 5‐day intervals after withholding water, detailed belowground and aboveground development was recorded. Results showed that the synchrony between aboveground and belowground development is strongly influenced by both water deficit and development stage at drought initiation. Under early drought, the aboveground development was hastened and belowground development was delayed. The opposite was found in later development stages. The earlier the drought was initiated, the longer the tuber filling phase was, while the bulking phase was shortened. We concluded that under terminal drought conditions aboveground development and belowground development need to be evaluated separately and cannot follow the standard evaluation system that uses aboveground phenology as a proxy for tuber formation belowground development rates.
Evaluating topsoil salinity via geophysical methods in rice production systems in the Vietnam Mekong Delta
(2023) Nguyen, Van Hong; Germer, Jörn; Asch, Folkard
The Vietnam Mekong Delta (VMD) is threatened by increasing saltwater intrusion due to diminishing freshwater availability, land subsidence, and climate change induced sea level rise. Through irrigation, saltwater can accumulate in the rice fields and decrease rice production. The study aims at evaluating topsoil salinity and examining a potential link between topsoil salinity and rice production systems in a case study in the Tra Vinh province of the VMD. For this, we applied two geophysical methods, namely, 3D electrical resistivity tomography (ARES II) and electromagnetic induction (EM38‐MK2). 3D ARES II measurements with different electrode spacings were compared with EM38‐MK2 topsoil measurements to evaluate their respective potential for monitoring topsoil salinity on an agricultural scale and the relationship between land‐use types and topsoil salinity. Results show that EM38‐MK2 is a rapid and powerful tool for obtaining high‐resolution topsoil salinity maps for rice fields. With ARES II data, 3D maps up to 40 m depth can be created, but compared with EM38‐MK2 topsoil maps, topsoil salinity was underestimated due to limitations in resolution. Salt contamination of above 300 mS m−1 was found in some double‐cropped rice fields, whereas in triple‐cropped rice fields salinity was below 200 mS m−1. Results clearly show a relation between topsoil salinity and proximity to the saline water sources; however, a clear link between rice production and topsoil salinity could not be established. The study proved that geophysical methods are useful tools for assessing and monitoring topsoil salinity at agricultural fields scale in the VMD.
Ion uptake and distribution in sweet potato genotypes subjected to salt stress is not driven by transpiration
(2023) Mondal, Shimul; Rahaman, Ebna Habib Md Shofiur; Asch, Folkard
Potassium is taken up actively by the plant, whereas sodium is often either competing for the same uptake mechanisms or uptake and distribution are driven by the transpirational volume flow in the shoots of plants grown under salinity. Reducing transpiration rate is regarded as an adaptation mechanism to reduce leaf tissue salt load. In combination with a high K uptake, plants may be able to maintain growth and are, thus, seen as salt‐tolerant. Little is known about these mechanisms in sweet potato (Ipomoea batatas L.). Therefore, cuttings of two sweet potato genotypes contrasting in salinity tolerance (CIP 188002.1, tolerant; CIP 189151.8, sensitive) were subjected to 0 and 50 mM NaCl root zone salinity in a hydroponic system and grown under low (0.76 kPa) and high (2.27 kPa) vapour pressure deficit (VPD) to create differences in transpiration. After 18 days of initial hydroponic growth, NaCl was added for another 33 days. Cumulative plant water loss and total uptake of Na, K and Cl were determined for all plants and treatments. Transpirational water loss was twice as high under high VPD as compared to low VPD conditions, but genotypic Na and Cl accumulation remained almost the same. In contrast to plants subjected to salt stress under low VPD conditions, genotypes under high VPD conditions differed significantly in transpiration. However, in both genotypes transpirational water loss from individual leaves and Na or Cl accumulation were not correlated, under high VPD younger leaves of CIP 188002.1 (tolerant) accumulated more than twice as much potassium than in CIP 189151.8 (sensitive). The distribution of the three ions across leaf positions and within one leaf position between petiole and leaf blade differed strongly between the two genotypes. Tolerant CIP 188002.1 accumulated up to five times more sodium and potassium in the leaf petioles in the middle‐aged and young leaf positions than in the leaf blade, whereas in sensitive CIP 189151.8 neither ion was preferentially accumulated in the petioles. This was independent of salinity treatment and VPD conditions. In contrast, hyperaccumulation of Cl in petioles only occurred under high VPD conditions in the petioles of the tolerant genotype, but not under low VPD conditions, indicating a VPD sensitivity for Cl distribution in sweet potato. While we conclude that transpirational volume flow is not a main driving force for Na and Cl uptake and distribution within the plant, we discuss potential pathways leading to the hyperaccumulation of sodium and potassium in the leaf petioles of the tolerant genotype. We suggest studies on HKT transporter activities in the petioles as an object of further studies in sweet potato.
Potassium content is the main driver for salinity tolerance in sweet potato before tuber formation
(2022) Mondal, Shimul; Rahaman, Ebna Habib Md Shofiur; Asch, Folkard
Sweet potato (Ipomoea batatas L.) is mostly grown in Asia, which accounts for 86% of global production. However, its production is under threat by salinity. Little is known about genotypic responses to salinity in sweet potato. Phenotypic responses or physiological processes linked to salt tolerance that could be developed into a reliable screening tool to assist breeding have not yet been developed for sweet potato. In a hydroponic cultivation system, 12 contrasting sweet potato genotypes were subjected to 0, 50, 100 and 150 mM root zone salinity (RZS). Genotypic thresholds for dry matter accumulation and the genotypic slopes for additional dry matter reduction when the RZS increased beyond the genotypic threshold were determined. Sodium, chlorine and potassium (K) were determined from above‐ground biomass and correlated with the genotypic thresholds found. Genotypic threshold levels were linearly negatively correlated with the difference in tissue K content at 75 mM RZS and the tissue K content at control levels. Based on the genotypic ability to retain high tissue potassium levels under increasing RZS, we propose a screening tool based on these experimental data that can distinguish between salt‐tolerant and salt‐sensitive genotypes and indicate the potential yield level of the sweet potato genotypes.
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.
Salinity effects on the activities of ROS scavenging enzymes in leaves of two sweet potato clones
(2023) Mondal, Shimul; Burgert, Susane; Asch, Julia; Rahaman, Ebna Habib Md Shofiur; Asch, Folkard
Sweet potato production, particularly in coastal areas is often prone to salinity. Salt‐tolerant clones will be needed to maintain production, but to date, little is known about salt tolerance traits in sweet potato. Salt stress may result in excessive uptake of unwanted ions into plant tissues leading to the formation of reactive oxygen species (ROS), which in turn may destroy membranes and reduce photosynthesis and growth. Antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX) scavenge ROS and early changes in the activities of such enzymes could be used to identify salinity tolerant genotypes. Therefore, cuttings of two contrasting cultivars of sweet potato, BARI SP 8 (tolerant) and BARI SP 4 (sensitive) were greenhouse‐cultivated in nutrient solution for 21 days and then exposed to 100 mmol NaCl for 7 days. Three, five and seven days after salt application the youngest leaves were sampled individually and enzyme activities, potassium (K) and sodium (Na) concentrations, and SPAD (as a proxy for chlorophyll content) were determined. In both varieties leaf growth was not affected by salinity and young leaves grown under salinity had higher SPAD values than older leaves. Na concentration increased over time, particularly in earlier and in older leaves, whereas K was reduced in younger leaves. In general, enzyme activities were strongly affected by leaf age and leaf position. SOD and APX showed varietal but no salinity effects, CAT increased under salinity in both varieties, whereas POX was strongly reduced and GR was strongly increased under salinity in BARI SP 8 with no effect in BARI SP 4. Enzyme activities were not correlated to leaf Na, neither in relation to leaf age, nor leaf number or duration of salt stress in both varieties. However, varietal differences were observed regarding leaf K. Activities of SOD were highly positive and of CAT highly negatively correlated with leaf K under salinity in BARI SP 8 but not in BARI SP 4, whereas activities of GR and POX were strongly positively correlated with leaf K in BARI SP 4 under salinity but not in BARI SP 8. We conclude that potassium may have a strong regulating role on leaf stress levels and therefore on the activities of antioxidant enzymes. Varieties may differ in their tolerance strategy and we have shown that salinity does not generally increase levels of ROS‐scavenging enzymes in sweet potato leaves under salt stress. Confounding factors such as leaf age and leaf position as well as maintaining high leaf level K concentrations need to be considered when evaluating metabolic traits for salinity tolerance traits.
Suitability of the stress severity index combined with remote‐sensing data as a tool to evaluate drought resistance traits in potato
(2023) Hoelle, Julia; Asch, Folkard; Khan, Awais; Bonierbale, Merideth
Potato is a drought susceptible crop and even short drought spells reduce tuber yields notably. In an earlier study we developed a stress severity index (SSI) based on the development stage of a genotype at the onset of drought and the soil water deficit based on soil water tension. Here, we test the suitability of the SSI combined with remotely sensed data as a screening tool to select drought‐tolerant potato genotypes. Normalized difference vegetation index (NDVI) and the photochemical reflectance index (PRI) were obtained from reflectance measurements and thermography. Temperature data from the thermography allow using the difference between leaf and air temperature (∆T) to estimate the transpirational cooling of the leaves. Via cluster analysis including SSI, tuber yield reduction under drought, NDVI, PRI and thermography, three groups were distinguished: 1. SSI < 1000 with fast decreasing NDVI, PRI and ∆T, 2. SSI 1000–2000 with almost constant NDVI and ∆T and 3. SSI > 2000 described by small changes of NDVI, PRI and temperature deficit. For SSI < 1000, ∆T, PRI and NDVI showed to be good indicators of genotypic performance under drought. Potential strategies for drought resistance in potato detectable through remote sensing are discussed.
Thermal imaging for assessment of maize water stress and yield prediction under drought conditions
(2022) Pradawet, Chukiat; Khongdee, Nuttapon; Pansak, Wanwisa; Spreer, Wolfram; Hilger, Thomas; Cadisch, Georg
Maize production in Thailand is increasingly suffering from drought periods along the cropping season. This creates the need for rapid and accurate methods to detect crop water stress to prevent yield loss. The study was, therefore, conducted to improve the efficacy of thermal imaging for assessing maize water stress and yield prediction. The experiment was carried out under controlled and field conditions in Phitsanulok, Thailand. Five treatments were applied, including (T1) fully irrigated treatment with 100% of crop water requirement (CWR) as control; (T2) early stress with 50% of CWR from 20 days after sowing (DAS) until anthesis and subsequent rewatering; (T3) sustained deficit at 50% of CWR from 20 DAS until harvest; (T4) late stress with 100% of CWR until anthesis and 50% of CWR after anthesis until harvest; (T5) late stress with 100% of CWR until anthesis and no irrigation after anthesis. Canopy temperature (FLIR), crop growth and soil moisture were measured at 5‐day‐intervals. Under controlled conditions, early water stress significantly reduced maize growth and yield. Water deficit after anthesis had no significant effect. A new combination of wet/dry sponge type reference surfaces was used for the determination of the Crop Water Stress Index (CWSI). There was a strong relationship between CWSI and stomatal conductance (R² = 0.90), with a CWSI of 0.35 being correlated to a 64%‐yield loss. Assessing CWSI at 55 DAS, that is, at tasseling, under greenhouse conditions corresponded best to the final maize yield. This linear regression model validated well in both maize lowland (R² = 0.94) and maize upland fields (R² = 0.97) under the prevailing variety, soil and climate conditions. The results demonstrate that, using improved standardized references and data acquisition protocols, thermal imaging CWSI monitoring according to critical phenological stages enables yield prediction under drought stress.
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.
Transplanting as an option to cope with abiotic stress in high‐altitude lowland rice production systems in East Africa
(2021) Abera, Bayuh Belay; Senthilkumar, Kalimuthu; Cotter, Marc; Asch, Folkard
The current practice of direct seeding in East‐African high‐altitude rice farming systems is constrained by water availability early in the season and low temperatures later in the season at the crop's critical reproductive stage. Thus, productivity is restricted as only short‐duration varieties can be grown due to the risk of crop failure. To fully exploit the yield potential of such rainfed systems, the best combination of crop establishment methods and climatic ‘best fit’ genotypes is required. In this study, nine rice genotypes were evaluated under direct seeding and transplanting in the 2016 and 2017 cropping seasons with the aim of identifying genotype by environment by management combinations best fitting the high‐altitude, rainfed rice production systems. On average across all genotypes, transplanting had a positive yield effect of 18% in 2016 and 23% in 2017. Regarding the phenological development, individual phenophases were not significantly affected by transplanting relative to direct seeding; however, vegetative development stages in transplanted rice tended to be about 15% longer than when direct seeded. Even though transplanting led to extended vegetative growth, the time in the nursery allowed the plants to escape the cold spell late in the season. The results from the current study provide options to adapt cropping calendars by combining genetic resources with targeted crop management, thus improving and stabilizing yields of rainfed lowland rice farming systems at high altitude.
Varietal effects on Greenhouse Gas emissions from rice production systems under different water management in the Vietnamese Mekong Delta
(2023) Vo, Thi Bach Thuong; Johnson, Kristian; Wassmann, Reiner; Sander, Bjoern Ole; Asch, Folkard
Rice production accounts for 15% of the national Greenhouse Gas (GHG) emissions and Vietnam aims at reducing emissions from rice production by focusing on changing farming practices. However, the potential for mitigation through the selection of different rice varieties is still poorly understood. A two‐year field screening of 20 rice varieties under continuous flooding (CF) and alternate wetting and drying (AWD) irrigation was conducted in the Vietnamese Mekong Delta (VMD), Vietnam, employing the closed chamber method for assessing GHG emissions. The results confirmed that varietal variation was the largest for methane (CH4) emissions under CF. Across the varietal spectrum, CH4 emissions were more important than nitrous oxide (N2O) (accounts for less than 2% of the CO2e) with the lowest emitting variety showing 243 kg CH4 ha−1 and the highest emitting variety showing 398 kg CH4 ha−1 emissions as compared to 0.07 kg N2O ha−1 and 0.76 kg N2O ha−1 emissions, respectively. Under AWD, CH4 emissions were generally strongly reduced with the varietal effect being of minor importance. Compared with IPCC default values, the data set from the two seasons yielded higher Emission Factors (EFs) under CF (2.92 and 3.00 kg ha−1 day−1) as well as lower Scaling Factors (SFs) of AWD (0.41 and 0.38). In the context of future mitigation programs in the VMD, the dry season allows good control of the water table, so varietal selection could maximize the mitigation effect of AWD that is either newly introduced or practised in some locations already. In the wet seasons, AWD may be difficult to implement whereas other mitigation options could be implemented such as selecting low‐emitting cultivars.
Varietal effects on methane intensity of paddy fields under different irrigation management
(2023) Vo, Thi Bach Thuong; Johnson, Kristian; Wassmann, Reiner; Sander, Bjoern Ole; Asch, Folkard
Alternate wetting and drying irrigation (AWD) has been shown to decrease water use and trace gas emissions from paddy fields. Whereas genotypic water use shows little variation, it has been shown that rice varieties differ in the magnitude of their methane emissions. Management and variety‐related emission factors have been proposed for modelling the impact of paddy production on climate change; however, the magnitude of a potential reduction in greenhouse gas emissions by changing varieties has not yet been fully assessed. AWD has been shown to affect genotypic yields and high‐yielding varieties suffer the greatest loss when grown under AWD. The highest yielding varieties may not have the highest methane emissions; thus, a potential yield loss could be compensated by a larger reduction in methane emissions. However, AWD can only be implemented under full control of irrigation water, leaving the rainy seasons with little scope to reduce methane emissions from paddy fields. Employing low‐emitting varieties during the rainy season may be an option to reduce methane emissions but may compromise farmers’ income if such varieties perform less well than the current standard. Methane emissions and rice yields were determined in field trials over two consecutive winter/spring seasons with continuously flooded and AWD irrigation treatments for 20 lowland rice varieties in the Mekong Delta of Vietnam. Based on the results, this paper investigates the magnitude of methane savings through varietal choice for both AWD and continuous flooding in relation to genotypic yields and explores potential options for compensating farmers’ mitigation efforts.

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