Effects of water-saving irrigation strategies on genotypic performance of irrigated lowland rice under different levels of salinity in the Mekong Delta in Vietnam

Kristian Johnson

Kristian was defending his PhD thesis in September 2023.

University of Hohenheim, Stuttgart

Institute for Agricultural Sciences in the Tropics (Hans-Ruthenberg-Institute) (490), Kien Giang University

Challenges

The projected increase in sea levels as a consequence of global climate change will put increasing pressure on coastal areas, such as Asian mega-deltas, which are both major population and food production centers. The Vietnamese Mekong Delta (VMD), with a population of around 20 million, contributes to 50% of national rice production and 90% of Vietnam’s rice exports. However, rice production is threatened by an increase in sea level, which leads to more frequent and extensive saltwater intrusion into coastal water tables. Parallelly, freshwater resources are dwindling due to competition from a growing population and hydropower. Faced with these conditions, rice farmers would be tempted to use water saving irrigation technologies (WSIT) during the dry season. The resulting reduction in standing water may lead to increased salinity, as the salt- contaminated groundwater in the coastal regions rises into the root zone, to be eventually leached during the rainy season. Thus, rice genotypes are required for systems using WSIT that take into account the related stresses of salinity and drought.

Objectives

• To measure the combined effect of WSIT’s and rice paddy salinity on 20 different rice genotypes’ phenology, morphology, and yield over the course of field trials in both dry and wet seasons in the Mekong Delta region, Vietnam.
• To determine within the same field trials, the water use efficiency according to rice genotype and irrigation method and how it relates to canopy temperature and salinity levels.
• To determine through a greenhouse experiment, using the rice genotypes from the field trials, the effect of salt stress on gas and water exchange and ultimately sink-source relationships within the plant.

Expected results

An assessment of the adaption capabilities of the selected rice genotypes in terms of growth and yield parameters to seasonal drought and saline conditions. Further knowledge into the development stage dependent adaption mechanisms of rice to salinity. Insight into the interaction between irrigation technology, freshwater resources, and salinity and their ultimate impact on rice production. The development of an irrigation schedule for farmers that combines choice of variety, seasonal water availability with water table depth and quality that also takes salinity into account.

Aims

• An assessment of the suitability of selected rice genotypes within a cropping system using WSIT.
• Insight into the physiological mechanisms behind adjustment to the combination of saline and drought stress.
• A characterization of the interaction between genotype, water management, and salinity, which could be further applied on a landscape level.

Methods

The field trials will be conducted within the RiSaWa project, which is collaborating with Kien Giang University in three districts in the Mekong Delta with contrasting salinity levels. Within each district, field sites with full irrigation control and a micro-meteo station for monitoring weather parameters will host field trials with around 20 contrasting rice genotypes during the dry and wet seasons over two years. In each location and season, the rice genotypes will be subjected to 2 WSIT and a fully irrigated control in a randomized split plot design with three replications. Phenology (emergence, onset of tillering, booting, heading, flowering, milking, physiological maturity) of the individual genotypes will be monitored visually. Daily irrigation water amounts will be recorded. Electric conductivity of the irrigation water and the topsoil (EC meter) and the soil moisture in the top 40 cm (FDR sensors) will be continually monitored. All genotypes will be destructively sampled in 10-day intervals for biomass, carbon partitioning (leaves, stems, panicles), and leaf area development. Tissue samples will be analyzed for Na and K content using flame photometry, and crop physiological responses will be monitored in 10-day intervals through SPAD (Konica Minolta, 502Plus) and transpiration and photosynthesis measurements (ADC, LCpro T). Yield and yield components will be determined at physiological maturity. Salt uptake and growth as well as yield will be linked to planting date, top soil salinity during specific development stages, and water use. In turn, yield, carbon partitioning and photosynthesis will be linked to sink-source relationships.

Between growing seasons, greenhouse experiments will be conducted in Hohenheim in which the same rice genotypes used in the field trials will be grown under contrasting salt and drought stress. These experiments will look in further detail into the mechanics of physiological adjustment according to genotype, with particular focus on dynamics within the leaf. This will be characterized by a combination of gas exchange measurements (Walz, GFS-3000), modelling, enzyme assays, and morphological characterization.

References

CGIAR. (2016) The drought and salinity intrusion in the Mekong River Delta of Vietnam: Assessment Report. Ben Tre, Tra Vinh, Kien Giang, Vietnam: CGIAR.

Peer reviewed publications:

1. Alejandro Pieters; Marcus Giese; Marc Schmierer; Kristian Johnson; Folkard Asch, 2022. Chamber-based system for measuring whole-plant transpiration dynamics. Plant-environment Interactions. 3:243–253

2. Julia Asch; Kristian Johnson; Shimul Mondal; Folkard Asch, 2022. Comprehensive assessment of extraction methods for plant tissue samples for determining sodium and potassium via flame photometer and chloride via automated flow analysis. Journal of Plant Nutrition and Soil Science. 185:308–316.

3. Kristian Johnson, Thuong Ti Bach Vo, Duong Van Nha,Folkard Asch, 2023. Genotypic responses of rice to alternate wetting and drying irrigation in the Mekong Delta. Journal of Agronomy and Crop Science, J Agro Crop Sci. 2023;209:593–612.

4. F. Asch; K.Johnson,T.B.T. Vo; B.O.Sander,V.N.Duong; R.Wassmann, 2023. Varietal effects on methane intensity of paddy fields under different irrigation management. Journal of Agronomy and Crop Science, J Agro Crop Sci. 2023;00:1–11.

5. Thi Bach Thuong Vo; Kristian Johnson; Reiner Wassmann; Bjoern Ole Sander;   Folkard Asch, 2023. Varietal effects on Greenhouse Gas emissions from rice production systems under different water management in the Vietnamese Mekong Delta. Journal of Agronomy and Crop Science, J Agro Crop Sci. 2023;00:1–17.