Understanding the physiological basis for improved wheat seedling growth on dispersive sodic soils

Abstract

Sodicity can have a considerable negative effect on crop yield. Sodic soils can impede seed germination, seedling emergence, root and shoot growth resulting in reduced crop yields. The relative contribution of these physical and chemical constraints on plant growth depends on a range of factors that were investigated in this project.Plants growing in sodic soils often experience reduced seedling emergence due to the formation of surface crusts and ion imbalances in the soil solution. This can result in delayed and poor crop establishment, and in reduced production. The main aim of this study was to better understand (A) the physical impact of sodic soils on seed germination and seedling emergence of wheat (Triticum aestivum. L) genotypes through surface crusts, and (B) the chemical impacts of ion imbalances in sodic soils on wheat seed germination and seedling emergence. This information was used to determine the characteristics of some sodicity-tolerant and non-tolerant wheat genotypes commonly grown in Australia.While considerable research has previously been undertaken into the physiological response of wheat genotypes to sodic soils, our understanding of the capacity of wheat genotypes to emerge from crusted soils is still largely theoretical. To address this, the seedling emergence of 38 wheat genotypes from a crusting sodic soil was investigated using glasshouse experiments. Crusts of differing strength (0.8 - 2.7 kg cm-2 or 76 -263 kPa) and thickness (0.51-2.16 cm) were created using simulated rainfall applied to a representative surface soil with 10% exchangeable Na. Wheat seedling emergence decreased as crust strength and thickness increased. Seedlings exhibited one of four developmental patterns associated with variation in emergence. These were; (1) rapid germination and emergence before full crust development, (2) emergence by displacing the crust, (3) emergence of the coleoptile by reaching a crack in the crust, or (4) failure to emerge from under the crust.To determine why genotypes differ in their ability to emerge from crusted soils, certain plant traits that might affect the seedling emergence were investigated. Firstly, germination of wheat genotypes was determined with Petri dish assays using solutions with sodium adsorption ratios (SARs) of 0, 10, 40, and 60. The emergence force of the seedlings was also measured at the same four SAR values. Seed germination and seedling emergence force varied between genotypes at different SAR levels, with the emergence force of the coleoptile correlated to the cross-sectional area of the hypocotyl.Variation between wheat genotypes in seedling coleoptile length and seminal root angle was also examined to determine the usefulness of these traits to identify genotypes likely to have improved tolerance to sodic conditions. Wheat genotypes were grown using soils of three exchangeable sodium percentage (ESP of 4, 10 and 17%) and three bulk densities (1.2, 1.4 and 1.5 g cm-3). The seedling coleoptile length of all genotypes decreased with increasing soil ESP and bulk density, but there was no significant difference between genotypes. The seminal root angle differed significantly between genotypes but was not affected by ESP or bulk density. There were inverse relationships between the seminal root angle of genotypes and both seedling emergence rate (R2=0.81, P<0.0001) and seedling emergence force (R2=0.61, P<0.0001). In addition to the physical constraints of sodic soils, we also evaluated the performance of four wheat genotypes in the presence of chemical constraints in the absence of soil crust using solution cultures with five SAR levels 0, 10, 20, 30 and 60. For all the four genotypes, seedling emergence and shoot dry matter (DM) decreased significantly with increasing SAR. A significant positive correlation was found between Ca concentration in roots, and root and shoot DM for all genotypes. Thus, selection for genotypes that are able to accumulate Ca in sodic conditions may be a useful trait to select genotypes tolerant of soils with high ESP values.Overall, the results suggest that seedling emergence and early growth of wheat genotypes can be adversely affected by the physical or chemical constraints of sodic soils, and that different wheat genotypes vary in their performance in the presence of these constraints. In the presence of physical constraints of crusted sodic soil, the genotypes with rapid seed germination, higher seedling emergence force, larger hypocotyl cross sectional area, and narrow root angle had improved seedling emergence. However, a stepwise regression showed that narrower root angle was the most significant trait to identify tolerant genotypes in crusted sodic soils. In the presence of chemical imbalances in sodic soil solutions, Ca concentration in the roots was mostly correlated to early plant growth of wheat genotypes in high SAR solutions.