Phytohormonal and physiological responses of Solanum lycopersicum to strong soils

Donaldson, Sarah and Dodd, Ian and Quinton, John and Whitmore, Andrew (2019) Phytohormonal and physiological responses of Solanum lycopersicum to strong soils. PhD thesis, Lancaster University.

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Abstract

Soil compaction occurs when external pressures (from heavy machinery or grazing animals) exerted on the soil surface increase soil bulk density, reducing porosity and aggregation. Nutrient, air and water holding capacities of the soil are reduced, and plant roots encounter increased mechanical resistance as they grow. Soil compaction also stunts shoot growth, with hydraulic and chemical signalling systems between below- and above-ground parts allowing the plant to adapt to this multi-stress environment. However, relatively few studies have characterised root-to-shoot signalling systems of plants with mechanically-impeded roots. Tomato plants (Solanum lycopersicum cv. Ailsa Craig) were grown under low and high soil bulk densities, and allowed to dry the soil to investigate plant physiological responses. Compact soil stunted plant growth, decreased stomatal conductance of well-watered plants and decreased plant water status at higher soil water contents. Multi-hormone analyses of root xylem sap and foliar tissues revealed that high bulk density soils attenuated the soil drying-induced increase in xylem [ABA]. Moreover, high bulk density soil increased xylem jasmonic acid concentrations and decreased foliar bioactive gibberellins, which were correlated with reduced shoot growth. Root drenches of bioactive gibberellic acid (GA3) were then applied to determine its ability to improve tomato shoot growth in compact soil. GA3 was transported from root to shoot tissues and significantly increased leaf expansion, but at the expense of plant water status. Further multi-hormone analyses indicated that GA3 application increased foliar cytokinin (trans-Zeatin) levels and decreased xylem jasmonic acid concentrations. Finally, to isolate soil strength from possible confounding effects of nutrient and water availability, tomato plants were grown in a sand culture system. A light foam block or 17 kg weight was placed upon the surface of the sand to increase substrate strength, while tanks were supplied with ample nutrients and water by capillary action. While GA3 again rescued shoot growth, shoot and leaf water potentials were reduced. Furthermore, xylem jasmonic acid concentration consistently decreased in both sand- and soil-grown plants as soil strength increased, which was not attributed to any decrease in leaf water status. Taken together, this thesis is the first to employ multi-hormone analyses on tissues and sap from plants growing in compact or strong soils. Novel roles for gibberellins and jasmonic acid in regulating plant growth when roots are mechanically impeded were discovered. GA3 appears to promote shoot growth against water potential gradients. Further study of the physiological significance of xylem-transported jasmonic acid and its cross-talk with gibberellins seem necessary to help determine how plants respond to soil mechanical stresses.