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Surfactants (surface-active agents) are amphiphilic molecules, possessing a polar hydrophilic head and a non-polar, hydrophobic, long-chain tail. They reduce the surface tension of water and are widely used by the turfgrass industry to mitigate against soil water repellency and alleviate localised dry spots. More recently, applying surfactants to soil has been considered as an alternative way of enhancing nutrient and water use efficiencies of arable crops. However, the mechanisms by which surfactants affect plant physiological responses to soil water deficit and nutrient status remain largely unknown. Thus, this thesis investigated surfactant effects on soil-plant water relations, water uptake and nutrient acquisition, in drying soil and/or elevated vapour pressure deficit. Initially, putative effects of surfactants on rhizosheath formation and nutrient uptake were investigated in two barley genotypes that either lacked (brb) or had (wild type ��� WT) root hairs, and thus had less and more rhizosheath respectively. Application of surfactant amplified rhizosheath formation when plants were grown in a sandy soil but did not affect nitrogen and phosphorus uptake. Generally, application of surfactant did not affect plant growth, which was 21% greater in WT than in brb plants. Thus, nutrient content (concentration x biomass) was significantly higher in WT than brb plants, indicating the importance of root hairs in nutrient acquisition. Although surfactant application did not affect plant nutrient acquisition, whether they affected plant response to soil water availability was next evaluated. The relationship between soil water potential and soil moisture was determined in surfactant-treated and untreated sandy soils by constructing soil moisture release curves via psychrometry, and by measuring base water potential (leaf water potential of non-transpiring plants) of plants grown in surfactant-treated and untreated soils. At the same bulk soil water content, surfactant-treated soils had a higher soil water potential and plants grown in these soils had a higher base water potential than plants grown in untreated soils. Since application of surfactant augmented rhizosheath development, WT and brb were grown in surfactant-treated and untreated soils and subjected to soil drying and/or elevated evaporative demand, to investigate whether additional rhizosheath development enhanced plant water availability. Surfactant-mediated or genotypic differences in rhizosheath development generally did not affect base water potential or leaf water potential. Surfactant application and genotype did not affect transpiration response to elevated evaporative demand (1-3.5 kPa), thus enhanced rhizosheath formation did not affect water uptake. Overall, applying surfactants enhanced soil water availability independently of rhizosheath formation. Although surfactant application did not affect plant transpiration during soil drying or under high evaporative demand, surfactant effects on whole plant gas exchange of different species (barley and maize) were determined after rewatering from the top or base of the pot. Surfactant application significantly increased shoot dry biomass by approx. 20% in both species. Although rehydration of upper soil layers was delayed following basal rewatering of surfactant-treated soil, whole plant gas exchange and leaf water potential recovered similarly irrespective of surfactant treatments. Thus, applying surfactant enhanced shoot dry biomass independently of plant gas exchange and leaf water status. This research showed that surfactant application can increase soil water availability to plants and enhance rhizosheath formation. However, these effects did not improve nutrient or water uptake capacity by the roots. Further research is needed to determine the mechanisms behind plant growth differences observed in some of the experiments. |
