Development and Functioning of Mycorrhizal Root Systems under Non-Uniform Rootzone Salinity
Abstract
Arbuscular mycorrhiza are obligate symbionts that live in association with most plant roots. The fungal mycelium extracts nutritional elements from the soil, and supplies these to plants in exchange for plant photoassimilates. The present study investigated the effects of root colonization by arbuscular mycorrhizal fungi on growth and nutrient uptake of tomato (Solanum lycopersicum L.) and Sudan grass (Sorghum hicolor L.) exposed to topsoil salinity through a horizontal split-root setup. Roots in the upper compartment were exposed to substrate salinity, while lower compartment roots had access to non-saline nutrient solution. Despite roots being well-colonized by mycorrhizal fungi, there was no difference between mycorrhizal and non-mycorrhizal plants in growth, nutrient and water uptake. The results of this study cannot support the hypothesis that mycorrhiza fungal root colonization facilitates host plant uptake of nitrogen, phosphorus, potassium and water from saline soil. Negative effects of salinity on the functioning of the mycorrhizal symbiosis, or a poor functional compatibility of the symbiotic partners under prevailing experimental conditions might be reasons for this. In another experimental approach, effects of partial rootzone salinity and partial rootzone drying on growth and nutrient uptake of young, mycorrhizal, clonal date palms (Phoenix dactylifera L.) were compared. Horizontal split root containers were used in this approach, with either topsoil or subsoil roots exposed to salinity or drought. Results from this experiment suggest that mycorrhizal date palms grow better and show a higher extent of mycorrhiza fungal root colonization when exposed to partial rootzone drying compared with partial rootzone salinity. However, plants exposed to subsoil salinity had a higher water use efficiency compared with the other partial rootzone stress treatments, eventually due to water saving mechanisms induced upon exposure of roots to a low osmotic potential. In addition to these experiments, a wide range of grafting techniques were tested on seedlings of Prosopis sp. for the production of double rooted grafts to be used in a vertical split-root assembly. In vitro grafting approaches produced the highest rate of success and this technique could be tested and developed further in future studies.