Arbuscular mycorrhizal fungi and their influence on growth and water relations of sweet cherry rootstock and tomato plants

This thesis presents a literature review and results of experimental studies related to the role of arbuscular mycorrhizal fungi (AMF) in plant water relations in sweet cherry and tomato (as a fast-grown model system). As AMF can assist host plants to increase water relations during drought stress, they may also help plants to moderate negative impacts of excess water, such as the splitting of soft fruit. The experimental chapters focus on two main areas. Firstly, the diversity and abundance of AMF in a conventional and organic sweet cherry orchard were investigated with a preliminary study. Secondly, the impact of AMF on cutting survival in sweet cherry, and growth and physiological functioning in both sweet cherry and tomato were assessed in several experiments. In all studies the level of phosphorous supplied to plants was low to encourage AMF colonisation and the amount of root colonisation was determined to ensure a substantial difference from the controls. To determine abundance and diversity of AMF in an organic and conventionally managed cherry orchard in two seasons, spores were extracted from soil samples, counted and spore morphology was examined by microscopy. Spore characteristics included three categorical variables (shape, colour, and pattern) and two continuous variables (spore diameter and the number of spore wall layers). The average number of spores in the conventional orchard soil was significantly higher than the organic site in both seasons. Canonical discriminant analysis of spore characteristics revealed that there was substantial overlap in the characteristics between each site and season, most likely reflecting a high similarity of species diversity. An objective of this study was to determine if the presence of AMF ‘Rhizophagous irregularis’ and local AMF species on sweet cherry rootstocks results in greater survival and establish more rapidly from cuttings inoculated with AMF than those not inoculated, and to have better regulation of water balance under conditions of water stress than for plants without mycorrhizae. It was found that inoculation with Rhizophagus irregularis (syn. Glomus intraradices) significantly increased the survival rates of cherry cuttings; however the effects on growth (height, stem diameter, leaf number and biomass) and on physiological variables (photosynthesis rate, stomatal conductance and leaf water potential) were limited. Treatments of low water exposure (50% of water holding capacity) followed by high water (150% of water holding capacity) were applied to established cherry plants. Local soil inoculum was used in an additional experiment and field soil was filtered to remove spores so that a complementary control treatment could be applied. Surprisingly, plants grown in the filtered soil had significantly higher AMF colonisation than unfiltered soil, and unfiltered soil had higher bulk density than filtered soil. The results showed that sweet cherry grown in filtered soil had significantly higher growth, photosynthetic rate and stomatal conductance than those grown in unfiltered. Due to the confounded effects of colonisation and bulk density, the role of AMF in these physiological responses is difficult to elucidate. Similar studies with tomato plants aimed to determine if the presence of mycorrhiza tomato plants would lead to better regulation of water balance under conditions of water stress than for plants without mycorrhizae. Tomato seedlings inoculated with R. irregularis showed enhanced growth in terms of height, number of leaves and biomass, compared with controls. Further, AMF tomato plants generally showed higher photosynthetic rate, stomatal conductance and an increased rate of transpiration which indicates better regulation of water balance under conditions of water stress than for plants without AMF. Therefore, the response of plants to AMF was more apparent for tomato than sweet cherry. In conclusion, several hypotheses were tested with experiments in this thesis to explore the role of AMF in sweet cherry and tomato growth and physiological functioning, particularly in relation to water fluctuations. While there were some experimental limitations, results reflected that AMF can assist plant establishment and potentially provide benefits to soft fruit crops to assist mediation of water stress.