The effects of ericoid inoculum source and nutrient addition on growth and reproduction in highbush blueberry

Ericoid mycorrhizal fungi (EMF) form symbiotic relationships with ericaceous plants such as Vaccinium corymbosum, or highbush blueberry and assist in nutrient acquisition. EMF help plants thrive in stressful environments by increasing the area in which roots can uptake water and nutrients. In plant-mycorrhizal symbioses, nutrient uptake may depend on the identity of the fungal partner. Therefore, differently sourced mycorrhizal fungi could show differences in nutrient uptake ability. Here, I hypothesized that inoculation of V. corymbosum with EMF would enhance plant growth and investment in reproduction, and that effect would be more pronounced for plants in low nutrient conditions. I also hypothesized that inoculum source would affect plant growth and reproduction under varying nutrient conditions. To test this, I used 135 potted highbush blueberry plants that were inoculated at planting with either 1) commercial inoculum, 2) local inoculum, or 3) uninoculated control. Within these inoculum treatments and the control, I haphazardly assigned 15 plants to one of three fertilizer treatments using SUPERthrive fertilizer (N:P:K ratio of 4:1:1) at: i) the amount recommended for field grown plants, ii) half the recommended amount of fertilizer, and iii) no fertilizer. I predicted that plants inoculated with EMF from local soils would be better able to access nutrients than those inoculated with commercial inoculum or non-inoculated controls, and the effects would be measurable through increased plant growth and reproduction.Inoculation enhanced plant size (F2,161=3.157; P=0.045), the number of flowers produced (F2,112=3.736; P=0.027), and the number of berries produced (F2,113=3.653; P=0.029). However, fertilization had no significant effects on any of the response variables measured. Plants that received the local soil inoculum were significantly larger than commercially inoculated plants (F2,161=3.157; P=0.045). Plants in the local inoculum treatment produced significantly more flowers (F2,112=3.736; P=0.027) and berries (F2,113=3.653; P=0.029) than plants in the commercial inoculum treatment, and plants responded differently to the inoculum treatments in the two years of the study (F2,121=6.371; P=0.002). Additionally, initial plant size had a significant effect on the total number of berries produced (F1,106=7.047; P=0.009). The relationship between plant size and number of berries differed between inoculum treatments. Plants in the commercial inoculum and control treatment showed a positive correlation between plant size and berries produced, while the number of berries on plants in the local inoculum treatment remained constant as plant size increased (F2,107=3.320; P=0.040). Despite the order of magnitude differences in fertilizer applied, concentrations of N, P, K measured in soils taken from plants in the different treatments were all within the optimal range recommended for highbush blueberry. However, NH4 and K soil concentrations differed between inoculum treatments, revealing that differently sourced mycorrhizal fungi may differ in nutrient uptake.