Can mycorrhizal inoculation stimulate the growth and flowering of peat-grown ornamental plants under standard or reduced watering?

• Peat-pot grown ornamental plants were inoculated with AMF.
• Plant growth response was positively correlated with the level of root colonization.
• AMF stimulated flowering and growth of several tested plant species.
• The effect of AMF on P. zonale was tested under two watering regimes.
• Mycorrhizal low-watered P. zonale plants grew better than well-watered control plants.

Although the growth of plants is often successfully stimulated by inoculation with arbuscular mycorrhizal fungi (AMF), the question remains whether AMF are beneficial under the specific conditions of peat-based pot cultivation of ornamental plants. A series of two greenhouse experiments aimed on this question. In the first experiment, we tested the effect of inoculation with AMF, applied as a commercial inoculum, on various biometric parameters including the flowering of eight ornamental plant species. Capsicum annuum, Dimorphoteca sinuata, Gazania splendens, Impatiens hawkerii, Pelargonium peltatum, Pelargonium zonale, Sanvitalia procumbens and Verbena × hybrida were planted in pots with a peat-based substrate. AMF were naturally absent in this substrate. The plant species differed in their mycorrhizal growth response (MGR) evaluated as the effect of inoculation on shoot biomass. The MGR was positively correlated with the level of root colonization, which ranged from 17% to 68% depending on the plant species. Inoculation with AMF also significantly increased other growth parameters important for ornamental plants, namely the number of flowers (S. procumbens, Verbena × hybrida), flower size (I. hawkerii), shoot dry weight (P. peltatum, P. zonale and S. procumbens), root dry weight (G. splendens, P. peltatum and S. procumbens), the number of leaves (C. annuum, G. splendens, P. peltatum and P. zonale), plant length (C. annuum, P. zonale and S. procumbens), the number of branches (P. zonale and S. procumbens) and the total length of branches (S. procumbens). In the second experiment, P. zonale was used as a model plant grown under two watering regimes: standard and low. A strong positive effect of AMF on plants was observed under both watering regimes for all measured parameters (shoot fresh and dry weight, plant length, leaf area, number of branches and flowers). In all the parameters, inoculated low-watered plants performed significantly better than well-watered plants without AMF. We conclude that (1) inoculation with AMF improved the general vitality and visual quality of ornamental plants, and that (2) for P. zonale this stimulation occurred even under the low watering regime.