Arbuscular Mycorrhizal Fungi Improve Tolerance to Water Deficit in Indian Pennywort (Centella asiatica) by Promoting Physio-morphological and Biochemical Adaptations

Arbuscular mycorrhizal fungi (AMF) are a consortium of symbiont fungi present in the root zone of plants, which provide phosphorus and enhance the soil water holding capacity, resulting in low-cost input for plant growth and development. The objective of this study was to promote growth and development of Indian pennywort (Centella asiatica (L.) Urb.) via arbuscular mycorrhizal fungi (AMF)-regulated water deficit tolerance. One-month-old stolon propagated stocks were cultivated under greenhouse conditions by following a regular irrigation schedule for a month. Two treatments of soil samples were prepared: i) sterilized soil—AMF and ii) sterilized soil + AMF. Three water regimes, (i) well-watering (control; 95% field capacity), (ii) moderately water deficit (73% field capacity) and (iii) severely water deficit (66% field capacity) for 14 days, were applied. Root colonization in + AMF plants was larger than—AMF, leading to the enhanced phosphorus content in the leaf tissues and better plant growth performances. Leaf temperature (T_leaf) (+ 0.92 °C) and crop water stress index (1.74 folds) were significantly increased, when subjected to severely water deficit. In contrast, leaf greenness was increased by 1.42 folds over the control and the maximum quantum yield of PSII and photon yield of PSII were retained. In the case of gas exchange parameters, net photosynthetic rate, and stomatal conductance were sensitive to severely water deficit. Under without AMF-inoculation, net photosynthetic rate was decreased by 52.2 and 79.1% when subjected to moderate and severe water deficit conditions, respectively. Increment of free proline and glucose in the water deficit stressed plants were evidently observed as major osmolytes to control the leaf osmotic potential, especially under without AMF-inoculation. Under AMF inoculation, free proline and glucose concentration in the leaf tissues was stable even subjected to moderate and severe water deficit. Madecassoside content in + AMF plants was the maximum at 120 mg g⁻¹ dry weight, and it was retained under moderately water deficit at 118.9 mg g⁻¹ dry weight, whereas it declined by 37.11% over the control when subjected to severely water deficit. Similarly, centellosides yield in + AMF plants under moderately water deficit was increased over the control (− AMF) and declined by 5 folds when subjected to severely water deficit. In the case of herbal yield traits, number of leaves, leaf area and plant biomass of AMF-inoculated plants were greater than those of without AMF inoculation and also retained even subjected to WD stress. In summary, AMF inoculation is an alternative way to promote madecassoside, high biomass yield and high centellosides content as well as alleviate moderate water deficit stress during the production of Indian pennywort.