Use of growth promoter microorganisms in substrates in order to improve seedling quality and reduce transplant stress

In horticulture, improving environmental sustainability and plant resilience is becoming more crucial. Most horticultural species are transplanted in soil rather than sown, unlike practice in the past. The aim of this work was to study the use of microorganisms as a tool to improve seedling quality in nurseries and their resilience in the field. During 2021 and 2022 in Northern Italy trials were carried out on tomato, lettuce, cucumber and leek seedlings. Bacterial inocula of Bacillus arayabattai strain AG 252 (BA), Azospirillum brasiliense strain Mex 22 (AB) and a combination of them (AB+BA) were added into growing substrates before use. Some quality descriptors (SPAD value, hypocotyl diameter, plant height, sturdiness and compactness index, foliage fresh and dry weight and dry matter) were collected in the nursery phase and in the open field, to evaluate vegetative recovery. At the end of the nursery phase treatments AB, BA and AB+BA improved hypocotyl diameter, AB+BA increased plant height, BA enhanced plant sturdiness, whereas AB+BA increased fresh weight in all the tested species. No statistical differences were shown in the dry matter, the chlorophyll content (SPAD value) and the root quality. However, when AB was applied to the cucumber and leek substrate, quality clearly improved. In the 20 days following transplant in the open field, AB performed better than BA, AB+BA and the test substrate, resulting in a higher fresh and dry weight. Despite of the great differences in the climatic conditions during the two years, Azospirillum brasiliense strain Mex 22 seemed to be effective by improving both nursery quality and resilience in the first phases after transplant. However, more studies are necessary to clarify the bacterial activity, mode of application, dosage, and their duration over time into substrate before use.