Responses of Hyssopus officinalis to bicarbonate stress and foliar application of green synthesized zinc nano-complex formed on Medicago sativa extract

The bicarbonate ions restrict the growth and productivity of crops by inducing nutrient deficiency in alkaline, calcareous soils. Zinc deficiency is widespread in these soils and is one of the factors limiting crop production. This study investigates the effects of foliar applications of green synthesized zinc nano-complex (Zn-NC: 0, 1, 2, and 4 mM) on growth, productivity, and bioactive compounds of pot-grown Hyssop ( Hyssopus officinalis ) under sodium bicarbonate stress (0, 10, 20, and 40 mM). Harvesting the plants at the flowering stage revealed that bicarbonate stress induced leaf area decreases and chlorophyll decline, which resulted in the loss of fresh root and shoot mass, the photosynthetic capacity of the plants (Fv/Fm), and root and shoot elongation. However, the mild bicarbonate stress induced the accumulation of essential oils (9.5%), phenols (18%), and flavonoids (19.2%) and increased antioxidant activity in the leaves (18.1%). Application of Zn-NC increased zinc content in the shoot (up to 112.1%), which consequently recovered plant growth and health. These improvements were in line with recovering protein content in leaves (up to 21.9%), which enhanced chlorophyll content and Fv/Fm. Some of these responses were associated with improved carbohydrate metabolism and auxin biosynthesis in the plant. This led to an increase in the production of essential oils (13.5%) and the biosynthesis of phenolics (10.9%) in the bicarbonate-stressed plants. Increased accumulation of these compounds and proteins in leaf tissue increased the plant's antioxidant capacity (10.6%). In this regard, the best results were obtained by the application of 2 mM Zn-NC.