Biochar derived from different sources ameliorates the phytochemical and physiological characteristics of foxtail millet (Setaria italica) under water stress

Organic material, such as biochar, can improve soil fertility and crop yield under water stress. A pot experiment was conducted to consider the effects of biochar on the pigment content, enzyme activity, physiological characteristics, and nutrient uptake of foxtail millet under water stress. Plants were tested under normal, mild, and severe irrigation regimes (100%, 75%, and 50% field capacity, respectively) using different biochar sources (licorice root biochar (LRB), cow manure biochar (CMB), cotton residue biochar (CRB), and municipal waste biochar (MWB)) and a control without fertilizer. Under severe water stress, the chlorophyll a content and total chlorophyll increased by 137.3% and 131.2%, respectively, after applying MWB compared to the control. Carotenoid content in the MWB treatment increased by 46.4 compared to LRB. As water stress intensified from normal to severe, the catalase (CAT), peroxidase (POD), and superoxide dismutase activity increased by 199.1, 86.1, and 67.4%, respectively. Severe stress enhanced CAT activity in the MWB group, while the POD activity decreased compared to other treatments. Under mild and severe water stress, proline content in the MWB group were 26.9 and 30.9% higher, respectively. The application of MWB increased the plant height, biological yield, and number of grains per spike by 102.6%, 271.4%, and 174.8%, respectively, compared to LRB. In addition, applying MWB increased the grain yield between 191.5% and 285.7% compared to the control. Higher relative water content and nutrient availability in MWB facilitate the maintenance of photosynthetic pigments and enzymatic balance under water deficit. This increases the number of grains per spike, P and K content, and ultimately the grain yield of millet.