Interaction of Water and Nitrogen Reveals Growth and Physiological Characteristics Responses of Isatis indigotica Fort. Seedlings

Water content of medium and nitrogen concentration of nutrient solution are considered crucial for the growth, photosynthetic characteristics and antioxidant enzymes in cultivation of medicinal plant. We investigated the effects of various combinations of water content of medium (60, 75 and 90%) and nitrogen concentration (0, 5, 15 and 25 mM) of the solution on the growth, photosynthetic characteristics, antioxidant enzymes activities, malondialdehyde (MDA), proline content, protein contents of Isatis indigotica Fort. seedlings. The results demonstrated that the I. indigotica seedlings had the largest root dry weight and leaf dry weight at the mild waterlogging of 75% water content of medium and high nitrogen of 25 mM. Suitable nitrogen nutrition could effectively improve the net photosynthetic rate (Pn) and water use efficiency (WUE) so as to promote the growth of I. indigotica seedlings. The higher level of nitrogen could significantly improve the antioxidant enzyme activity of leaves under 90% water content of medium, and could also significantly improve the antioxidant enzyme activity of root under 75% water content of medium. The higher nitrogen level could increase the ability to resist waterlogging and reduce the damage degree by reducing the MDA and improving proline contents of I. indigotica seedlings. The leaf dry weight was positively correlated with net photosynthetic rate, stomatal conductance, transpiration rate, intercellular CO₂ concentration, water use efficiency, superoxide dismutase, and catalase activities, proline and protein contents, which indicated that the dry matter accumulation of leaf was directly affected by leaf photosynthetic parameters, water use efficiency, antioxidant enzymes, and osmotic adjustment substances. The results indicated that the photosynthetic parameters had the most significant effect on leaf dry matter after water and nitrogen treatment.