Drought is the major yield-limiting stress factor, and it will remain so due to increased water demands of crop land driven by evapotranspiration increases related to climate change
[89,90][47][48]. Drought or water deficit conditions stimulate an oxidative burst in the cells
[27][49]. Drought-induced stomatal closure leads to photorespiration which accounts for up to 70% of H
2O
2 generated in the leaves
[91][50]. However, it is not completely harmful; crop performance requires a delicate balance between ROS generation and its detoxification
[24][51]. In many crops, water deficit occurs transiently and exogenous application of biostimulants is widely used for enhancing crop performance under different abiotic stresses
[60,67][18][25]. For instance, HA application increased antioxidant enzymatic protection and improved the expression of tonoplast intrinsic proteins (OsTIP), a sub-family of aquaporins which assist in the movement of water, small uncharged solutes, and gases, which contributed to drought tolerance
[88,92][46][52]. Humic acid is a product of the biodegradation of plant parts and microbes and has direct effects on plant growth and metabolism. Its growth promotive effects are reported in many crops, which cannot be attributed solely to hormone-like activity
[93][53]. Humic acid is also found to minimize oxidative stress in plants. Its application on millet seedlings which were subjected to water stress by withholding irrigation at the three–five leaf stage, increased seedling growth and antioxidant properties. In the study, water stress increased levels of O
2•− and H
2O
2 in the leaves of millet; however, HA application reduced the production rate of oxygen radicals along with the decreased activity of SOD and POD. Without the regulation of antioxidant enzymes, HA showed its effect on stomatal conductance, photosystem I and photosystem II activity which improved the photosynthetic performance while also helping to decrease ROS production and maintain membrane stability
[94][54]. The effect of PGPB and HA was examined in drought-affected sugarcane. In sugarcane, HA helped plants to recover from drought stress by enhancing the activity of SOD, CAT, and APX. On the other hand, PGPB-induced osmoregulation contributed to regulate leaf water potential and RWC by closing stomata efficiently, resulting in plant water preservation. These are involved in maintenance of the cellular microenvironment for continuing the metabolic and physiological activities in a better way, so that ROS production and oxidative stress are decreased
[95][55]. Another biostimulant, seaweed extracts, are widely used in many crops for increasing crop production
[96,97][56][57]. Application of commercial extract of
Ascophyllum nodosum on soybean exhibited higher water content, reduced wilting and better recovery as well as improved ROS scavenging under drought conditions
[98][58]. Similarly, the application of
A. nodosum seaweed extracts also increased antioxidant activity and reduced lipid peroxidation in
Paspalum vaginatum grass during water stress
[71][29]. Regulation of Pro, protein and carotenoid contents, and activity of CAT, APX, guaiacol peroxidase, and GR activity were conferred by foliar application of beeswax waste and licorice extract which were directly involved in ROS scavenging, reduction in malondialdehyde (MDA) level and prevention of chl breakdown. In addition, improvement in the quantum efficiency of photosystem II (
Fv/
Fm), net photosynthetic rate, stomatal conductance, transpiration, and water use efficiency were responsible for a reduction in ROS production
[99][59]. In field-grown maize, HA application with S-containing soil amendment, significantly increased SOD and CAT activities and reduces and H
2O
2 content under water stress conditions
[100][60]. Soil microbial community also improves above-ground plant health under stress and the use of PGPR has been gaining importance as a drought management strategy
[101][61]. Microbial inoculum of
P. fluorescens and
Bacillus amyloliquefaciens were reported to increase antioxidant scavenging in peppermint
[102][62]. Thus, growing evidence suggests that the application of biostimulants may be a cost-effective strategy to overcome drought-induced oxidative stress and improve crop health under stress. Some examples of the application of biostimulants and their effects on plant ROS generation and antioxidant system are summarized in
Table 1.