Does humic acid foliar application affect growth and nutrient status of water stressed maize?

Maize (<i>Zea mays</i> L.) is one of the most important crops worldwide, but its productivity is at high risk due to the advancing climate change, increasing both drought and waterlogging events. Thus, developing mitigation strategies to combat the negative effects of water stress is fundamental in agriculture to guarantee food security. Humic acids are well-known to positively affect drought tolerance, but data on its efficacy under waterlogging are lacking. Therefore, this study aimed to elucidate the effect of a HA product on maize growth and nutrient status under different water stress scenarios. Maize was soil-grown under three water stress scenarios (compared to control) for two weeks: waterlogging, alternate waterlogging–drought stress, drought. On the day of stress application, HA (1% v/v) were sprayed to the leaves. SPAD values were recorded during the stress treatments. Plants were harvested right after the stress ceased and fresh weight, P and Zn status were analyzed. Drought decreased shoot fresh weight, while it was unaffected under waterlogging. This is contradicting SPAD readings, which showed a significant decrease over time under submergence, but not under drought. Under alternate stress no growth reduction was apparent, though SPAD values declined under waterlogging but stabilized upon alteration to drought. Humic acid application was in all cases not effective. Although anthocyanin discoloration occurred under waterlogging stress, P deficiency can be excluded in direct comparison with the control. Interestingly, Zn concentration dropped under waterlogging but not the other stresses, but which was alleviated by HA application. However, no HA effect was apparent under alternate and drought stress. It can be concluded that the tested humic acid product has the potential to mitigate waterlogging stress by improving the Zn status of maize, thus contributing to a higher tolerance against waterlogging induced oxidative damage.