Drought stress is one of the major abiotic stresses that drastically affects crop production around the globe. Exposure to drought stress poses serious challenges for the survival of plants because it affects plant growth and reduces the harvestable yield of plants.
Plants have adopted certain mechanisms to respond to various environmental stresses. Maintenance of turgor through the accumulation of osmoprotectants, decreases the transpiration rate and closure of stomata helps to minimize the drastic effects of drought stress. The limited water conditions decrease the uptake and translocation of nutrients, therefore the foliar application may be an alternate and effective approach to improve the nutrients availability to plants. Foliar applied NPK fertilizers significantly contribute towards improved yield through increase in biomass of the plants.
The mechanistic processes by which foliar applied nutrients are taken up are through leaf stomata and hydrophilic pores within the leaf cuticle. The decline in nutrient concentration of water-stressed plants can be described by the fact that under water-limited conditions, diffusion rate and mass flow of nutrient from rhizosphere to the root surface becomes slow due to the replacement of water by air in the soil pores, ensuing in less availability of these nutrients to the plants.
The diffusion coefficient of P in soil is very low, hence the root zone P is depleted and plants cannot absorb P. Therefore, the utilization of P as a foliar application becomes increasingly important. Under water-limited conditions, P increases the early root growth and K root weight and root length. High root density plays a crucial role in obtaining water from the deeper soil layers, whereas deeper roots decrease the moisture loss in the soil. The promotion of root growth might increase water and nutrient uptake by plants.
Foliar application of NPK based fertilizers helps in maintaining the dry matter stress tolerance index, maintaining the water status of plants possibly through osmotic adjustment. Increase in leaf K concentration decreased the water potential of plants through maintaining the turgor pressure in sunflower. Foliar-applied N increased the photosynthetic rate under water stress conditions in various plants, like maize and wheat. It is well recognized that N enhances the cell number and cell size and increases the efficiency of photosynthetic rate in leaves. The decrease in photosynthetic rate might be due to the stomatal closure which restricts the carbon uptake by the leaves.
Potassium has been shown to play a significant role in the opening and closing of leaf stomata which control the movement of CO2 into the plant and water into the air, and would, therefore, have an effect on stomatal conductance. Potassium application improved the photosynthetic rate, transpiration rate, and intercellular CO2 concentration in sweet potato-under water stress conditions.
Phosphorous under mild water deficit improved water use efficiency in P treated wheat plants. The encouraging effects of P on plant growth under water stress have been ascribed as to enhancing the efficiency of P, stomatal conductance and water use efficiency.
Therefore, NPK based foliar fertilizers improve plant’s performance in drought situations.