Stress combination: When two negatives may become antagonistic, synergistic or additive for plants?

Plants in their natural environment are constantly subjected to various abiotic and biotic stressors and, therefore, have developed several defense mechanisms to maintain fitness. Stress responses are intricate and require various physiological, biochemical, and cellular changes in plants. The reaction mechanisms in plants subjected to drought, salinity, or heat stress alone have been explained in numerous studies. However, the field conditions are significantly different from the controlled laboratory conditions. In the field, crops or plants are simultaneously exposed to two or more abiotic and/or biotic stress conditions, such as a combination of salinity and heat, drought and cold, or any of the abiotic stresses combined with pathogen infection. Studies have shown that plants' reactions to combinations of more than two stress factors are distinct and cannot be explicitly deduced from their responses to different stresses when applied separately. Therefore, additional research is needed to understand the complete mechanism of plant responses to stress by analyzing data between single stress and multiple stress responses. This review aims to provide an overview of current research on plant responses to a combination of various stress conditions and their influence on the metabolic, transcriptional, and physiological characteristics of plants.