Integrated analysis of multifactorial stress combination impact on citrus plants.

MAIN CONCLUSION: Multifactorial stress combination negatively affects citrus performance, especially when these plants are under three or more stresses, affecting citrus growth at different levels including phenotypic, physiological, biochemical and molecular levels. In nature, biotic and abiotic factors affect plant growth and development. "Multifactorial Stress Combination" (MFSC) refers to situations in which three or more stressors occur simultaneously or sequentially on plants. Its importance lies in the drastic reduction in plant survival under such complex stress scenarios. In this work, we studied the effect of five stresses and their combinations (deficiencies in nitrogen, phosphorus and potassium with water and heat stress) on Carrizo citrange, a citrus genotype widely used in physiological studies. Nutrient deficiencies were applied for three months using specific irrigation solutions. To impose heat stress, plants were maintained for three days in environmental chambers set at 24 °C (control) and 40 °C (heat), while drought was simulated by transferring them to dry perlite. MFSC clearly impacted plant phenotype, increasing leaf damage and decreasing shoot weight, particularly under three or more stressors. Gas exchange parameters and total pigment content were only affected under the combination of four or five stressors, respectively. Oxidative damage increased in plants subjected to five stresses, as indicated by increased malonaldehyde content. A progressive rise was observed in abscisic acid, jasmonic acid, salicylic acid, phaseic acid and indole-3-acetic acid as stress complexity increased, highlighting their involvement as key regulators of the plant stress response. The observed upregulation of galactose metabolism suggested an alternative pathway for energy production and sugar accumulation as essential responses to a complex stress scenario. Overall, results demonstrate the severe impact of MFSC on citrus development, with plant damage increasing exponentially under three or more stressors.