Regulation of citrus responses to the combined action of drought and high temperatures depends on the severity of water deprivation

Plants grown in natural environment are regularly subjected to different combinations of abiotic stresses. Recent studies revealed that citrus plants subjected to a combination of severe drought and high temperatures displayed specific physiological, hormonal, molecular and metabolic responses. In the present study, we have performed a long-term experiment combining moderate drought and heat in Cleopatra mandarin to evaluate the impact of the stress-sequence, intensity and duration. Our results support previous observation of high sensitivity of Cleopatra mandarin to abiotic stresses that include high temperatures. In this sense, a combination of drought and heat stress negatively impacts Cleopatra seedlings independently of the drought intensity. However, some responses to combined drought and heat depend on drought intensity, especially those involved in stomatal regulation. The intricate natural environment, abiotic stress combinations and global climatic changes increase the complexity of studying plant responses to stress factors in the laboratory. Consequently, new experimental approaches taking in consideration different stress combinations should be implemented to study the viability of Cleopatra mandarin as a rootstock in a rapidly changing environment.