Physiological Responses of Sunflower (<i>Helianthus annuus</i> L.) to Multiple Combined Prolonged Drought Stress, Salinity Stress and Boron Toxicity: Insights from Pre‐ and Post‐Recovery Stages

ABSTRACT The study aimed to determine how the physiological responses of the sunflower ( Helianthus annuus L.) plant were affected by prolonged drought stress, salinity stress, and boron application, as well as to assess the recovery dynamics following re‐watering. The experimental design included well‐watered (WW 80% watering), drought stress (DS, 20% watering) salinity stress (SS, 0 control and 13 dS m −1 ), boron toxicity (Na 2 O 5 B 2 O 3 .10H 2 O, at different doses of 0 and 8 mg L −1 ) and re‐watering after a long‐term period of drought stress (24 days). The well‐irrigated (80% WW) treatment, which included all factors as a the non‐stressed control treatment during the experiment was carried out with five replications. Morphological, physiological and biochemical analyses of plants were measured at four time points: at the 10th and 24th days after the onset of the drought stress period and after re‐watering, at 2nd and 7th days following. The relative membrane permeability was increased and relative water content was decreased because drought and salinity stress limited water availability and caused an imbalance in the water status of the leaves and stem of the plant. Even though high levels of Na + and Cl − ions interfered with essential nutrient uptake under drought stress and boron application, Ca +2 ion levels in the leaves significantly increased in the leaves of plants in areas treated with drought, salt, and boron after re‐watering. Extended or intense drought and salinity conditions harmed the phloem and xylem tissue cells of the stem by changing cell size and density, which in turn disrupted biochemical processes, including the functioning of water channels under challenging circumstances. Particularly under conditions of salt and drought stress, the vascular bundles in the plant stem were observed to either shrink significantly or assume an irregular shape. Long‐term drought reduced relative water content (RWC) values, resulting in plant dehydration and increased osmotic pressure (RMP) in leaf cells, further exacerbated by salinity and drought stress. The plant attempted to regain some of its characteristics in response to these severe stress conditions after re‐watering. However, 24 days after the long dry period, even if watering was re‐applied, the growth power of the plant was reduced due to the disturbance in membrane permeability as a result of excessive cell damage.