Physiological, antioxidative and yield responses of three fennel genotypes to exogenous proline under drought stress condition

Proline is an amino acid with a crucial role in ameliorating environmental stresses, including drought. In order to study the effect of external proline on the response of fennel to drought stress, three fennel genotypes (namely Urmia, Shiraz, and Kerman) were exposed to two levels of irrigation, i.e. control (34-45% of maximum allowable depletion in soil water) and drought (75-85% of maximum allowable depletion in soil water) and two levels of foliar-applied proline (0 and 20 mM) in a 4-replicate factorial randomized complete block pot experiment. Drought led to significant decreases in chlorophyll (Chl) and carotenoids (Cars) concentrations, leaf relative water content (RWC), ascorbate peroxidase (APX) and peroxidase (POX) activities, leaf water potential (ѱw), plant dry mass (DM) and seed weight and significant increases in leaf proline and polyphenols concentrations, and catalase (CAT) activity. Foliar-applied proline resulted in significant increases in membrane stability index (MSI), RWC, ѱw, proline and polyphenols concentrations, APX activity, plant DM and seed weight but it led to significant decreases in CAT and POX activities. Genotype Urmia out-numbered the remaining genotypes in terms of Chl and Cars concentrations, MSI, CAT and APX activities. There were significant correlations between leaf proline concentration and plant DM and seed weight under drought condition. Foliar-applied proline affected Chl and Cars differently in the drought-stressed and non-stressed plants. Furthermore, it affected plant seed weight in a genotype-dependent manner, as evidenced by 150% and 110% increases in Urmia and Shiraz and a 42.5% decrease in Kerman. It may be concluded that drought suppresses fennel growth, but foliar-applied proline is potent to mitigate the depression through increasing the leaf Chl, Cars, proline, and polyphenols concentrations, RWC and certain antioxidative enzymes activities in genotypes such as Urmia and Shiraz.