Osmoregulation-mediated differential responses of field-grown fennel genotypes to drought

•Drought increased seed essential oil content and harvest index of fennel genotypes.•Drought tolerant genotypes exhibited high levels of proline and soluble carbohydrates.•Water use efficiency for seed and oil yield for the majority of genotypes increased with mild drought.

This study was aimed at examination of physiological and agronomic responses of fennel (Foeniculum vulgare Mill.) to water deficit. Twelve fennel genotypes namely 'Kashan', 'Urmia', 'Hamadan', 'Kerman', 'Shiraz', 'Birjand', 'Mashhad', 'Ardabil', 'Bushehr', 'Avicenna', 'Isfahan', and 'Yazd' were subjected to four levels of irrigation (irrigation after 35%, 55%, 75%, and 85% depletion of available soil water) in a field study in two years. Leaf water potential, relative water content (RWC), proline, total soluble sugars, chlorophyll a (Chl a), b (Chl b), total (Chl a + b), a/b (Chl a/b) along with dry mass (DM), seed yield and its attributes and seed essential oil content were measured. Water deprivation left significant effects on all characteristics, i.e. in contrast to seed essential oil content and harvest index, the rest of the attributes were decreased significantly with drought intensification. Varietal differences in response to drought were meaningful for most of the traits; i.e. genotypic variations for DM, grain yield and stress susceptibility index (SSI) were consistent with differences among the genotypes in physiological traits such as leaf water potential, RWC, proline, soluble sugars and chl content. Genotypes were discriminated according to their response to drought and SSI. Drought tolerant genotypes ('Yazd', 'Kerman, 'Mashhad' and 'Shiraz') exhibited a greater capacity for accumulation of osmotic solutes associated with higher leaf water potential and RWC, compared to drought sensitive genotypes ('Ardabil', 'Avicenna', 'Hamadan' and 'Birjand'). From our findings, fennel could be appreciated as a promising species in potentiating alternative industrial-medicinal crops in the face of the eminent challenge of water scarcity in arid and semi-arid climatic regions.