Assessing photosynthetic performance of fennel (Foeniculum vulgare Mill) influenced by plant growth regulators and drought stress imposed at vegetative and reproductive stages

Maintaining crop productivity under limited irrigation water requires some new strategies. This study investigated the influence of drought stress and the application of plant growth regulators (PGRs) including jasmonic acid, brassinosteroids, and putrescine on photosynthetic performance of fennel (Foeniculum vulgare Mill). The results indicated that fennel exhibited active osmoregulation which prevented a sharp decrease in relative water content. Fennel successfully maintained high leaf chlorophyll index, Net photosynthesis rate, and transpiration under mild stress, however, severe stress reduced the photosynthetic parameters by 22%, 24%, and 50%, respectively. Drought stress increased chlorophyll a fluorescence when fennel plants exposed to the stress condition. V_k and W_k parameters related to the donor side of photoinhibition of photosystem II (PSII) increased by 44% when severe drought stress imposed at the vegetative stage and 34% when occurred during the flowering. The elevation of V_k and W_k indicated a failure in water splitting in PSII. The V_J and V_I parameters of acceptor sides increased by 16% and 22%, respectively when drought stress imposed at the vegetative phase and to 19% and 30%, when drought stress occurred during reproductive phase. Using PGRs resulted in reduced V_J, V_I, V_k, and W_k, suggesting that some degree of recovery of damages occurred. All three PGRs stimulated biomass production and on average, plants yielded roughly 1.6 fold higher than the control plants. The influences of PGRs were mainly independent of drought stress level.