Morphological and physiological responses of black cumin to biochar and different irrigation regimes

Climate change accelerated the negative effects on plant production; Therefore, agricultural researchers are focused on identifying strategies that minimize the effects of climate change on crop production. Black cumin (Nigella sativa L.) is an annual herbal plant belonging to the Ranunculaceae family. It has been used in traditional medicine in the past. Black cumin has a lot of industrial and medicinal uses. Nowadays, regarding the importance of medicinal and aromatic plants, such as black cumin, different strategies should be applied to improve their traits in different conditions. Drought stress is an effective climatic factor. Biochar as an ecological input is introduced in recent years. It was hypothesized that biochar may alleviate drought effects. Accordingly, the aim of this study was to investigate the effect of biochar on morphological characteristics, physiological parameters activity, and yield of black cumin affected by drought stress.

To evaluate the effect of biochar and different irrigation levels on the morpho-physiological traits of black cumin an experiment was carried out in the greenhouse of the University of Kurdistan in 2018. The experimental factors included three irrigation levels (100%, 70%, and 40% of the field capacity of the soil) and two biochar consumption levels (0, and 15 t ha-1). The experimental design was a factorial based on a completely randomized with three replications. The amounts of field capacity and permanent wilting point were separately calculated for two different levels of biochar. Fourteen seeds of black cumin were sown in each pot. All of the pots were irrigated after sowing. The morpho-physiological traits included relative water content, total chlorophyll, photosystem II efficiency, plant height, flowering and secondary branches number per plant, capsule and seed number per plant, and biological and grain yield were measured. The statistical SAS software (version 9.3; SAS Institute; USA) was used for ANOVA. The mean of treatments was compared with the LSD test.

The results showed that irrigation and biochar interaction had a significant effect on the morpho-physiological characteristics and yield of black cumin. Biochar application decreased the negative effects of drought stress. The greatest RWC (86.13) belonged to 100% irrigation and biochar application. Given the increased weights of pots containing biochar and increasing the leaf water potential, it can be concluded that biochar application ultimately increased RWC due to increasing water holding capacity. At the 40, 70, and 100% irrigation levels, biochar increased total chlorophyll by 1.43, 13.64, and 5.31% compared with non-application of biochar, respectively. The maximum (83.2%) and minimum (70.72%) photosystem II efficiency were observed in 100% irrigation with biochar and 40% irrigation and non-application of biochar, respectively. Biochar application enhances chlorophyll content consequently increasing photosynthesis compared with no biochar application under drought stress. The highest number of flowering and secondary branches per plant was obtained from 100% irrigation and biochar application and the lowest number of mentioned traits belonged to 40% irrigation and non-application of biochar. Drought stress reduced capsule number per plant. The minimum number of capsules was obtained from 40% irrigation level. Biochar increased capsule number per plant and seed number per plant at 70% and 100% of irrigation levels and 40% and 70% of irrigation levels, respectively. The greatest biological yield belonged to 100% irrigation and biochar application. This treatment increased biological yield due to enhancing morphological traits and yield components. At the 40, 70, and 100% irrigation levels, biochar increased grain yield by 33.51, 34.12, and 10.72% compared with non-application of biochar, respectively.

The usage of biochar improved the relative leaf water content, total chlorophyll content, photosystem II efficiency, morphological traits, biological and grain yield of black cumin under drought stress. Overall it can be said that biochar not only can reduce the negative impacts of drought stress but only improve the growth of black cumin. Therefore, biochar can be used as a useful input in sustainable agricultural systems under drought stress.