Abstract
Castor bean is considered as an important non-edible oilseed crop and source of castor oil, which has many applications ranging from cosmetics to the biofuels industry. Humic acid (HA) results from organic matter decomposition and is beneficial to plant growth and development. In the present study, a two-year experiment was conducted in Damghan, Iran, to study the physiological responses of castor bean to foliar application of zinc nano-chelate (Zn-nano) and HA under drought stress. The drought stress was used as the main treatment in three levels: normal irrigation as control, irrigation up to 75 BBCH scale (Biologische Bundesantalt, Bundessortenamt und Chemische Industrie) (mild stress), and irrigation up to 65 BBCH (severe stress). Foliar application of HA in three levels (non-application, application of the recommended rate and two times more than the recommended rate), as well as Zn-nano fertilizer in two levels (application at 1.5 part per thousand (ppt) and non-application) as subplots. The drought stress, HA, and Zn-nano fertilizer could significantly affect the number of capsules, the number of seeds, 100-seed weight, seed yield, oil yield, protein percentage and yield, activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), and chlorophyll a (Chl a), chlorophyll b (Chl b), and total chlorophyll (total Chl) contents. In addition, severe drought stress resulted in reducing the number of capsules (33.9%), the number of seeds (32.7%), 100-grain weight (16.0%), as well as seed (43.0%), oil (59.3%), and protein (29.9%) yield. Based on the results, the highest yield components, oil and protein contents, and photosynthetic pigments were achieved in the foliar application of HA (recommended rate) and Zn-nano fertilizers under normal irrigation during the second year. Further, the foliar application of Zn-nano fertilizer led to a decrease in the activities of CAT, SOD, and POD enzymes. According to partial regression analysis, the recommended rate of HA application the changed the nature of relationships governing the characteristics, especially under drought stress conditions. Finally, the foliar application of HA (recommended rate) and Zn-nano fertilizers could create an excellent resistance to drought stress in castor under dry and semi-arid climate conditions by improving yield and yield components and physiological traits.
Highlights
Castor bean is an indeterminate, non-edible industrial oilseed crop belonging to the Euphorbiaceae family, which is found in most tropical and subtropical parts of the world (Anjani et al, 2018)
Yield and yield components Based on the ANOVA results (Table 3), the effects of year (Y), drought stress (D), Humic acid (HA), and zinc nano-chelate (Zn-nano) fertilizer were significant on the number of capsule and seed, 100-seed weight, and seed yield
Regarding yield and yield component characteristics, the results indicated that mild stress with normal irrigation treatment had insignificant effects
Summary
Castor bean is an indeterminate, non-edible industrial oilseed crop belonging to the Euphorbiaceae family, which is found in most tropical and subtropical parts of the world (Anjani et al, 2018). The oil content of castor bean accessions was reported to vary between 45.7 to 54.0% (Roman-Fiueroa et al, 2020). The oil of the plant is regarded as the only commercial source of ricinoleic acid (over 85% of the oil), which has many industrial and pharmaceutical applications such as aviation fuels, fuel additives, paints, dyes, biopolymers, perfumes, and biodiesel (Liv et al, 2012; Ramanjaneyulu et al, 2013). Planting and researching castor bean have attracted a lot of attention due to an increased demand for its oil in the world, along with its growth in marginal lands (Sujatha et al, 2008). Mozambique, China, and Brazil are the major castor bean producing countries. The results of surveying planting statistics of this plant in Iran indicated that its production and cultivation increased (Sadeghi-Bakhtavari and Hazrati, 2021)
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