The effect of cytokinin foliar on morpho-physiological traits, yield and yield components of black cumin (Nigella sativa L.) under salinity stress conditions

Salinity as one of the most important abiotic stress that reduce the growth, development and yield of crops and  the use of plant growth regulator is one of the beneficial methods to reduce unfavorable effects of salinity stress. Black cumin (Nigella sativa L.) is an annual plant from the buttercup family that used widely in traditional and industrial pharmacology and seeds or their extracts contain anti-diabetic, antihistaminic, antihypertensive, anti-inflammatory, anti-microbial, antitumour, galactagogue and insect repellent effects.

In order to investigate the effects of foliar application of cytokinin on morpho-physiological traits, yield and yield components of black cumin (Nigella sativa L.) a factorial experiment was conducted based on randomized complete block design with four replications at the greenhouse condition. The experimental factors included salinity at five levels of 0, 3, 6, 9 and 12 dS m-1 and foliar application of cytokinin at three concentrations of 0, 100 and 200 μM. This experiment was performed inside 5 kg plastic pots with a height 21 and openings diameter 23 cm under greenhouse conditions. The substrate composition consisted of a 2:1:1 ratio of arable soil, rotted and sifted manure and aerated sand. The average day and night temperatures of the greenhouse were 27±2 and 18±2°C with relative humidity between 65 and 80%, respectively. Four plants were kept inside each pot and the rest were thinned. Foliar application of cytokinin was performed one stage at the beginning of flowering and the second stage one week after the first foliar application. Physiological traits such as electrolyte leakage, relative water content, chlorophyll pigments and proline were measured one week after the second spraying. Plant height, number of number secondary branch and yield components traits were measured after complete plant maturity.

The results of analysis of variance showed that salinity treatment has significant effect on all traits. Salinity stress increases electrolyte leakage and proline, though it has a declining effect on other traits. Was not significant reduction in traits up to 3 dS m-1 of salinity. The highest grain yield with 2.42 g pl-1 was obtained in the control treatment and the lowest grain yield with 0.81 g pl-1 at a concentration of 12 dS m-1 of salinity. Cytokinin treatment has significant effect on all traits except chlorophyll a, proline and number of capsule in plant. Foliar application of cytokinin reduced unfordable of salinity stress in black cumin, and 100 μM concentration of cytokinin has high efficiency than 200 μM. The highest percentage of electrolyte leakage, proline and carotenoid content was observed at a salinity level of 12 dS m-1. The lowest of plant height, relative water content, chlorophyll pigments, number seed in capsule, number secondary branch, number capsule in plant, weight of thousand seeds and biological yield observed in 12 dS m-1 of salinity level. The lowest grain yield (1.51 g pl-1) was observed in the control treatment and the highest grain yield (1.83 g pl-1) was observed in the concentration of 200 μM of cytokinin. Foliar Cytokinin was effective on all studied morpho-physiological traits under salinity stress. Under salinity stress, cytokinin application only affected the number of grains in capsule and had no significant effect on yield and other yield components traits.

The results obtained in this study showed that the black cumin can tolerate salinity up to 3 dS m-1 without any significant reduction in its yield. 1000-grain weight, number of grains per capsule and proline content were the least sensitive to different salinity levels. Biomass yield was also the most sensitive, so that it showed a significant decrease in all salinity levels. The concentration of 100 μM cytokinin was better than 200 μM. The results showed that foliar application of cytokinin under salinity stress may be improve some traits, however, this does not constitute a definite increase in yield and yield components under salinity stress conditions and may not result in a change in grain yield.