The critical periods of weed control on the weed species composition, morphological, physiological, and biochemical characteristics responses of black seed (Nigella sativa L.)

A field experiment was conducted to the determine the critical period of weed control and its effect on the weed composition, the yield components, seed yield, oil yield, oil content, and the major secondary metabolites accumulation in the black seed (Nigella sativa L.). Two treatment series: weed-free (WF) and weed-infested (WI) periods, were performed from the seed emergence of the black seed until its maturity. The plots were kept weed -free for 0, 14, 28, 42, 56 and 70 days after emergence of the black seed and vice versa. In the weed community composition, 19 species from 14 families were recorded in the studied experimental field. Among the weeds, the Poaceae family (5) had the highest number of species, followed by the Fabaceae family (2). The major species were Avena sterilis L., Rapistrum rugosum L. (All), and Convolvulus arvensi L. during the harvesting of black seed. Results also revealed that the seed yield, yield components, seed oil content, and oil yield of the black seed significantly decreased due to the longer periods of WI or shorter periods of WF. In this study, weed-black seed competition periods promoted the total alkaloid and total phenolic accumulation, compared to WI periods, which indicated the plant's defensive response against the stress caused by the weeds. However, the decrease in seed yield and oil content indicates the insufficient levels of these compounds to cope with the weed stress. This study also concluded that the highest seed yield was obtained by keeping the plots WF from 42 or 70 days after emergence of the black seed to prevent yield loss higher than 10%. Therefore, this period could be the critical period of WF in the black seed.