the effect of different weed-free and weed-infested periods on growth indices of black seed (nigella sativa l.)

in order to investigate the effect of different weed-free and weed-infested periods on the growth indices of black seed, a field experiment was conducted at the agricultural research station of ferdowsi university of mashhad, during 2009-2010 growing season. the experiment was laid out in randomized complete block design with 12 treatments and 3 replications. twelve experimental treatments consist of six initial weed-free periods in which plots were kept free of weeds for 0 (wf 0), 14 (wf 14), 28 (wf 28), 42 (wf 42), 56 (wf56), 70 (wf 70) days after crop emergence (dae), and then weeds were allow to grow until harvesting, and six initial weed-infested periods in which weed allowed to grow for 0 (wi 0), 14 (wi 14), 28 (wi 28), 42 (wi 42), 56 (wi56), 70 (wi 70) dae, then plots kept free of weeds to harvesting. the results indicated that black seed height was strongly affected by different weed free and weed-infested periods. interestingly, weed interference for entire growing season caused increase in black seed height by 39% compared with the treatment that was weed free throughout the growing season. also, these periods had significant effects on leaf area index, green area index, dry matter accumulation, crop growth rate and relative growth rate. mentioned traits increased with increasing duration of weed- free periods and were reduced by increasing length of weed- infested periods. however, these periods had no significant effects on net assimilation rate, specific leaf area and specific leaf weight.