Does the shade avoidance response contribute to the critical period for weed control in maize (Zea mays)?

P>The effect of early weed emergence on crop seedling development has not been analysed within the context of a critical period study. Experiments were conducted to quantify the influence of a low light quality environment (i.e., low ratio of reflected red to far-red light (R/FR)) on maize seedling growth and development under non-limiting resource conditions. Weed-addition and -removal series were constructed, such that the effects of R/FR on seedling growth and development were isolated from those of direct competition. Maize seedlings responded to the presence of weeds within 24 h of addition by increasing plant height, which was followed by a subsequent reduction in the rate of leaf appearance. Seedling biomass and leaf area decreased linearly in the weed removal series with increasing duration of weed presence. Conversely, seedlings in the weed-addition series were unaffected. These results demonstrate that early exposure to weeds reduced the rate of seedling growth and development and that this effect was most pronounced if it was initiated upon emergence. This suggests that the existence of a period of developmental sensitivity to R/FR precedes the defined critical period for weed control in maize. These early physiological changes triggered by the R/FR ratio may contribute to the onset of the critical time of weed removal.