Effect of the relative time of emergence on the growth allometry of Galium aparine in competition with Triticum aestivum

The impact of the relative time of emergence on the growth allometry of Galium aparine L. (cleavers) in competition with Triticum aestivum L. (spring wheat) was studied in a greenhouse experiment. The hypothesis that changes in growth allometry, particularly in the leaf area/height ratio, caused by a shift in the relative time of emergence, could lead to plants overcoming the competition or to competitive suppression was tested. The plant height, dry weight, total leaf area and vertical distribution were analyzed during the first 83 days of growth. The late emergence of G.aparine substantially influenced the growth dynamics for the dry weight and total leaf area. The dynamics of height growth were affected only following a significant delay in G.aparine emergence. The ratio of dry weight to total leaf area was almost unaffected by the competition. By contrast, the allometric relationship between the plant height and total leaf area was highly sensitive to the relative time of emergence. The results suggest that height is the most important growth trait for the plants to resist increasing competition. Even though the plants responded to competition with elongation, the physiological costs linked with shade avoidance caused reductions in the dry weight and leaf area growth that resulted in the total suppression of the late-emerging individuals, in particular when their emergence was >10 days after that of the crop.