Effect of crop species and mixed ratios on morphological plasticity and competitiveness of Phalaris minor Retz.

Phalaris minor Retz. is considered as one of the world's most destructive weed species in farmlands and its control and management have attracted tremendous attention globally. Understanding the morphological plasticity and competitiveness of P. minor under different environmental conditions has been crucial for early warning and control of the weed. To study the morphological plasticity and competitiveness of the invasive P. minor plant, a greenhouse experiment was conducted with the de Wit replacement series method. In the experiment, P. minor was cultivated together with native crops of wheat (Triticum aestivum L.), broad bean (Vicia faba L.) and rape (Brassica campestris L.) in different mixed ratios (2∶1, 1∶1, 1∶2), respectively. Two-way ANOVA results indicated that the planting ratio of the native and invader species had no effect on P. minor branch number and main spike length (P ≥ 0.05), while plant species, mixed ratio and their interaction significantly affected P. minor plant height, seed number of main ear, flag leaf area and biomass (P ≤ 0.05 or 0.001). When planted with rape, P. minor plant height was significantly higher than when planted with wheat, broad bean or monoculture. The number of branches, length of main spike, seed number of main ear, flag leaf area and biomass significantly decreased when P. minor was planted with rape than when planted with wheat or broad bean at the same mixed ratio. When planted with P. minor, wheat, broad bean and rape had significantly decreased relative yields (RY) (below 1.0), indicating inter-specific competition between P. minor and native species was dominant. RY of P. minor was larger than 1.0 when planted with broad bean at all three mixed ratio, and with wheat with 1∶1 and 1∶2, indicating that intra-specific competition was dominant. RY of P. minor was below 1.0 when planted with rape, indicating competition between two species was dominant. Competitive balance index (CB) between P. minor and native species was significantly greater than 0, implying that P. minor had a stronger competitive ability than native crops. However, when P. minor was planted together with rape under mixed ratio greater than 1∶1, CB between P. minor and rape was less than 1, rape became stronger than P. minor. Consequently, rape had a higher competitive ability than P. minor. The branch number, length of main spike, seed number of main ear, flag leaf area and biomass of P. minor significantly dropped when planted with rape. This suggested that it was possible to use rape to inhibit the expansion and outbreak of P. minor.