Co-planted barnyardgrass reduces rice yield by inhibiting plant above- and belowground-growth during post-heading stages

Barnyardgrass (Echinochloa spp.) is the most common noxious weed in rice paddies as it inhibits rice growth and reduces grain yield. To date, little information is available on above- and belowground-growth changes in rice due to neighboring barnyardgrass. This study aimed to investigate the changes in root traits and shoot growth of rice when it is grown with different kinds of barnyardgrass. Japonica rice plants (var. Nanjing 9108) were co-cultured with two varieties of Echinochloa crusgalli (L.) Beauv. (EP, var. mitis (pursh) Petern; EH, var. zelayensis (H.B.K.) Hitchc), and E. colonum (L.) Link (EL) in the field in 2017 and 2018. Four treatments included control (i.e., weed free rice plants) and co-cultures with each of three barnyardgrasses (EP, EH, and EL). The results revealed that EP, EH, and EL treatments significantly reduced rice grain yields by 30.6%–36.2%, 42.5%–46.5%, and 10.6%–14.3%, respectively. Shoot growth including shoot dry weight, leaf photosynthetic rate, zeatin (Z) and zeatin riboside (ZR) in grains, and activities of key enzymes involved in sucrose-to-starch conversion in grains and root traits, such as length density, root dry weight, total absorbing surface area, active absorption surface area, oxidation activity, and Z + ZR contents in roots were dramatically reduced during post-heading stages of rice when grown with the three kinds of barnyardgrass. Moreover, above-mentioned rice shoot growth indices were strongly and positively correlated with root traits. These results suggested the decrease in rice shoot growth and root traits during post-heading stages contributes to the reduction in the rice yield when it grows with barnyardgrass neighbors.