Abstract
The legume white clover (Trifolium repens L.) is considered the most important forage plant in New Zealand. It significantly contributes to increase the quality of pastures through its ability to fix atmospheric nitrogen; providing high nutritional feed for livestock. Trifolium repens is a common host for the Potexvirus White clover mosaic virus (WClMV), the latter being readily mechanically transmitted via animal trampling and grazing, as well as cropping practises. WClMV infection may strongly impact white clover’s performance, hence affecting its ability to compete with other plants in the pasture. In the field, the legume may also experience important stress induced by excess water in soils mainly due to over-irrigation or heavy rainfall, a phenomenon likely to occur at higher frequency as a consequence of climate change. When plants are subjected to biotic and abiotic stresses, the outcome often involves the formation of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), superoxide anion (O2•-) and hydroxyl radical (HO•). As highly reactive species, ROS can damage the cell’s components, including DNA, proteins and membrane lipids. To counteract the effect of ROS, plants have evolved non-enzymatic and enzymatic antioxidant mechanisms that detoxify ROS and prevent cellular damage. The objective of the current study was to investigate the effects of WClMV infection and root-flooding on Trifolium repens’ performance, oxidative damage and the induction of antioxidants. These experiments were conducted using two cultivars of white clover, cv. Huia and cv. Kopu II.
In response to both WClMV infection and root-flooding, white clover experienced oxidative stress as evidenced by the accumulation of lipid peroxides and protein carbonyls. Biomass accumulation, given as fresh weight and dry matter content, was affected by both treatments but differently between varieties, cv. Kopu II being more sensitive than its counterpart. Exposure to both stresses induced an increase in ascorbate and glutathione levels, as well as enhancing the activity of the enzymatic antioxidants superoxide dismutase, ascorbate peroxidase, glutathione peroxidase, catalase and glutathione reductase. Glutathione S-transferase activity was not affected by both treatments. The levels of reduced ascorbate and glutathione were unchanged in WClMV-infected plants, whereas the root-flooding treatment significantly decreased their availability for the plant cell. Interestingly, the interactive effect of Virus x Root-flooding was more damaging for the clovers, showing higher levels of oxidative stress and a down-regulation of antioxidants when compared to uninfected/root-flooded plants. Under flooding treatment, white clover developed aerenchymatous tissues in the roots, an important feature that may significantly improve the legume’s tolerance towards submergence-induced stress. Furthermore, from pasture feeding analyses, WClMV infection was shown to have no effect on the nutritional composition of the clovers, in either cultivar.
Understanding how some biotic and abiotic factors can adversely affect the ability of Trifolium repens to grow and compete with other plants in the field is an important aspect for farmers, as it would provide valuable information for improving the quality of the pastures in New Zealand.