Effects of drought on competition, growth, and resistance of the lawn species Zoysia tenuifolia to the invasive weed Imperata koenigii.
In subtropical areas, there is frequent functional degradation of Zoysia tenuifolia lawn by invasion of Imperata koenigii during summer drought. To explore the relationship between these two species under drought, we planted Z. tenuifolia and I. koenigii in five different proportions (9 : 1, 8 : 2, 7 : 3, 6 : 4, 5 : 5), and after 40 days of growth, imposed four different drought treatments (adequate water, mild drought, moderate drought, and severe drought). After 60 days of the drought treatments, competitive indicators including relative yield, total relative yield, attacking power, and competitive equilibrium index were determined for both plant species. We also analyzed growth indexes including plant height, tiller number, leaf number, biomass, and the root : shoot ratio, as well as physiological indexes including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, and the contents of soluble sugars (SS), soluble protein (SP), proline (Pro), and malondialdehyde (MDA). These analyses allowed us to compare the effect of drought on the competition, growth, and resistance systems of the two plant species. The results showed that : (1) The degree of drought and proportion of I. koenigii significantly affected the aboveground and underground competition index of both plant species (P<0.05). Under mild drought conditions and with a small proportion of I. koenigii (≤30%), the underground attacking power index and competition balance index of Z. tenuifolia were both positive. Moderate and severe drought caused the loss of the interspecific competitiveness of Z. tenuifolia. When the proportion of I. koenigii was ≥20%, its antagonistic effect was strong enough to exclude Z. tenuifolia from the community. (2) Drought mainly affected the underground competitiveness of Z. tenuifolia, yet the invasion of I. koenigii mainly affected the aboveground competitiveness of Z. tenuifolia. The synergistic effect of drought and invasion of I. koenigii had a greater impact on the attacking power and balance index of underground parts of Z. tenuifolia than on its aboveground parts. When the degree of drought and the proportion of I. koenigii increased, the aboveground and underground competitiveness of Z. tenuifolia decreased to negative values. (3) Mild drought was beneficial for the growth of both plant species. Moderate and severe drought reduced the plant height, tiller number, leaf number, and biomass of Z. tenuifolia, but had little effect on I. koenigii. At proportions of >10% I. koenigii restricted the numbers of tillers and leaves of Z. tenuifolia, and at proportions of >20% I. koenigii reduced the biomass and root = shoot ratio of Z. tenuifolia. The synergistic effect of drought and invasion of I. koenigii inhibited the growth and regeneration ability of Z. tenuifolia. (4) The SOD, POD, and CAT activities of Z. tenuifolia significantly increased with increasing drought severity (P<0.05). Mild drought decreased the CAT activity of I. koenigii but did not affect SOD activity. More than 10%, 20%, and 30% I. koenigii increased the SOD, CAT, and POD activity of Z. tenuifolia, respectively. The enzymes were ranked, from strongest response to drought to weakest, as follows: CAT>SOD>POD in Z. tenuifolia; and SOD>POD>CAT in I. koenigii. In both plant species, the proportion of I. koenigii had the strongest effect on SOD activity, followed by CAT and then POD activity. Drought affected the antioxidant enzyme system of Z. tenuifolia more than did the proportion of I. koenigii. (5) Mild drought increased SS and SP contents of Z. tenuifolia and the SP content of I. koenigii. Moderate and severe drought increased the contents of SS, SP, Pro, and MDA in both plants (P<0. 05). More than 10% I. koenigiiincreased SS, SP, and MDA contents of Z. tenuifolia, and more than 20% I. koenigii increased Pro content of Z. tenuifolia. The synergistic effect of drought and proportion of I. koenigii aggravated the degree of injury in both plant species. The physiological indexes were ranked, from strongest response to habitat stress to weakest, as follows; SS>Pro>SP>MDA in Z. tenuifolia; and MDA>Pro>SS>SP in I. koenigii. Therefore, the basic requirements to maintain the health of Z. tenuifolia lawn are to avoid severe drought in summer and maintain the proportion of I. koenigii at less than 10%.