Leaf and root respiration of Lolium perenne populations selected for contrasting leaf respiration rates are affected by intra- and interpopulation interactions

Autotoxicity and allelopathy affect the yield of GL66 and GL72, two populations of perennial ryegrass (Lolium perenne L. cv. S23) that were originally selected for contrasting rates of mature leaf dark respiration, but under conditions where allelopathic effects could not occur and autotoxic effects were minimal. To investigate if the respiration properties are also affected, the populations were subjected to two conditions (monoculture and mixed culture) and two treatments (grown in `renewed', i.e. solution completely replaced every week, and `replenished', i.e. nutrients added to the solution every week, to compensate for the nutrients that had been absorbed from the solution). When plants were grown in mixed culture with a `renewed' nutrient solution, the rate of mature leaf respiration in the absence of inhibitors was higher in GL66 than in GL72. In GL66, this rate was unaffected by condition or treatment, whereas in GL72 it increased in the `replenished' cultures, due to autotoxic and allelopathic effects. In contrast with GL66, for GL72 the effect of an inhibitor of the alternative path (salicylhydroxamic acid, SHAM) differed between conditions and treatments. Effects of an inhibitor of the cytochrome path (KCN) were affected by treatment in both populations. The increase in the rate of root respiration of GL66 due to autotoxic and allelopathic effects was accompanied by a higher rate of the SHAM-resistant component, i.e. maximum activity of the cytochrome path. Roots of GL72 showed a higher control respiration rate and a higher rate of the SHAM-resistant component when long-term accumulation of root exudates was allowed to occur in mixed cultures (allelopathy). We conclude that allelochemicals can modify both leaf and root respiration and that there is no consistent correlation between yield and respiration for GL66 and GL72 in the present study. This shows that selection for low rates of mature leaf respiration is not an appropriate method to select for high-yielding cultivars in perennial ryegrass.