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 populat ions 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 affect ed, 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 solutio n every week, to compensate for the nutrients that had been absorbed from t he solution). When plants were grown in mixed culture with a 'renewed' nutr ient solution, the rate of mature leaf respiration in the absence of inhibi tors 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' c ultures, due to autotoxic and allelopathic effects. In contrast with GL66, for GL72 the effect of an inhibitor of the alternative path (salicylhydroxa mic acid, SHAM) differed between conditions and treatments. Effects of an i nhibitor of the cytochrome path (KCN) were affected by treatment in both po pulations. The increase in the rate of root respiration of GL66 due to auto toxic and allelopathic effects was accompanied by a higher rate of the SHAM -resistant component, i.e. maximum activity of the cytochrome path. Roots o f GL72 showed a higher control respiration rate and a higher rate of the SH AM-resistant component when long-term accumulation of root exudates was all owed to occur in mixed cultures (allelopathy). We conclude that allelochemi cals can modify both leaf and root respiration and that there is no consist ent correlation between yield and respiration for GL66 and GL72 in the pres ent study. This shows that selection for low rates of mature leaf respirati on is not an appropriate method to select for high-yielding cultivars in pe rennial ryegrass.