Response to Selection for Dark Respiration Rate of Mature Leaves in Lolium perenne and its Effects on Growth of Young Plants and Simulated Swards

Selections for slow and for fast rate of dark respiration ofmature leaves were made from within Lolium perenne cv. S23.Selected parents were pair-crossed to provide 15 F₁ familieswith slow respiration and 15 with fast. Dark respiration was inherited and families with contrastingrates were subjected to sequentially harvested growth analysesfrom the third leaf stage to that of 95 per cent light interceptionin a growth room. Seven periods of regrowth of simulated swardsof the families were then recorded. During development of theprimary canopy, growth of the selections did not differ untilthe final harvest interval. At this stage slow respiration familieshad faster (P < 0.05) net assimilation rate and greater plantdry weight (P < 0.05) and leaf area index (P < 0.05) thanthe fast respiration group. Relative growth rate followed thesame trend. In the swards after each regrowth dry matter yieldof the slow respiration group was greater than that of the fast. In another experiment, simulated swards of six slow respirationfamilies yielded more than swards of six fast respiration familiesover sequential regrowth periods in a glasshouse from May toNovember: S23 was intermediate. Differences were most duringAugust and September. Crop growth rate at each harvest correlated(P < 0.05 or P < 0.01) with previously determined leafrespiration at 25 °C. Leaf protein levels in August weaklycorrelated (r = +0.57, P < 0.05) with respiration rate perunit dry weight but there was a significant residual negativecorrelation (r = –0.67, P < 0.05) between the rateper unit protein and growth at that time. Results are discussedin relation to the concept of ‘maintenance-relàted’respiration. Lolium perenne L., perennial ryegrass, respiration, maintenance respiration, relative growth rate, leaf area ratio