Light and heavy turfgrass seeds differ in germination percentage and mean germination thermal time

Seed weight is often related to germination percentage (GP) and mean germination time (MGT) within seed lots, but the relationships are poorly described. In this study, we described these relationships in the species slender creeping red fescue (Festuca rubra L. subsp. litoralis Vasey), perennial ryegrass (Lolium perenne L.), and Kentucky bluegrass (Poa pratensis L.). Seeds from one seed lot of each species were graded into nine seed weight fractions in an air separator. For each seed weight fraction, the 1000-seed weight (TSW) was determined and the weight distribution was described for each seed lot. Seeds from each seed weight fraction were germinated in standard laboratory tests at 5/15degreesC and 15/25degreesC. For each species and temperature regime, the relationship between TSW and GP and between TSW and MGT could be described for whole populations by two different nonlinear functions, with GP increasing and MGT decreasing with increasing TSW. When the seed fraction with the lightest seeds was excluded, both relationships could be described for the remaining seeds by linear functions with positive effect of TSW on GP and negative effect on MGT in all but one case. Mean germination thermal time (MGTT) was calculated for the two temperatures. With exclusion of the lightest seeds, there was a linear relationship between TSW and MGTT for the two temperatures with light seeds of all three species requiring more degree days for germination than heavy seeds.