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Growth of Kentucky Bluegrass Leaves and Tillers With and Without Defoliation¹

Tillering rates, leaf emergence rates, and responses to defoliation are important to productivity, density, and ersistence of grasses. Breeding programs might benefit a more precise understanding of these responses. Studies were conducted in the greenhouse on individual plants of Kentucky bluegrass (Poa pratensis L.) ‘Merion’, ‘Newport,’ and ‘Windsor’ to precisely describe leaf and tiller growth and the effects of defoliation on growth of these structures.

In the first of three studies, six plants of each cultivar, grown under uniform controlled conditions, were examined daily and leaf, tiller, and rhizome emergence recorded for 60 days. The second study differed from the first in that five plants of each cultivar were defoliated and five plants left intact. To provide uniform defoliation for all cultivars, entire leaf blades were removed at the time they reached full expansion. In the third study CO₂ exchange as a measure of net photosynthesis was determined for all leaf surfaces and for leaf blade surfaces only by coating leaf sheaths with grease.

Under these fairly constant conditions, leaf and tiller emergence on the main shoot proceeded at constant rates characteristic for each cultivar. On all cultivars the first leaf of the main shoot was dead before the sixth leaf had emerged. Thereafter leaf senescence and leaf emergence proceeded at the same rate so each shoot had a maximum of five functioning leaves. The same growth patterns were observed on primary and secondary tillers until affected by intertiller competition.

Defoliation did not affect leaf emergence rates on any cultivar while reducing the rate of tiller emergence on Newport and Windsor but not on Merion.

The cultivar Merion had a higher rate of net photosynthesis than Newport. Leaf blade photosynthesis rate did not differ significantly between the two cultivars. The higher rate for Merion was thus attributed to a higher rate for leaf sheaths. The greater tolerance to defoliation displayed by Merion could be due to the greater photosynthetic activity of its leaf sheaths.

These results show that it may be possible to develop Kentucky bluegrass cultivars with higher rates of leaf and tiller emergence and greater tolerance to mowing. The potential for breeding strains more tolerant of mowing by selection of genotypes with higher leaf sheath photosynthesis should be explored.

Key Words: Poa pratensis L. • Net photosynthesis • Clipping • Leaf senescence

² Professor, staff research associate, and former graduate research assistant (now at Texas Technological Univ., Lubbock, Tex.), respectively.

Received for publication April 14, 1975.