LEAF APPEARANCE RATE AND FINAL LEAF NUMBER OF SWITCHGRASS CULTIVARS

In switchgrass (Panicum virgatum L.), maximum yields occur with late f lowering genotypes that have an extended duration of vegetative growth . However, little is known about the developmental basis for a long ve getative growth phase in perennial grasses. Studies were carried out o n the leaf development of five switchgrass cultivars with varying flow ering times (Cave-in-Rock, Caddo, Kanlow, NC2, and Alamo) to provide i nputs for crop simulation models and to determine if a long duration o f vegetative growth was associated with a reduced rate of leaf appeara nce or a high final leaf number. The studies were carried out in the f ield at College Station, TX, (30 degrees 38'N, 96 degrees 20'W), durin g 1994 and 1995. Thirty tillers emerging in spring were tagged and vis ible leaves were recorded weekly until panicles emerged. Final leaf nu mbers were recorded on the flowering tillers which had emerged in spri ng and after a June defoliation. Tillers of all cultivars emerged in M arch and panicles emerged during a 2-mo period from May to July. Final leaf number of spring-emerging tillers of all cultivars ranged from n ine to 11, whereas summer-emerging tillers powered after seven leaves had appeared. The phyllochron [growing degree days (GDD) between the a ppearance of two successive leaves] for all cultivars increased with a dvancing leaf number. High average phyllochron values (GDD base 10 deg rees C) were associated with late panicle emergence and a long duratio n of vegetative growth. Cave-in-Rock, the earliest flowering cultivar, averaged 79 GDD per leaf and required 634 GDD to attain panicle emerg ence, while Alamo, the latest to power, averaged 152 GDD per leaf and required 1777 GDD to attain panicle emergence. A slow rate of leaf app earance was identified as the primary developmental trait associated w ith a long duration of vegetative growth in switchgrass.