PHYTOMER POSITIOn AFFECTS SEED PRODUCTIOn AnD SEED WEIGHT In A CLEISTOGAMOUS GRASS

Tall dropseed [sporobolus compositus Poir. (Merr.)] is a warm-season grass that occurs throughout the USA, with the exception of the southeastern and southwestern states. It is a common associate of the dominant tallgrass prairie and mixed-grass prairie grasses and is commonly found along roadsides, railroad rights of way, and other disturbed areas. It tends to increase in cover and abundance under drought. Cleistogamy is self-fertilization in an enclosed flower, and occurs in about 5% of the total number of grass species. Tall dropseed is clesitogamous. Infloresecences are located inside the sheaths of the aerial phy tomers. Seed heteromorphism occurs across nodes in cleistogamous grasses. Our objective was to determine if seed number and seed weight varied among phytomers of tall dropseed. Seventy-five plants from each of two genotypes were transplanted on 0.9-m centers at Aurora, SD, during spring of Year 1. During August of Year 2, 3 tillers from each plant were harvested and inflorescences were removed from inside sheaths and seeds were extracted and weighed. During Year 3, 5 plants of each genotype were sampled as in Year 2. In addition several natural populations were also sampled. Seed number increased acropetally, with the apical inflorescence having the greatest number of seeds and the largest seeds. This pattern was similar for seed number but opposite for seed weight relative to results from previous studies of other cleistogamous grass species.