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Response of Pearl Millet Inbreds and Hybrids to Inoculation with Azospirillum brasilense¹

Plant yield and acetylene reduction were measured on six hybrids and 15 inbreds (including the hybrid parents) of pearl millet, Pennisetum americanum (L.) K. Shum., after field inoculation with Azospirillum brasilense, Sp 13t (reclassified from Spirillum lipoferum Beijerinck). During the first year of testing, inoculation responses were investigated among all plant genotypes by measuring plant dry weight, % N, total N, and acetylene reduction activity. A smaller population of genotypes were tested a second year for repeatability of inoculum response. The first year, one hybrid, Tift 23DA x Tift 186 (‘Gahi 3’), gave significantly higher plant dry weight and total N in response to inoculation. Inoculated Gahi 3 produced 31.7% more dry weight and 37.4% more total plant N when compared to autoclaved inoculum controls. No inbred was found to respond. Acetylene reduction values were low among all genotypes (range 0.54 nmole/[g dry root x hour] and did not support yield effects or confirm inoculum treatments. Inoculation responses were found to be repeatable for all genotypes tested during the second year (positive or negative) but not of a statistically significant magnitude. Of the genotypes tested both years, a combined analysis of the 2-year yields revealed significant dry weight increases after inoculation of 19.2 and 14.0% with Gahi 3 and the inbred, Bil 3B, respectively. Again, acetylene reduction values did not explain any data. A N balance study was conducted in greenhouse containers on inoculated Gahi 3 plants in an attempt to repeat the yield differences observed in the field and to monitor inputs of N into the soil-plant system. No significant yield increase of N into the soil-plant system was found due to inoculation. Sampling error possibly negated an accurate measurement of all N.

Key Words: Acetylene reduction • Grass N₂-fixation • Pennisetum americanum (L.) Shum. • Plant yield • Plant N • Spirillum lipoferum

² Former graduate research associate, Dep. of Agronomy, Univ. of Florida, Gainesville, now assistant professor of agronomy, Univ. of Georgia, Athens, GA 30602; associate professor of agronomy, professor of agronomy, Univ. of Florida, Gainesville, FL 32611; research geneticist, SEA, USDA, and Univ. of Georgia Coastal Plain Experiment Stn., Tifton, GA 31794; professor of microbiology, associate professor of statistics, associate professor of agronomy, and assistant professor of agronomy, Univ. of Florida, Gainesville, FL 32611, respectively.

Received for publication March 17, 1978.