Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Allen Knapp


A copious quantity of pollen production is desirable from the male parent in hybrid maize (Zea mays L.) seed production. However tassel size and presumably pollen production varies across environments. This research was conducted to better understand if variation in pollen production is due to stress during initiation of specific pollen production components. If component compensation for pollen production occurs, then components that form later should show plasticity with reduced production of earlier formed components. This was also investigated. To conduct such studies, a pollen production measurement technique was devised since current methods do not adequately quantify total pollen produced per tassel. In addition, knowledge of the morphological development of tassel components that lead to pollen production is critical for these studies. Thus tassel morphological development was characterized relative to plant development and heat units.;To develop best management practices for high pollen production, a reliable pollen production measurement tool is necessary. A new method was developed involving extraction of pre-shed pollen from tassels through grinding tassels, wet sieving to separate pollen from tassel debris, and counting pollen. This technique compared favorably with other established pollen yield measurement techniques. Benefits of the new technique include the quantification of all pollen produced by a single tassel, a direct count of pollen grains, and a comparatively low CV. This technique is resource intensive compared to other methods.;Tassel developmental events leading to pollen production were characterized in relation to plant development and heat unit accumulation in two modern dent inbreds. Tassel initiation occurred from fourth leaf stage, six leaf tips (V4:T6) to V5:T7. Branch meristems appeared promptly after tassel initiation. Pollen mother cells entered meiosis from V9:T14 to V12:T15. In this study the rate of tassel development relative to leaf emergence was the same between inbreds, but the rate of plant development relative to accumulated heat units differed between inbreds. Using these inbreds, chilling was applied during tassel and pollen formation events. These events included branch initiation, spikelet pair initiation, and meiosis of microsporogenesis. The 113 relative maturity (RM) inbred produced 60 percent less branches and 42 percent fewer spikelets for branch and spikelet chilling treatment, respectively. The 103 RM inbred was generally not responsive to chilling. Chilling applied during meiosis decreased the percentage of starch filled pollen grains only in the lower florets on the lowest branch of the 113 RM inbred. The 113 RM inbred produced 43 and 29 percent fewer pollen grains per tassel when chilling was applied during branch and spikelet initiation, respectively. When branch or spikelet production was reduced, compensation by later forming pollen production components was not identified in this study.;A novel pollen quantification technique was developed that can be used by those characterizing male parents of hybrids for pollen production per tassel. This new technique now allows for investigations of all pre-shed pollen produced per tassel without confounding effects of percent of pollen shed. The tassel formation events allows seed producers to better understand potential stress effects on pollen production and flowering. The diameter of the apical meristem at tassel initiation for the modern inbreds studied was smaller than previously reported. Further investigations are required to elucidate implications for variation in the apical meristem size at tassel initiation. Tassel development relative to leaf emergence corresponded for these inbreds which have a range in maturities and growth rates relative to heat units. These relationships require confirmation through testing under various environments as well as across a wider range of genotypes. A coordinated rate of tassel development to leaf emergence improves our understanding of factors affecting time to flowering. Dent inbred lines vary in their response to chilling during tassel formation for pollen production. Plasticity in pollen production components to compensate for decreased production of earlier formed pollen production components was not identified in these studies.



Digital Repository @ Iowa State University,

Copyright Owner

Dean Michael Tranel



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104 pages