M. A. Hussey

COMPARATIVE GENE EXPRESSION IN SEXUAL AND APOMICTIC OVARIES OF PENNISETUM CILIARE (L.) LINK.

Limited emphasis has been given to the molecular study of apomixis, an asexual method of reproduction where seeds are produced without fertilization. Most buffelgrass (Pennisetum ciliare (L.) Link syn = Cenchrus ciliaris L.) genotypes reproduce by obligate apomixis (apospory); however, rare sexual plants have been recovered. A modified differential display procedure was used to compare gene expression in unpollinated ovaries containing ovules with either sexual or apomictic female gametophytes. The modification incorporated end-labeled poly(A)+ anchored primers as the only isotopic source, and was a reliable and consistent approach for detecting differentially displayed transcripts. Using 20 different decamers and two anchor primers, 2268 cDNA fragments between 200 and 600 bp were displayed. From these, eight reproducible differentially displayed cDNAs were identified and cloned. Based on northern analysis, one cDNA was detected in only the sexual ovaries, two cDNAs in only apomictic ovaries and one cDNA was present in both types of ovaries. Three fragments could not be detected and one fragment was detected in ovaries, stems, and leaves. Comparison of gene expression during sexual and apomictic development in buffelgrass represents a new model system and a strategy for investigating female reproductive development in the angiosperms.

Soil organic carbon pools under switchgrass grown as a bioenergy crop compared to other conventional crops.

Abstract Switchgrass ( Panicum virgatum L.) has been proposed as a sustainable bioenergy crop because of its high yield potential, adaptation to marginal sites, and tolerance to water and nutrient limitations. A better understanding of the potential effects of biomass energy crop production practices on soil biological properties and organic matter dynamics is critical to its production. Our objective was to evaluate changes in C pools under a warm-season perennial switchgrass in different soils compared to typically-grown crops collected at College Station, Dallas, and Stephenville, TX in February 2001. Sampling depths were 0–5, 5–15, and 15–30 cm. Switchgrass increased soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C, and particulate organic matter C (POM-C) compared to conventional cropping systems. Soil C concentrations were in the order: long-term coastal bermudagrass [ Cynodon dactylon (L.) Pers.] Panicum coloratum L.) planted in 1992 > switchgrass 1997 > conventional cropping systems. Soil C concentrations tended to increase with increasing clay content. Greater microbial biomass C followed the order of Dallas > College Station > Stephenville, and ranged from approximately 180 mg C kg −1 soil at Stephenville to 1900 mg C kg −1 soil at Dallas. Particulate organic C was more sensitive than other fractions to management, increasing as much as 6-fold under long-term coastal bermudagrass compared to conventional cropping systems. Our study indicated that conversion of conventional cropping systems into switchgrass production can sequestrate more SOC and improve soil biological properties in the southern USA.

Cytology of Paspalum malacophyllum and Its Relationship to P. juergensii and P. dilatatum

This study describes the meiotic chromosome pairing behavior and method of reproduction of 16 Paspalum malacophyllum accessions and their F1 hybrids with diploid P. juergensii (2n=2x=20) and tetraploid P. dilatatum (2n=4x=40). All P. malacophyllum accessions were tetraploids with 40 chromosomes. Their mean meiotic chromosome pairing was 0.82 I + 13.40 II + 0.02 III + 1.58 IV. As many as seven quadrivalents were observed, implying autotetraploidy. An examination of megasporogenesis and the subsequent gametophyte development revealed that all accessions were facultative apomicts with the form being apospory. All P. juergensii x P. malacophyllum hybrids were triploids with 30 chromosomes that associated primarily as 10 I + 10 II. The P. dilatatum x P. malacophyllum hybrids had 40 chromosomes that associated essentially as 20 I + 10 II. The 10 bivalents in both groups of hybrids represent autosyndetic pairing of the P. malacophyllum chromosomes, thus supporting autotetraploidy. The 10 univalents in the P. juergensii x P. malacophyllum hybrids are members of the J genome from P. juergensii (JJ), and the 20 univalents in the P. malacophyllum x P. dilatatum hybrids are members of the I and J genomes from P. dilatatum (IIJJ). Because P. malacophyllum does not possess the I or J genomes and its chromosomes pair autosyndetically, it was assigned the genome formula MMMM. Cytologically, both groups of F1 hybrids reproduced as facultative aposporous apomicts. In ovules with only sexual development, the female gametophyte frequently deteriorated prior to maturity. Several F2 progeny were recovered from both groups of F1 hybrids and had the same chromosome number as their meiotically unstable parental F1 hybrids. These F2 plants are the products of apomictic reproduction, demonstrating that apomixis is effective in perpetuating highly sterile, meiotically irregular interspecies hybrids.

Influence of mefluidide on growth, development, and cell wall digestibility of sorghum

Application of mefluidide (N-[2,4-dimethyl-5-([(trifluoromethyl)sulfonyl]amino) phenyl]acetamide) inhibits plant development in perennial grasses. This study examined the effect of mefluidide on the morphological development and digestibility of sorghum. In the greenhouse, 5.9 × 10−5 g active ingredient (a.i.) plant−1 applied at the seedling, eight-leaf and boot stages reduced mean plant height 70%, 59%, and 2%, respectively. Heights were also reduced 14%, 15% and 35% by 5.9 × 10−8, 5.9 × 10−7 and 5.9 × 10−6 gram a.i. plant−1 applied at the eight-leaf stage. Field application of 0.26 or 0.52 kg ha−1 mefluidide at either the eight-leaf or flagleaf stage reduced mean plant height of all cultivars. Basal tiller numbers increased 319% 28 d, and dry matter production was reduced 65% 42 d following mefluidide application at the eight-leaf stage. Treated stems were 34% higher and treated leaves were 7% higher in cellulase dry matter digestibility than control plants following mefluidide application at the eight-leaf stage. These results indicate that mefluidide application to vegetative stages in sorghum may enhance the forage value of the plants while it inhibits normal plant growth.