Photosynthesis, Morphology, Leaf Anatomy, and Cytogenetics of Hybrids between C3 and C3/C4Panicum Species

The Laxa group of the Panicum genus contains species which have CO 2 exchange and anatomical characteristics intermediate to C 3 and C 4 photosynthetic types (C 3 /C 4 ), and also species characterized as C 3 . Hybrids were made between two of the C 3 /C 4 species and two C 3 species. Carbon dioxide exchange and morphological, leaf anatomical, and cytogenetic characteristics of F 1 hybrids between Panicum milioides Nees. ex Trin (C 3 /C 4 ) and P. laxum Mez. (C 3 ), P. spathellosum Doell (C 3 /C 4 ) and P. boliviense Hack. (C 3 ), and P. spathellosum and P. laxum were studied. There were no consistent differences in apparent photosynthesis, although two of the three hybrids had higher net CO 2 uptake than the C 3 parent. Values of inhibition of apparent photosynthesis by 21% O 2 , CO 2 loss in the light, and CO 2 compensation concentration for the hybrids were between those of the parents. All three hybrids showed leaf anatomical traits, especially organelle quantities in the bundle sheath cells, between those of their respective parents. Linear regression of CO 2 compensation concentration on the percentage of mitochondria and chloroplasts in vascular bundle sheaths of the parents and hybrids gave correlation coefficients of −0.94. This suggests that the reduction in CO 2 loss in the C 3 /C 4 species, and to a lesser degree in the F 1 hybrids, was due to development of organelles and perhaps a higher proportion of leaf photorespiration in bundle sheaths. The overall morphology of the hybrids was so different from the parents that they could be described as new taxonomic forms. The chromosomes in the hybrids were mainly unpaired or paired as bivalents indicating possible homology between some parental genomes.