Photosynthesis, Morphology, Leaf Anatomy, and Cytogenetics of Hybrids between C₃ and C₃/C₄ Panicum Species

The Laxa group of the Panicum genus contains species which have CO2 exchange and anatomical characteristics intermediate to C3 and C4 photosynthetic types ($\text{C}_{3}/\text{C}_{4}$), and also species characterized as C3. Hybrids were made between two of the $\text{C}_{3}/\text{C}_{4}$ species and two C3 species. Carbon dioxide exchange and morphological, leaf anatomical, and cytogenetic characteristics of F1 hybrids between Panicum milioides Nees. ex Trin ($\text{C}_{3}/\text{C}_{4}$) and P. laxum Mez. (C3), P. spathellosum Doell ($\text{C}_{3}/\text{C}_{4}$) and P. boliviense Hack. (C3), 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 CO2 uptake than the C3 parent. Values of inhibition of apparent photosynthesis by 21% O2, CO2 loss in the light, and CO2 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 CO2 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 CO2 loss in the $\text{C}_{3}/\text{C}_{4}$ species, and to a lesser degree in the F1 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.