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Photosynthetic Characteristics of Panicum milioides, a Species with Reduced Photorespiration¹

A species (Panicum milioides Nees ex. Trin.), has been found which apparently has a reduced rate of photorespiration compared to other C₃ plants. It was found in preliminary studies that the response of net photosynthesis (Pn) to O₂ concentration was less for this species than for other C₃ plants. Therefore, this work was conducted to characterize for five accessions of P. milioides the CO₂ exchange related to photorespiration. Panicum maximum Jacq. (C₄), P. laevifolium Htack. (C₄), and tall fescue (Festuca arundinacea Schreb. (C₃) were included for comparison. Leaf cross-sections of P. milioides were examined for characteristics known to be related to photosynthetic carbon pathways. ¹³C/¹²C ratios were determined on P. milioides tissue.

P. milioides photorespired less than tall fescue and more than P. maximum as determined by the three criteria: Pn response to O₂, CO₂ evolution in the light, and CO₂ compensation concentration. The change in Pn for each 1% increase in O₂ was from –0.36 to –0.48 mg CO₂ dm^–2 h^–1 for five accessions of P. milioides compared to –0.10, –0.13, and –0.61 mg dm^–2 h^–1 for P. maximum, P. laevifolium, and tall fescue, respectively. P. milioides evolved an average of 0.85 mg CO₂ dm^–2 h^–1 in the light at 20% O₂ compared to 0.02 mg dm^–2 h^–1 for P. maximum and 2.96 mg dm^–2 h^–1 for tall fescue. CO₂ compensation concentrations determined by a Mylar bag technique were 0.4, 15.8, and 54 ppm for P. maximum, P. milioides, and tall fescue, respectively. Determination of CO₂ compensation from Pn response to CO₂ gave a similar trend, although actual concentrations were different. P. milioides tissue had ¹³C values of about –25 , characteristic of C₃ species. A leaf anatomical study showed that P. milioides had enlarged parenchyma sheath cells with centripetally located chloroplasts and mesophyll cells arranged in a radial manner around the parenchyma sheath. The parenchyma sheath cell walls appeared to be thicker than those of most C₃ species. It is concluded that while P. milioides exhibits mostly C₃ characteristics, its reduced photorespiration rate and some C₄-like anatomical characteristics indicate a degree of intermediacy between C₃ and C₄.

Key Words: CO₂ compensation • C₃ and C₄ species • Oxygen response • ¹³C/¹²C ratios • Leaf anatomy

² Professor of agronomy, Univ. of Ga. and professor of botany, Univ. of Tex., respectively.

Received for publication March 21, 1975.

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