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Photosynthetic Responses of Tall Fescue to Selection for Longevity below the CO₂ Compensation Point¹

There have been many attempts to identify efficient C₃ plants by survival in CO₂ environments below their normal CO₂ compensation point (). Thus far there have been no reports on characteristics of final survivors from such a treatment. Our objective was to determine relationships between CO₂ exchange parameters in tall fescue (Festuca arundinacea, Schreb.) and to determine if selection for longevity under sub- conditions would be feasible for decreasing photorespiration or increasing net carbon exchange (NCE).

About 4,400 seedlings of ‘Alta’ and 2,200 of ‘Kentucky 31’ were exposed to sub- conditions for seven to eight days and 36 and 22 final survivors, respectively, were selected. In both cultivars, the selected population was not different from the base in rate of NCE in 21% O₂, NCE in 0% O₂, or photorespiration estimated as NCE enhancement in 0% O₂ over 21% O₂. Rate of dark respiration was significantly increased in both Alta (P < 0.01) and Kentucky 31 (P < 0.10).

Rate of dark respiration was not correlated with rates of other CO₂ exchange parameters except in the base population of Kentucky 31, where it was positively correlated (P < 0.05) with photorespiration, NCE in 21% O₂, and NCE in 0% O₂. Rate of photorespiration was positively correlated (P < 0.01) with NCE in 21% O₂ in all four populations and regression coefficients were similar. Combining all populations gave an overall correlation coefficient of 0.649 (P < 0.001) and a regression coefficient of 0.54.

This suggests that in plant breeding programs selection for increased NCE will probably also increase photorespiration, unless genetic control is independent and selection pressure is placed on both characters.

Key Words: Net carbon exchange • Respiration • Photorespiration • Grass breeding • Festuca arundinacea Schreb

² Associate professor, former associate professor (now research geneticist, ARS USDA, Logan, UT 84552), and former graduate assistant, Dept, of Agron., Univ. of Mo., Columbia, MO 65201

Received for publication July 19, 1974.