6.5.6  Growth efficiency and crop selection

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A priori, increased respiratory efficiency via either energy production or utilisation should have an impact on RGR, and some selection has been attempted. Wilson and Cooper (1969) first demonstrated that wide variation in RGR among Lolium perenne (perennial ryegrass) populations from diverse habitats was attributable to differences in NAR. Wilson (1982) sub-sequently suggested that improved growth (10–20% increase in yield) of L. perenne grown in high-density swards could be achieved by genotypes whose mature leaves respired more slowly. Mature leaves had been selected as a measurement criterion in the belief that maintenance respiration would predominate compared with growth respiration.

Wilson (1982) and Robson (1982a,b) found genetic variation in maintenance respiration where slower dark respiration accounted for greater dry matter production, prompting the expectation that low maintenance require-ments would generally increase the amount of fixed CO2 that could be invested in growth. High-density swards of ryegrass behaved this way, but sadly the relationship between low respiration rates and improved growth disappeared when perennial ryegrass was grown as low-density swards (Kraus et al. 1993). Similarly, fast-growing pea cultivars exhibit less alternative pathway respiration than slow-growing cultivars in some studies (Musgrave et al. 1986) but not in others (Obenland et al. 1988). In view of such experiences, selection for faster RGR via respiratory efficiency due either to reduced alternative oxidase activity and/or decreased costs remains an attractive goal, but useful outcomes are not yet assured. Selection criteria will certainly have to be based on respiratory features that are maintained under a variety of growth conditions.