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A rapeseed ideotype for high yields

N.J. Mendham and M.S.S. Rao

Department of Agricultural Science, University of Tasmania, Hobart, 7001.

In addition to the desirable features common to most annual seed crop ideotypes (1), rapeseed has particular requirements as a result of its characteristic profuse flowering above the vegetative canopy. Previous work in the United Kingdom (2) showed that the typical European winter cultivar is very inefficient when sown in early autumn. The crop produced a mass of vegetative growth, followed by large numbers of flowers and pods, with heavy abortion of both and the reduction of seed numbers per pod to low levels. Some late sowings which grew well but produced only modest numbers of pods were shown to have a much higher yield potential due to better seed retention. Breeders have since been aiming to limit pod numbers to increase yield.

Studies in Tasmania (3) comparing new Victorian lines of Japanese parentage with Canadian or Western Australian cultivars have also shown that the ability to retain more seeds per pod on well grown crops was the key to high yield, although under dry finishing conditions grain size and oil content may suffer. Japanese and Chinese lines also grew well at low temperatures, particularly in dry conditions.

An apetalous line showed even better seed retention (4), apparently because the much greater amount of radiation penetrating through the flower canopy allows leaves to persist and the first-formed pods to retain their seeds through the normal phase of abortion. Benefits were also seen in greater extraction of soil water and tolerance of drought stress.

The characteristic upright pod habit in Brassica campestris "Chinoli" lines was also investigated to determine whether this was of benefit to pod photosynthesis and hence seed fill, by analogy with upright leaves in cereals. Where lodging was controlled, the characteristic apparently conferred a benefit. The generally poor seed set on Chinoli, however, probably a result of the wide cross used to produce it, prevented proper expression of its potential.

An ideotype for rapeseed would therefore combine vigorous early growth to maximise light interception with moderately early flowering on short stems. High seed numbers per pod can be achieved by combining the apetalous type with the inherently high seed retention of Japanese lines2 Yield can then be maximised on relatively sparse pod canopies, 4000 per m being adequate and achievable by restriction of branching. Upright pods should allow maximum pod photosynthesis and hence seed size and oil content.

1. Donald, C.M. and Hamblin, J. (1983). Adv. Agron. 36, 97-143.

2. Mendham, N.J., Shipway, P.A. and Scott, R.K. (1981). J. Agric. Sci. (Camb.) 96, 389-416.

3. Mendham, N.J., Russell, J. and Buzza, G.C. (1984). J. Agric. Sci. (Camb.) 103, 303-316.

4. Rao, M.S.S. and Mendham, N.J. (1987). Proc. 4th Agron. Conf., p. 335.

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