COMPARISON OF PHOTOSYNTHESIS IN NORMAL AND TRIAZINE-RESISTANT Brassica

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Abstract
In spaced field field plantings, triazine-resistant types of Brassica campestris L. and B. napus L. had a carbon dioxide exchange rate (CER) 28% lower in 1983 and 25% lower in 1984 than normal (triazine-susceptible) types. In plots stimulating agronomic spacings in 1984, the difference between CER in normal and resistant types was 17% for B. campestris, 14% for B. napus and 13% for B. juncea L. Differences were apparent throughout the season and were not associated with any particular stage of growth. Resistant progeny from reciprocal crosses between resistant and susceptible plants of B. napus exhibited reduced CER at all levels of photosynthetically active radiation and at all temperatures. There was no significant difference between plant types for chlorophyll a+b content or chlorophyll a/b ratio. Shoot dry weight, stomatal resistance and specific leaf weight were higher in the normal types, but there was no difference between types in either relative growth rate or net assimilation rate. The reduced biomass was not therefore linked to reduced CER.