The Photosynthetic Net Carbon Dioxide Exchange Potential in Conventional and ‘Leafless’ Phenotypes of Pisum sativum L. in Relation to Foliage Area, Dry Matter Production and Seed Yield

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Abstract
The effect of the ‘leafless’ mutation (in which tendrils replace leaflets and the stipules are reduced to a vestigial form) upon foliage area, photosynthetic net CO 2 uptake potential, dry matter production and seed yield in Pisum sativum was studied by comparing two near-isogenic lines of genotype afafstst and ++++. The mutation is of potential agronomic value in that it offers improved lodging resistance, crop drying and harvester throughput. In the conventional phenotype the total foliage area of the main axis attained a plateau (456 cm 2 ) at day 56 from seedling emergence, whereas corresponding values for the ‘leafless’ mutant showed a total area of 208 cm 2 at day 68 with no indication of a plateau. The agronomic consequence of this is discussed. During the vegetative phase of the plant the maximum CO 2 uptake potential in the fully expanded conventional leaf was 8·5 mg CO 2 leaf −1 h −1 and in the ‘leafless’ mutant this value was 7·0 mg CO 2 leaf −1 h −1 . For most ‘leaves’ of the latter phenotype this value was between 30 and 60 per cent less than for their conventional counterpart. There was a consistently higher photosynthetic potential per unit area in tendrils of the ‘leafless’ mutant than in leaflets of the conventional phenotype. The respective mean specific values for the two phenotypes were 53 and 37 mg CO 2 dm −2 h −1 . The problem of obtaining a meaningful surface area value for tendrils is discussed and the cylindrical nature of tendrils is taken into account. The ‘leafless’ mutant consistently accumulated 50 per cent less dry matter than did conventional plants in the period from seedling emergence to anthesis and yield of mature dry seed per plant showed a reduction of 50 per cent both in seed number and total seed weight. The implications for future breeding and selection programmes aimed at haulm reduction are discussed in relation to evaluating the ability of the background genotype to produce adequate tendrils in the presence of afafstst .