A Field Study of the Carbon Economy of Normal and ‘Topless’ Field Beans (Vicia faba)

Abstract

The growth and yield of Maris Bead spring bean and Topless, a population incorporating the terminal inflorescence mutant, were compared in irrigated and non-irrigated experiments at Cambridge in 1979. In both experiments, Topless, initially grew more rapidly than Maris Bead, but there was little difference between the two genotypes in total dry matter yield at maturity. The yield of grain from Topless was 89% of that of Maris Bead, in both experiments. Irrigation, given in one experiment until the onset of pod growth, greatly stimulated the vegetative growth of both genotypes but increased their grain yields only slightly. At maturity, Topless plants were shorter than those of Maris Bead, particularly in the irrigated experiment. The yield of vegetative dry matter in Topless depended on the production of tillers and axillary branches, many of which did not bear grain and were sinks for assimilate during grain filling.

Patterns of photosynthesis and trans location were studied by ¹⁴C methods. Until the terminal inflorescence in Topless was produced and pod growth began, the patterns of assimilation by both genotypes were similar. During pod growth, the leaves contributed most to the photosynthesis of the canopy, and the large, upper leaves were the main source of assimilates in Topless. In Maris Bead more Leaves contributed assimilate and no single leaf was a dominant source. Pods contributed a maximum of 20% to the gross assimilation of the crop. The rate of photosynthesis per unit dry weight of pod, judged from ¹⁴C fixation, was greater in Topless than in Maris Bead.

It is concluded that the ‘topless’ plant type is not inherently inferior in productivity to the normal indeterminate type. It is suggested that in future breeding to produce determinant varieties having the terminal inflorescence character, selection should be practised for increased production of tillers which develop synchronously with the main shoot and against the production of infertile branches.