Pisum sativum L., the pea, is at present the only pulse crop suitable for direct human consumption which can be grown reliably in the United Kingdom. There is thus a need for additional leguminous grain crops, both to increase the production of home-grown plant protein and to provide alternative break-crops for cereal growing areas. The aim of this programme was to investigate the potential of Phaseolus coccineus L. and P. coccineus x P. vulgaris L. hybrids for this role. The principal requirements for the use of P. coccineus as a grain legume were seen to be as follows: dwarf growth habit, since the plants should be self-supporting; adequate ground clearance for the pods, to permit direct combine harvesting; early maturity, to allow acceptable yield; reliable harvesting under adverse Autumn weather conditions; and reliable pollination, since P. coccineus is cross-pollinated and reliant on insect pollen vectors for adequate pod set. There may be a potential market for P. coccineus beans as a substitute for imported butter beans, in which case varieties will be needed which also have large white seeds.
Of the above requirements, the most important was considered to be growth habit and so, since only two dwarf varieties were known, a breeding programme was carried out in which the dwarf habit was introduced into a wider range of genetic backgrounds. Six climbing parents were chosen and these were crossed with Hammond's Dwarf White. The resultant material was self-pollinated to the F3 or F4 generation. It was then grown in a trial, together with material derived from crosses made by other workers and with an advanced generation interspecific hybrid between P. coccineus and P. vulgaris. One dwarf and six climbing varieties were included as controls. Four characters were chosen for selection namely: potential yield per plant, earliness of maturity, seed size and pod length; the latter character being chosen for its effect on the ground clearance of the pods. Two methods of selection were used. Firstly, selection was applied to both individual plants and single plant progenies for the four criteria using a ranking procedure. Large improvements were found in all four characters over the control varieties. The second method of selection involved the construction of genetically optimised indices. Five indices were produced, involving up to eight additional characters, but little improvement was obtained from their inclusion. The greatest predicted genetic advance was for the reduction of pod length, since this was the most heritable character, and advances in the other traits were limited by adverse intercorrelations.
During the course of the programme, a number of populations were produced which segregated for growth habit, and from these the nature of its genetic control was deduced. The production of either climbing (> 7 mainstem nodes) or dwarf ( <7 mainstem nodes) plants appears to result from the action of a single gene (named B), the dwarf form being recessive. A second gene (named A) when in the recessive form modifies this action, such that plants with genotype aaBB produce more mainstem nodes than normal climbing plants, and plants with genotype aabb are stunted, chlorotic and have very short internodes. In one of the F2 populations segregating for growth habit, the dwarf segregants were shown to be earlier maturing than the climbing plants but they produced a higher proportion of diseased seeds and were much lower yielding, through a reduction in the number of pods borne.
A new male sterile P. coccineus is described which is genetically controlled by a single recessive allele. No mature pollen is produced and there appears to be a delay and disruption of meiosis either at or before first metaphase.
The role of the interspecific hybrid P. coccineus x P. vulgaris is seen as being twofold. In the first place it may be used as a vehicle for the transfer of genes from P. vulgaris to P. coccineus. This might be done by either back-crossing the hybrid onto P. coccineus or possibly by using the hybrid as a bridge between the two species, crossing it first to P. vulgaris and then back-crossing with P. coccineus. The transfer of two characters, dwarf growth habit and the absence of stigmatic surface barrier to pollen tube penetration (which gives an increase in the degree of self-pollination), was achieved using the former method. The hybrid was also crossed successfully with plants from both parental species, which indicates that the latter method might also be feasible. The second potential role of the interspecific hybrid is as a cultivar in its own right, since several of the hybrid progenies were extremely vigorous and out-yielded most of the dwarf P. coccineus material in trial.
No information was available concerning the cultural practices to be used with dwarf P. coccineus when grown for seed. A small trial was therefore conducted to determine the optimum density and the effect of irrigation on yield and earliness of maturity. Irrigation was found to have no significant effect, but potential yield per plant declined in a linear fashion with reduced plant spacing. This was largely due to a reduction in the number of pods borne per plant. Acceptable yields per unit area were produced at high density, but the high seed rate and low multiplication ratio were likely to make the crop uneconomic. It was concluded that varieties were needed which could produce a reasonable yield when grown at low densities.
The work in this project has indicated that considerable advances are possible in this crop with breeding, and that any further work should be concentrated on increasing the ground clearance of the pods and in producing earlier maturing forms.
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