Carrying out the crossing in a mesh house or greenhouse is preferable, to:
However, potted plants are small and number of pods per plant is low.
In tropical climates, an expensive greenhouse is not essential for crossing purposes. Commonly available wire or nylon mesh over a simple wood frame serves well (Figure 3).
Have the ceiling 2.0-2.5 m high to permit staking of spreading or climbing cowpea types, as slightly reduced light promotes the climbing tendency in many Vigna species as well as etiolation. Pitching the roof somewhat helps shed heavy rains, but, even with a flat roof the mesh disperses rain drops and minimizes splashing during heavy downpours.
A considerable proportion of Vigna germplasm is daylength sensitive. Inclusion of such types in the crossing program creates problems because of asynchronous flowering. At Ibadan (7° 30' North), planting from late August through early February results in good synchronization of flowering. However, substantial differences between genotypes do occur and staggered plantings about 1 week apart of early parents, particularly when used as females, is usually desirable.
A delay in flowering can be achieved on a limited scale by nipping off the developing flowers and fruits or by pruning the plant more severely. New plants are easily started by placing stem cuttings with a leaf in flats of coarse sand. Cover the flats with plastic to maintain high humidities around the developing plantlet.
Cuttings taken from plants during active fruiting will flower almost immediately on resuming active vegetative growth. Such cuttings are effective as pollen parents, though they are often poor pod setters.
The first developing buds on the plant tend to set pods more easily than later developing buds. Remove other buds on the same raceme and peduncle and leave only one for crossing purposes. This diverts all assimilates in the peduncle into one pod and avoids confusion in labeling.
When making interspecific crosses, a weak solution of silver thiosulfate (AgTS, approximately 4 mm Ag) sprayed on the whole plant till dripping, can reduce the abscission of crossed flowers. The AgTS solution, by interfering with ethylene oxidation, reduces ethylene-induced abscission of buds and flowers.
|Objectives, Study materials, Practicals|
|1||Principles of cowpea crossing|
|7||Suggestions for trainers|