Christopher Ochieng Ojiewo

Male-sterility induced by gamma-ray irradiation of African nightshade (Solanum nigrum L. ssp. villosum) seed

Summary The leaf yields of vegetables belonging to the Solanum nigrum complex are limited by prolific early flowering and berry set. Source-sink limitations and imbalances occur after anthesis, with dry matter partitioning directed more to pollen, seed and fruit formation and development. Each of these processes is associated with high respiratory costs, in competition with leaf expansion and productivity. To use male-sterility to redistribute biological biomass towards greater economic leaf yield, a range of mutants inducible by irradiation of seed with gamma-rays was investigated. On the basis of overall plant development from germination, seedling survival, production of M2 selfed-seed and the frequency of male-sterile mutants, 100 Gy of gamma-rays was determined to be the optimum dose. Four male-sterile mutant types were isolated during the Spring-Summer season from the M2 screen: (i) an acetocarmine-stained non-viable pollen type which stained black with potassium iodide solution (T-1); (ii) a defective pollen type not stainable with acetocarmine (T-2); (iii) a pollen-less type with defective anthers (T-3); and (iv) an extremely low pollen-producing type (T-4). An intermediate type (T-1/2) had a mixture of acetocarmine stainable and defective non-stainable pollen. All T-1, and some T-3 mutants were partially restored and set berries with selfed-seed, while T-1/2 set non-seeded berries in the Autumn-Winter season. Eliminating the reproductive function and subsequently fruits, which are the major sink organs of the plant, is expected to enable the male-sterile mutants isolated in this study to use the biomass no longer allocated to pollen (and eventually to berry and seed production), for extra leaf production.

Chromosome duplication and ploidy level determination in African nightshade Solanum villosum Miller

Summary Octoploids were induced in vivo from wild-type tetraploid Solanum nigrum ssp. villosum plants using colchicine sprays. Seedling survival rates and numbers of induced octoploids from 72 seedlings were 95.8%, 73.6% and 48.6%, and 4, 2 and 1, from 0.01%, 0.05% and 0.25% colchicine treatments, respectively. The applicability of pollen area and stomatal length, as indirect methods to determine ploidy level, was investigated. Further confirmatory tests involving direct chromosome counts (in root tip cells) and flow cytometric analysis revealed that pollen and stomatal cell size may not correlate accurately with ploidy level. Although octoploids generally had larger pollen and larger stomata, plants that were identified in the first generation (G1) progeny on a large-pollen and large-stomata basis were not necessarily octoploids. In addition, a number of tetraploids also had “large” stomata or “large” pollen. Flow cytometric analysis revealed that this species exhibits polysomaty in leaf tissues (i.e., the tissues consist of cells with different ploidy levels), which could affect the size and morphology of both pollen grains and stomatal (guard) cells, thus explaining the inconsistency observed. We conclude that plant pollen and stomatal size can provide a good general guide to ploidy level determination in this species; but confirmatory tests, including direct chromosome scoring in root tip cells and flow cytometry in young leaves, are indispensable.