In vitro polyploidization of selected indigenous plant species.
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Many plant species indigenous to South Africa have ornamental, medicinal and horticultural value. Polyploidization is one technique that has been used to artificially produce superior genotypes, particularly in horticultural species. In the current investigation two antimitotic substances, colchicine at concentrations of 0.1% and 0.01% and oryzalin at concentrations of 0.01% and 0.001%, were used in an attempt to polyploidize microshoots of Dorotheanthus bellidifonnis (Burm.f.) N.E.Br and Mondia whiteii (Hook.f.) in vitro. Microshoots of D. bellidifonnis and M. whiteii obtained from nodal cuttings of in vitro germinated seedlings were maintained for 48 hours in liquid medium containing the antimitotic substances and thereafter cultured on sucrose-supplemented MS medium. The treated microshoots were evaluated for elongation, necrosis, contamination and phenolic exudation. Best results were observed in M. whiteii microshoots treated with antimitotic substances and transferred on to solid sucrose-supplemented MS medium containing 0.2% activated charcoal for 4 weeks. Leaves from the surviving treated plants were excised and used for flow cytometric analyses to evaluate changes in chromosome number. Shoots of M. whiteii treated with 0.01 % colchicine showed no changes in chromosome number, while the higher concentration used produced polyploids and mixaploids. However, oryzalin at 0.01 % concentrations produced a comparatively higher number of microshoots that were polyploids and mixaploids. Shoots of M. whiteii that have altered chromosome number have been transferred onto multiplication medium, for future evaluation of changes in phenotypic characteristics. The germination response of seeds of D. bellidifonnis was evaluated in the presence of oryzalin (0.01% and 0.001%) and colchicine (0.1% and 0.01%). Poor germination was observed in seeds germinated in the presence of 0.01 % oryzalin. Upon transfer of the germinated seedlings treated with antimitotic substances onto sucrose-supplemented MS medium, subsequent growth and development was restricted. Shoot and root development was different for the seedlings germinated in the presence of the two antimitotic substances. Shoot elongation and root development was vigorous in seedlings geminated in the presence of 0.01 colchicine and stunted development was observed in seedlings germinated in 0.1 % colchicine. On the other hand restricted rootind was observed in seedlings germinated in the presence of oryzalin, and the shoots lacked pigmentation. Meristematic cells excised from the shoot tips of the treated seedlings showed several ultrastructural changes including abnormal mitochondrial development, endomembrane formation and vacuolation. It was concluded that oryzalin and colchicine influence ultrastructure in plant cells differently. Practical constraints associated with plant tissue culture also influence the rate of in vitro polyploidization. Since different plant species require different conditions for optimal growth, it was also noted that no unique polyploidization treatment can be used for a wide range of plant species, individual species require different growth conditions.