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Structure and morphology of spines of Mammillaria gracillis Pfeiff. (Cactaceae) shoots.

Mammillaria gracillis Pfeiff. cactus was propagated in vitro on MS medium without growth regulators. The culture spontaneously switches from an organized to unorganized way of growth, producing a habituated organogenic callus which regenerates morphologically normal and hyperhydric shoots. We studied changes in morphogenesis of cactus spines related to artificial environmental conditions of in vitro culture. Spines of pot-grown Mammillaria plants and of in vitro regenerated shoots were studied by optical magnifier (Zeiss, Stemi 2000-C) and scanning electron microscop (SEM, Vega TS-5136). It was noticed that tissue culture conditions influence the organization of areoles and number of spines. In comparison to spines of pot-grown shoots spines of regenerated shoots are more flexible, without characteristic defensive properties and practically not pricking the skin. Optical magnifier revealed the highest number of spines per areole in the pot-grown plants (17) while it decreased in normal (11-12) and hyperhydric regenerants (six). The SEM analysis revealed that the surface of each spine of pot-grown shoots is covered with stacked cuticule cells that protect the spines from drying. The spines of in vitro-grown normal regenerants are found to contain numerous circulary shifted cuticule cells partially detached from the surface of the spine. Due to high relative humidity in tissue culture cuticule cells might be filled with water and therefore are arising from the spine surface. These detached cells are even more elongated in spines of hyperhydric regenerants. Similar structures were not observed in the spines of pot-grown plants.

artificial environmental conditions; cacti; morphogenesis; plant tissue culture; SEM

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