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Spine micromorphology of normal and hyperhydric Mammillaria gracilis Pfeiff (Cactaceae) shoots

Artificial conditions of tissue culture affect growth and physiology of CAM plants which often results in formation of hyperhydric shoots. In in vitro conditions Mammillaria gracilis Pfeiff. (Cactaceae) growth switches from organized to unorganized way, producing a habituated organogenic callus which simultaneously regenerates morphologically normal as well as altered hyperhydric shoots. In this study, influence of tissue culture conditions on morphology of cactus spines of normal and hyperhydric shoots was investigated. Spines of pot-grown Mammillaria plants and of in vitro regenerated shoots were examined with stereo microscope and scanning electron microscope. The pot-grown plants had 16-17 spines per areole. In vitro-grown normal shoots, even though they kept typical shoot morphology, had lower number of spines (11-12) and altered spine morphology. This difference was even more pronounced in spine number (six to seven) and morphology of the hyperhydric shoots. SEM analysis revealed remarkable differences in micromorphology of spine surface between pot-grown and in vitro-grown shoots. Spines of in vitro-grown normal shoots showed numerous long trichomes, which were more elongated on spines of the hyperhydric shoots ; the corresponding structures on spine surface of pot-grown plants were noticed only as small protrusions. SEM morphometric studies showed that the spines of pot-grown plants were significantly longer compared to the spines of shoots grown in tissue culture. Moreover, transverse section shape varies from elliptical in pot-grown plants to circular in normal and hyperhydric shoots grown in vitro. Cluster and correspondence analyses performed on the SEM-obtained results suggest great variability among spines of pot-grown plants. Spines of in vitro grown normal and hyperhydric shoots showed low level of morphological variation among themselves in spite of the significant difference in shoot morphology.

cactus tissue culture; Mammillaria gracilis Pfeiff.; scanning electron microscope; spine; trichomes

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