There has been increased use of herbal drugs in recent years. Because of increasing demand and wider use, it is essential that the quality of plant-based drugs should be assured prior to use. When heavy metals contaminate the plants from which herbal drugs are derived, they affect both plant growth characteristics and production of secondary plant metabolites. Plants exposed to heavy metal stress show changes in production of secondary metabolites. High levels of heavy metal contamination in medicinal or other plants may suppress secondary metabolite production. Alternatively, the presence of heavy metals in medicinal plants may stimulate production of bioactive compounds in many plant species. Moreover, some research results suggest that heavy metals may play an important role in triggering plant genes to alter the titers or nature of secondary plant metabolites, although the exact mechanism by which this happens remains unclear. Oxidative stress induced by heavy metals triggers signaling pathways that affect production of specific plant metabolites. In particular, reactive oxygen species (ROS), generated during heavy metal stress, may cause lipid peroxidation that stimulates formation of highly active signaling compounds capable of triggering production of bioactive compounds (secondary metabolites) that enhances the medicinal value of the plant. As usual, further research is needed to clarify the mechanism by which heavy metals induce responses that result in enhanced secondary metabolite production.

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Garlic extract contains alliin (S-allyl-L-cysteine sulfoxide), an organosulphur compound that contributes to its therapeutic value and pharmacological importance. Alliin production in plant organs (leaf and root), plantlet, callus (non-embryogenic and embryogenic), embryo (proliferated, matured and germinated embryos), grown under in-vitro conditions was evaluated after 8 weeks. Highest alliin content was recorded in leaves. Clove callus and germinated embryos also showed alliin production. Evaluation of alliin content of in-vitro grown tissues both in normal (control) and sulphur supplemented conditions (4, 8, 16, 32 mg l -1 ) showed that sulphur treatment at supply of 16 mg l -1 gypsum (CaSO 4 ) significantly enhanced the production of alliin content in all in-vitro grown tissues and organs. These findings suggest that production of alliin is dependent upon type and age of tissue and can be altered by growth conditions.

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Direct somatic embryo formation (without intervening callus) from garlic clove basal tissue was induced in which the influence of plant growth regulators (PGRs) on various explants was examined. Medium added with 2.0 mg/l 6-benzylaminopurine (BAP) and 0.5 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) were the most effective PGR combination for somatic embryo induction. It induced embryos directly in 85.5% of the basal clove explant. Callus induction was also obtained from other parts of explant and 2.0 mg/l 2,4-D induced callusing in 86.5% of the inoculated explants. Protein, amino acid and alliin content were measured in callus and in embryos. Somatic embryos had more soluble protein and free amino acid compared to callus. HPTLC analysis revealed that alliin was significantly high in somatic embryos compared to undifferentiated callus tissue; the content was even more in older embryos. The present study of Allium indicates that the event of morphogenetic development including in vitro embryogeny can effectively be analysed by monitoring the changes of biochemical profiles.

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