Analysis of rhizome colour content, bioactive compound profiling and ex-situ conservation of turmeric genotypes (Curcuma longa L.) from sub-Himalayan terai region of India

Turmeric (Curcuma longa L.), the golden spice of India, offers a myriad of health benefits primarily due to the presence of polyphenolic curcuminoid pigments. It is believed that the dark orange colour of turmeric rhizome has wide-spread health benefits; but no reliable evidential proof for this is available till date. It is crucial to discern the association of rhizome colour with health benefits. It is also of prime importance to investigate whether the curcuminoids present in the rhizomes are the sole bioactive compounds contributing to its medicinal properties. In the present study, forty five turmeric genotypes were collected from the sub-Himalayan terai region of India which differed in their rhizome colour. Here, the Turmeric Redness Index (TRI) in terms of rhizome colour content (a*/b*) of the genotypes revealed strong correlation with curcuminoid content (+0.93), total phenol content (+0.76), total flavonoids content (+0.82), iron content (+0.55) and antioxidant activity (–0.90 and –0.92 for DPPH and ABTS assays and +0.82 and +0.95 for TAC and CUPRAC assays, respectively, with p < 0.001). Among the studied genotypes, TCP 2 (Turmeric Collection Pundibari 2) having dark orange coloured rhizome turned out to be a superior genotype in terms of its antioxidant potential, curcuminoid and iron content. Quantification of phenolic and flavonoid compounds of TCP 2 revealed that other than the three isoforms of curcuminoids, different therapeutically important bioactive compounds like p-coumaric acid (162.46 mg/kg), catechin (107.67 mg/kg), sinapic acid (417.36 mg/kg) and flavonoid like quercetin (2746.21 mg/kg) are also present in the rhizome, suggesting that along with curcuminoid these compounds also contribute towards its antioxidant potential. The GC–MS analysis of the essential oil of TCP 2 revealed the presence of thirty three volatile compounds with significantly high ar-turmeron (28.57 %), curlone (10.05 %), eucalyptol (10.13 %) and chemigran (13 %). A rapid and improved micro-propagation protocol was also developed for TCP 2 using single rhizome bud sprout as explants. A media composition of MS with 6 ppm BAP and 2 ppm NAA was found to be the best for rapid and efficient micro shoot development.