Determinants of colour and fragrance characteristics of Polianthes tuberosa Linn. Flower

Abstract

Tuberose (Polianthes tuberosa Linn.) is one of the summer flowers grown by smallholder farmers in Kenya, for export. It is a high value perennial crop grown in the field with no plant support structures or shading. A survey in 2007 provided the baseline information on distribution and production indicating that rejection of cut flowers was attributed to poor colour, pest damage and stem length. High rejections of up to 27.5 % lead to low export volumes and low returns to investment since the rejected flowers fetched very low prices on the local market or cannot be marketed at all. Also flowers produced out of the export schedule are marketed at a loss on the local market. Colour was a factor considered for this study and when tuberose are not sold on the export market, to extract essential oil for export would contribute to increased productivity, commercialization and competitiveness therefore its abundance was considered one of the factors to be investigated. Volatiles are synthesised from amino acids, membrane lipids and carbohydrates and their formation is dependent on availability of C2-C8 acids and alcohol which is similar for essential for pigment synthesis. A nutritional study was set up to investigate the effect of soil applied magnesium, nitrogen and light quality on colour development. This was followed by investigating the influence of floral applied magnesium on colour development and stability under transient temperatures in a controlled environment at temperature regimes of day/night temperature of 19ºC/9ºC, 25ºC/15ºC, and 32ºC/22ºC. Controls of fragrance in tuberose florets were studied by determining the composition of the floral volatiles under varied environmental conditions using a portable volatile collection technique in situ. The volatiles were analyzed and the chemical compounds identified using gas chromatography and mass spectrometry.The nutritional study showed that soil pH was not significantly linked to anthocyanin accumulation and colour development in the florets. Neither did magnesium appear to have increased in the tissues, but there was an increase in accumulation of anthocyanins by 2.61% and colour intensity by up to 53 radians. Florets with no net cover had the highest colour intensity but were not significantly different from those under red shade net. The red shade net cover or growing in the open with no net cover gave the best colour intensity of 53.1 radians; the measure of redness found on the petals significantly decreased when flowers without magnesium treatment were moved from 19ºC to 32ºC while those with magnesium remained the same. The two major chemical components identified in the tuberose floral volatile were methyl benzoate and 1,8 - cineole which accounted for 69–84% of the total fraction. These volatile components are used for the industrial manufacture of cosmetics, pharmaceuticals and confectionery. The number and type of volatiles emitted were location dependant: Juja at an altitude of 1350 m at 18–21°C emitted 21 compounds; Sagana at altitude 1214 m at 17–17.5°C emitted 27 with Cyclopentanone, 2- cyclopentylidene specific to this location; and Meru at altitude 2060 m at 7–9°C emitted 26 compounds with Muurola-4(14),5-diene<trans> and 2-Pyrrolidinone, 1-methyl- 1specific to the location.When soil amendments were applied, the ammonium sulphate and magnesium nitrate lowered the soil pH but that lightness, chroma, hue, and anthocyanins in the florets were not significantly linked to the soil pH, there was no increase in magnesium in the plant tissues but an in accumulation of anthocyanins. Investigations of various light conditions on colour of the florets confirmed the full sun conditions enhanced colour development more that all the other light conditions. For temperature and magnesium concentration, it was determined that magnesium was not required for colour to develop at low temperature but when temperatures increased magnesium appeared to contribute to the maintenance of the developed colour. The number and type of volatiles emitted were different according to altitude and temperature, thus the effect of altitude and temperature have to be taken into account when growing tuberose for the volatiles. Thus further studies are recommended on nutritional studies incorporating different rates of magnesium as a soil amendment and /or floral spray. Value addition applications for industrial purposes have a great potential for cottage industry and also medium and large enterprises.