African Crop Science Conference Proceedings, Vol. 9. pp. 227 - 234 Printed in Uganda. All rights reserved ISSN 1023-070X/2009 $ 4.00 © 2009, African Crop Science Society Effect of pH and magnesium on colour development and anthocyanin accumulation in tuberose florets A. NAKHUMICHA MURIITHI, L. S. WAMOCHO & J. B. M. NJOROGE Jomo Kenyatta University of Agriculture and Technology, Faculty of Agriculture, Department of Horticulture, P. O. Box 62000, Nairobi, Kenya Abstract Anthocyanins are a group of plant pigments responsible for colors ranging from red to violet and blue. Anthocyanins are pigments that accumulate in the vacuoles of plant epidermal cells. Chroma and hue are dependent on conditions inside and outside the vacuoles. Also conditions within the vacuole that facilitate formation of complexes with several metal ions. The aim of this study was to examine changes in accumulation of anthocyanins and the resultant colour in tuberose (Polianthes tuberosa Linn.) after application of amendments to the soil. Treatments were applied six weeks after planting as a top dress. Magnesium was given as magnesium nitrate and nitrogen was given as calcium ammonium nitrate CAN was neutral and ammonium sulphate (AS) was acidic. Soil and plant tissues were analysed every two weeks for nitrogen, phosphorus, potassium, magnesium, and pH. Colour was determined using a photoelectric tri-stimulus colorimeter and expressed in CIE L*a*b* colour space coordinates. In addition, chlorophyll and anthocya- nins in the florets were quantified. The amendments lowered the soil pH, especially for the AS and CAN. However, chroma and hue values as determined by the space coordinates, as well as the concentrations of anthocyanins in the floret, were not significantly linked to the soil pH following the amendments. Equally, the tissue elements and chlorophyll contents were similar between the amendments. The results of this study show that supplying magnesium through fertiliser application to the soil does not necessarily increase accumulation of Mg in tissues, and may ultimately not lead to accumulation of anthocyanins. Key words: Chroma, hue, metal complexes, Polianthes tuberosa, plant pigments Introduction Anthocyanins are a group of plant pigments responsible for colors ranging from red to violet and blue (van Tunen & Mol 1991). These pigments accumulate in the vacuoles of epidermal cell, and both their chroma and hue are dependent on external conditions, as well as on the pH in the vacuoles (Harborne & Grayer, 1988). Anthocyanins are able to accumulate in epidermal vacuoles and blend with the plastid pigments to give various hues that vary with light exposure and night and day temperatures (Sachray et al., 2002). In most flowers, anthocyanin synthesis occurs with petals growth and is under developmental control. For example, in petunia it occurs during the corolla elongation ( Weiss & Halevy, 1989) while in Lisianthus colouration occurs prior to the unfurling of the petals after the buds have reached their final size (Oren- Shamir et al., 1999). Temperature is one of the main external factors affecting anthocyanin accumulation in plant tissues: low temperatures increase, and elevated temperatures decrease, anthocyanin concentration (Zhong & Yoshida, 1993; Oren-Shamir & Nissim-Levi, 1997; 1999; Zhang et al., 1997). In petunia (Shvarts et al., 1997) temperature has been shown to have a significant effect on the expression of anthocyanin genes: low temperature conditions were accompanied by a several fold increase in transcript levels of genes whose products are either key enzymes in the general phenylpropanoid pathway such as phenylalanine ammonia lyase (PAL), or genes whose products catalyse reactions specific to flavonoid and anthocyanin biosynthesis, such as chalcone synthase (CHS), chalcone isomerase (CHI) and dihydroflavonol reductase (DFR). In addition, temperature may also affect the stability of anthocyanins. Therefore, the decrease in anthocyanin concentration at elevated temperatures may result from both a decrease in synthesis and an increase in degradation. Several studies have examined the effect of different metals on anthocyanin stability and hue in solutions. Mazza & Miniati (1993) reported that tin, copper, and aluminium ions are capable of forming stable complexes with anthocyanins. Stable ternary complexes containing anthocyanin, an unidentified colourless compound and magnesium (or magnesium plus ferric ion or aluminium) have also been described (Takeda et al., 1990; 1994; Kondo et al., 1992). The main known effect of metals on anthocyanins in flowers is a change in hue of the flower colour (Kondo et al,. 1992; Takeda et al., 1994). The red colour in many fruits and flowers has been found to be due to anthocyanins with a common aglycone (cyanidin) bound to different sugars, thereby producing different cyanidin glycosides (Montefiori et al., 2005). Huang et al. (2002) investigated the environmental effects on flower anthocyanin pigmentation using reddish-purple tuberose, and established that the primary component of the anthocyanin pigment in tuberose is cyanidin. The cut flower value of tuberose can be improved by increasing the reddish pink colouration attributed to anthocyanins. To identify the soil related factors associated with the formation of this colour in tuberose the study was carried out. The combined effect of low pH