Understanding the biochemical basis of flower colour in Australian native Ptilotus and Gomphrena

Summary of final report on the Australian Flora Foundation funded project:

Dr Dion K. Harrison, Dr Jitka Kochanek and Professor Daryl C. Joyce
Centre for Native Floriculture – School of Land, Crop and Food Sciences, The University of Queensland Gatton Campus  Grant Details    Final Report

Native Australian species of Ptilotus and Gomphrena have great ornamental potential. For example, P. nobilis and P. exaltatus have large and showy cream and pink coloured inflorescences but their limited flower colour range currently limits their ornamental potential. Smaller, less showy Ptilotus species with vibrant coloured flowers such as P. gaudichaudi (bright yellow) and P. spicatus (deep red) do not readily cross hybridise with other Ptilotus species. Gomphrena flaccida and G. leontopodioides have many small spherical to globular pink/purple inflorescences, but also lack colour variation. In order to devise the most appropriate breeding and selection strategies for manipulating flower colour, an understanding of the biochemistry of the pigments involved in colour is first required. While flower colour in most plants results from anthocyanin or carotenoid pigments, Ptilotus and Gomphrena plants have betalain pigments instead. There are two basic types of betalains, red-purple betacyanins and yellow-orange betaxanthins. Knowledge of the major betalain pigments in Australian Ptilotus and Gomphrena is lacking. In this project we used HPLC and LCMS methods to characterise the biochemical basis of flower colour for seven Australian Ptilotus and three Australian Gomphrena accessions. We showed that betalains were present in flowers of all the accessions studied. The highest betalain content was in the dark red flowers of P. spicatus (4.2 mg/g DW) while the pale yellow-green coloured flowers of P. nobilis cv. Purity contained the lowest betalain pigment content (0.09 mg/g DW). Flowers with purple-red colours typically contained only betacyanins, while those with yellow colours contained only betaxanthins. The exception was P. nobilis cv. Passion which contained mostly betacyanins but also trace amounts of betaxanthins. In total, nine major betacyanins (including two unknown pigments), one major betaxanthin and three minor unknown betaxanthins (present in trace amounts) were found in this study. Both acylated and non-acylated betacyanins were found. Flowers predominantly containing acylated betacyanins were more purple in colour (e.g. all Gomphrena, P. helipteroides, P. exaltatus var. semilanatus) while those with non-acylated betacyanins were more red or pink (e.g. P. spicatus, P. exaltatus cv. Joey). Interestingly, the majority of betacyanin containing flowers had acylated forms as the major pigment. In contrast, previous studies on exotic species in the Family Amaranthaceae found that acylated betacyanins were typically absent or present as minor pigments. Given the diversity of betalain pigments in showy Australian Ptilotus such as P. nobilis, there is real scope for breeding cultivars with novel vibrant flower colours through selecting for higher betalain content and altering the relative ratios of the various different betalain pigments.