Evaluating the functional role of betacyanin in salinity tolerance of Horokaka (Disphyma australe)
Yield loss in agriculture due saline soils is a growing problem in arid and semi-arid regions as traditional crop species are inherently sensitive to salinity in the root zone. In the face of diminishing fresh water resources it is necessary to explore the traits which allow naturally salt tolerant species to exploit high saline environments. In the hope of transferring these traits via genetic modification to traditional crop species, or utilising these species as niche crops in their own right. While a majority of plants appear green, red pigmented plants are commonly associated with marginal environments. In these leaves anthocyanins or less commonly betalains are responsible for leaf reddening. The betalains are small class of tyrosine derived chromo alkaloids found in the core Caryophyllales and in some Basidiomycetes. There are two structural groups: the red/violet betacyanins and the yellow/orange betaxanthins. Due to this distribution pattern, betalain pigments are thought to function in salinity stress tolerance. However, minimal research has been conducted to support this salinity tolerance hypothesis due to a lack of an appropriate model species. Horokaka (Disphyma australe) exhibits colour dimorphism among populations, green and red morphs grow contiguously in coastal environments where the frequency of red morphs positively correlates with increased substrate salinity. Betacyanins have previously been implicated in serving a photo protective for D. australe. In dimorphic populations D. australe along the south Wellington coastline, the red morph has been shown to be more tolerant to the combination of high light and salinity, as measured by higher CO2 assimilation rates, reduced inhibition of PSII and enhanced water use efficiency relative to the green morph. In these studies, betacyanin production in the red morphs was shown to depend on duel exposure to both salinity and high light, however the green morph was unable to produce betacyanin under the same conditions (Jain & Gould, 2015). This easy manipulation of leaf colour by salinity and high light offers a system to study whether betacyanin pigments aid salinity tolerance. I aimed further investigate the photo protective hypothesis of betalain using D. australe, and how this may influence distribution patterns by focusing on three areas: the capacity for new root growth along a salinity gradient, germination capacity under saline conditions, and ion content in the roots at low, moderate and high NaCl concentrations. Shoots with no roots and a minimum of two mature leaf pairs were cut from green and red morphs of D.asutrale growing in the greenhouse facilities at Victoria University of Wellington. The shoots were grown hydroponically in 10% Hoaglands solution supplemented or not with (50, 100, or 150 mM) NaCl. To test the light screening capacity of leaf betacyanin a red filter was secured of half the green shoots, the cuttings were grown for 5 weeks under a controlled 16h light/ 8h dark photoperiod. Final weights of the shoot and roots, along with tissue water content of the shoots and roots were obtained to establish the relative capacity for new root growth when subjected to increasing salinity. Seeds were germinated in vitro in the presence of increasing NaCl concentrations (0, 100, 200, 300, and 400 mM NaCl), and subject to recovery tests after stress. The germination percentages and velocity were determined to establish te relative tolerance and competitiveness of the two D. australe morphs. Salt treatments were also applied to plants with an established root system, by 14-day treatment with increasing NaCl concentrations (0, 200, 400, 800 mM). The tissue water content of the shoots and ion contents (Na+ and K+) in the roots were determined in the control and the stressed plants of the two colour morphs. The different germination behaviour of the two morphs and capacity for root development appears to contribute to their distribution along a salinity gradient. Despite some differences under the control treatment, the concentrations of the two ions (Na+ and K+) were similar in the two morphs, not explaining differences in salinity tolerance, except for the increase of K+ in the roots of the green morph in the absence of NaCl. This specific response may be relevant for distribution patterns in D. australe. The ecological implications of these findings, which can contribute to vegetation distribution of D. australe in coastal environments, and the relevance of betacyanin accumulation in salinity tolerance for halophytes, and potential application for improved crop vigour are discussed.