Functional significance of betalain biosynthesis in leaves of Disphyma australe under salinity stress

Shoots of Disphyma australe, a coastal succulent plant native to New Zealand, vary in colour from entirely red to entirely green. We hypothesised that the red pigmentation develops in response to salinity stress, and that these betalain pigments contribute to salt tolerance. Effects of salinity on betalain content, CO2 assimilation, stomatal conductance, chlorophyll content and chlorophyll fluorescence were measured in leaves from red and green-leafed morphs. Newly formed leaves of both morphs were entirely green when grown under control conditions in a glasshouse. NaCl treatment increased betalain concentration 10-fold in leaves of the red, but not of the green morphs. The red leaves held six betacyanins (betanin, isobetanin, betanidin, isobetanidin, lampranthin-II, isolampranthin) but no betaxanthins; in the green morphs, neither betacyanin nor betaxanthin was present. In contrast, betalains were present in the petals of both morphs. Photosynthetic CO2 assimilation and water use efficiency were greater, and stomatal conductance was lower, in leaves of the red than of the green morphs following NaCl treatment. Photosystem II quantum yields and photochemical quenching were both greater in red than in green NaCl treated leaves under white actinic light. The data indicate that betalain accumulation in red morphs is a direct response to salinity, but that the green morphs, although possessing the genetic potential to biosynthesise betalains, lack the mechanism for the induction of betalain in response to salinity stress. Foliar betalains appear to ameliorate responses to salinity stress in D. australe. (C) 2014 Elsevier B.V. All rights reserved.