Functional role of betalains in Disphyma australe under salinty stress

Foliar betalainic plants are commonly found in dry and exposed environments such as deserts and sandbanks. This marginal habitat has led many researchers to hypothesise that foliar betalains provide tolerance to abiotic stressors such as strong light, drought, salinity and low temperatures. Among these abiotic stressors, soil salinity is a major problem for agriculture affecting approximately 20% of the irrigated lands worldwide. Betacyanins may provide functional significance to plants under salt stress although this has not been unequivocally demonstrated. The purpose of this thesis is to add knowledge of the various roles of foliar betacyanins in plants under salt stress. For that, a series of experiments were performed on Disphyma australe, which is a betacyanic halophyte with two distinct colour morphs in vegetative shoots.  In chapter two, I aimed to find the effect of salinity stress on betacyanin pigmentation in D. australe and it was hypothesised that betacyanic morphs are physiologically more tolerant to salinity stress than acyanic morphs. Within a coastal population of red and green morphs of D. australe, betacyanin pigmentation in red morphs was a direct result of high salt and high light exposure. Betacyanic morphs were physiologically more tolerant to salt stress as they showed greater maximum CO₂ assimilation rates, water use efficiencies, photochemical quantum yields and photochemical quenching than acyanic morphs. Contrary to this, the green morphs, although possessing the ability to synthesise betalains in flower petals, did not produce betalains in vegetative shoots in response to salt stress. Moreover, green morphs, in terms of leaf photosynthesis, performed poorly under salinity stress.  In chapter three I further investigated the physiological benefit of betacyanin accumulation in D. australe. I postulated that betacyanin in the leaves of D. australe can protect the salt stressed chloroplasts from harmful excessive light by absorbing significant amount of radiation. To test this, a novel experimental approach was used; the key biosynthetic step for betacyanin synthesis was identified, which was deficient in vegetative shoots of the green morphs. By supplying the product of this enzymatic reaction, L-DOPA, betacyanin synthesis could be induced in the leaves of green morphs. This model system was used to compare the photoprotective responses of red vs. green leaves. The L-DOPA induced betacyanic leaves showed similar responses (such as smaller reductions and faster recoveries of PSII and less H₂O₂ production than in the green leaves) to naturally betacyanic leaves when exposed to high light and salinity. The differences in photoinhibition between red and green leaves were attributed to the light absorbing properties of betacyanins. L-DOPA treated and naturally red leaves showed lower photoinactivation than green leaves when exposed to white or green light, although not when exposed to monochromatic (red) light.  In chapter four, I used a similar experimental model to that in the third chapter and showed that other than photoprotection, betacyanins in leaves may be involved in salt tolerance by enhancing toxic ion (such as Na⁺) sequestration in betacyanic epidermal cells, storing Na⁺ away from sensitive mesophyll tissue. The Na⁺ localization between red and green leaves was compared after salinity treatment by using a sodium binding stain (SBFI-AM) and Cryo-SEM analysis. L-DOPA treated and natural red leaves sequestered Na⁺ ions to the epidermal cell layer. In contrast, green leaves retained Na⁺ in the mesophyll tissue, which suggested that red leaves were better equipped to tolerate salt-specific effects. Therefore, betacyanic plants were more tolerant to applied salinity stress and showed relatively higher growth rates than green morphs.  The findings of this thesis provide a significant contribution to our understanding of the role of betacyanins in plants under salinity stress. My data suggest that the multi-faceted properties of betacyanins (such as their photoprotective function, and their involvement in sequestration of toxic ions) clearly provide a benefit to plants under salinity stress.