The implausibility of the osmotic adjustment hypothesis for anthocyanin synthesis in leaves

Background/Question/Methods

The association between plant water stress and synthesis of red anthocyanin pigments has led plant biologists to consider an osmotic role of anthocyanin pigments in leaves. According to this hypothesis, anthocyanins function as a solute in osmotic adjustment, contributing to depression of cell osmotic potential and maintenance of turgor pressure in drought-tolerant plants.  Here, we argue that anthocyanins concentrations measured in vivo are inadequate to significantly affect the osmotic potential of plant cells.  Calculations are based on mean anthocyanin concentrations and leaf osmotic potentials of three, drought-tolerant anthocyanic (red-leafed) angiosperm evergreen species experiencing winter drought stress in the field (i.e. mid-day Ψ_W between -1.8 and -3 MPa). 

Results/Conclusions

Anthocyanin concentrations ranged from 1-4 mM, and are within the range of anthocyanin concentrations reported in other studies.  Total solute content, estimated from Ψ_π,100 via pressure-volume methodology, ranged from 700-1100 mM during winter reddening, a 120-440 mM increase relative to summer.  Assuming anthocyanins are synthesized completely de novo, we estimate that they can only account for ~0.1-0.3% of total cell solute content and Ψ_π during winter.  When maximum solute-solute interaction coefficients are included in calculations, we arrive at an upper limit of anthocyanin contribution to cell Ψ_π near 1%.  We conclude that anthocyanins most likely do not significantly affect cell osmotic potential or perform an osmotic function in drought-tolerant tissues, and more likely play another role within the context of drought acclimation, e.g. light attenuation and/or antioxidant functions.