Causal mechanisms by which sulphate, nitrate and acidity influence frost hardiness in red spruce: review and hypothesis.

This paper summarizes results from four experiments in which red spruce seedlings (Picea rubens Sarg.) were exposed to simulated acid mist containing SO , NH , NO and H ions. Seedlings were grown in compost, with or without fertilizer, in charcoal filtered air in open-top chambers near Edinburgh, Scotland. Plants were sprayed from bud burst between May and November with mist containing different concentrations and combinations of the four major ions to provide a range of doses, which were applied at different frequencies. Reductions in frost hardiness expressed in terms of the temperature which killed 50% of shoots (LT ) were significantly correlated with the dose of S received by the seedlings. Differences in foliar S concentrations between the controls and treated plants were correlated with S dose. Absolute S concentrations were, however, of limited use for predictive purposes. Seedlings appear to be more sensitive than older trees to S toxicity because the former have the greatest proportion of newly expanding needles which optimize conditions for foliar uptake. Seedlings are also least well equipped to export SO ions since they have a smaller resource of older foliage to supplement their assimilate pool. In conditions which promote uncontrolled SO ion uptake by foliage, i.e. high external SO concentrations and incompletely formed cuticles, the potential exists for the internal build up of SO ions. It is proposed that in the absence of sufficient assimilate and K the presence of these high concentrations of SO ions in the apoplast or cytosol can lead to protein denaturation and loss of membrane integrity. Reductions in frost hardiness appear to result through direct attack by SO ions on membrane proteins which impairs their function. Indirect effects on hardiness occur through both an increased consumption of sugars reducing the'pool'available for cryoprotection and a reduction in photosynthetic function, the ability to produce sugars. The presence of NO N mitigates the toxic effects of SO because SO ions are consumed in assimilation processes which both utilize and are facilitated by the presence of large amounts of fertilizer N. High concentrations of SO and H+ are found to be particularly toxic because of the synergistic effects these ions exert on their mutual uptake with devastating consequences for the control of cellular pH. Trees growing at high altitude sites are likely to be particularly sensitive to SO toxicity because (1) their carbon balance is low, (2) cuticle development is poor and (3) levels of soil available Ca tend to be low relative to Al so that membranes may already be weakened as a result of insufficient Ca ions for protein bridging.