CAUSAL MECHANISMS BY WHICH SULFATE, NITRATE AND ACIDITY INFLUENCE FROST HARDINESS IN RED SPRUCE - REVIEW AND HYPOTHESIS

This paper summarizes results from four experiments in which red spruc e seedlings (Picea rubens Sarg.) were exposed to simulated acid mist c ontaining SO42-, NH4+, NO3- and H+ ions. Seedlings were grown in compo st, with or without fertilizer, in charcoal filtered air in open-top c hambers near Edinburgh, Scotland. Plants were sprayed from bud burst b etween May and November with mist containing different concentrations and combinations of the four major ions to provide a range of doses, w hich were applied at different frequencies. Reductions in frost hardin ess expressed in terms of the temperature which killed 50% of shoots ( LT(50)) were significantly correlated with the dose of S received by t he seedlings. Differences in foliar S concentrations between the contr ols and treated plants were correlated with S dose. Absolute S concent rations were, however, of limited use for predictive purposes. Seedlin gs appear to be more sensitive than older trees to S toxicity because the former have the greatest proportion of newly expanding needles whi ch optimize conditions for foliar uptake. Seedlings are also least wel l equipped to export SO42- ions since they have a smaller resource of older foliage to supplement their assimilate pool. In conditions which promote uncontrolled SO42- ion uptake by foliage, i.e. high external SO42- concentrations and incompletely formed cuticles, the potential e xists for the internal build up of SO42- ions. It is proposed that in the absence of sufficient assimilate and N the presence of these high concentrations of SO42- ions in the apoplast or cytosol can lead to pr otein denaturation and loss of membrane integrity. Reductions in frost hardiness appear to result through direct attack by SO42- ions on mem brane proteins which impairs their function. Indirect effects on hardi ness occur through both an increased consumption of sugars reducing th e 'pool' available for cryoprotection and a reduction in photosyntheti c function, the ability to produce sugars. The presence of NO3- N miti gates the toxic effects of SO42- because SO42- ions are consumed in as similation processes which both utilize and are facilitated by the pre sence of large amounts of fertilizer N. High concentrations of SO42- a nd H+ are found to be particularly toxic because of the synergistic ef fects these ions exert on their mutual uptake with devastating consequ ences for the control of cellular pH. Trees growing at high altitude s ites are likely to be particularly sensitive to SO42- toxicity because (1) their carbon balance is low, (2) cuticle development is poor and (3) levels of soil available Ca2+ tend to be low relative to Al3+ so t hat membranes may already be weakened as a result of insufficient Ca2 ions for protein bridging.