Fine Structure of Acid Mist Treated Sitka Spruce Needles: Open-top Chamber and Field Experiments

Sitka spruce grafts (clones DF and 141) grown in open-top chambers(OTC) and ‘mature ’, 6 –8m tall Sitka spruce(clone DF) grown in the field were exposed to acid mist containingan equimolar ion mixture of H₂SO₄and NH₄NO₃at pH2.5. Mist wasapplied 4 times a week (4 x1mm) in the OTC experiment and twicea week (2 x2mm) on average in the field experiment, betweenMay and Oct. 1991. Samples for light and electron microscopywere collected in Nov. and Jan. following acid mist treatment.Acid mist significantly decreased the amount of calcium depositedin the outer epidermal cell walls, the reduction being mostpronounced in the OTCs. Ultrastructurally, acid mist causeda significant increase in chloroplast and grana width. Othersymptoms associated with acid mist included swelling of chloroplastthylakoids, chloroplast protrusions, cytoplasm vacuolization,increase in large lipid accumulations and sickle-shaped chloroplastthylakoids. In the OTCs, acid mist hastened the acquisitionof frost hardening in both clones. In the field, the controltrees exhibited more frost injury than the acid mist treatedtrees suggesting, again, that acid mist had either hastenedor enhanced the stage of frost hardiness of treated trees. Ingeneral, acid mist induced changes were more pronounced in theOTCs than in the field. Picea sitchensis; Sitka spruce; acid mist; fine structure; calcium oxalate; frost hardening