Foliar Response of Six Clones of Hybrid Poplar

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Abstract
After exposure to simulated acid rain at pH levels of 2.7-3.4, lesions of several types were produced on foliage of 6 clones of Populus spp. hybrids [P. nigra .times. P. laurifolia; P. maximowiczii .times. P. berolinensis no. 43 and no. 44; P. deltoides .times. P. trichocarpa no. 207 and no. 211; P. balsamifera cv. Candicans .times. P. berolinensis]. The types of adaxial leaf surface lesions were observed at low magnification, by scanning electron microscopy and via leaf histology. Galls resulting from hyperplasia and hypertrophy of parenchyma cells predominated on 2 clones. Two clones exhibited neither hyperplasia nor hypertrophy. In these clones the upper epidermis, palisade parenchyma and sponge parenchyma were injured in succession. In a 3rd set of 2 clones, hyperplasia and hypertrophy were present in areas between injured and apparently noninjured tissues. In general, percent leaf area with lesions and percent leaves injured were similar among all 6 clones at all pH levels tested. At pH 2.7, up to 10% of the leaf area was injured after 5 daily exposures of 6 min each. Injury decreased to about 1.0% at pH 3.4. Lesions developed mostly near stomata and vascular tissues and occurred most frequently on leaves just prior to maximum leaf enlargement. Very young and older leaves were less affected. The results uspport the hypothesis that the adaxial leaf surface is the most affected after exposure to simulated acid rain.