Recently, premature yellowing and abscission of pear leaves believed to be the typical symptoms of photochemical smog injury, have been observed in our orchard in the summer. This study was carried out by comparing the leaves injured in the field with those of trees kept in a controlled smog chamber.
The results obtained are summarized as follows:
1) The symptoms of chlorosis, browning and necrosis were found on the upper surfaces of mature leaves of growing shoots.
2) Leaf lesions caused a notable decrease of the total chlorophyll and chlorophyll b contents in the leaf. When the leaf was slightly injured, starch accumulated remarkably in the leaf and then gradually disappeared as the injury worsened.
3) The slightly injured leaf showed increased O₂ uptake for respiration, but the respiratory quotient (RQ) remained at a lower level than that of the normal leaf on account of decreased CO₂ production. Although the O₂ uptake increased to some extent, the RQ of heavily injured leaf was low because of the decrease in CO₂ production.
4) Nutrient contents in the injured leaves decreased on the whole. In particular, N and Mg contents notably decreased even in the slightly injured leaves. Though N and Ca contents declined in proportion to the degree of injury, further reduction in P, K, and Mg contents did not occur in the heavily injured leaves.
5) High correlation coefficients were found between RQ and amounts of total chlorophyll, and chlorophyll a and b respectively. Correlation coefficients between N and total chlorophyll, between Ca and total chlorophyll, between N and RQ, and between Ca and RQ indicated significance at 0.1% level.
6) The symptoms observed on Japanese pear leaves appeared to be caused by ozone or peroxyacetyl nitrate (PAN). However, since the photochemical smog of Tokyo contains a large percentage of SO₂ and dust, it is possible that the injury observed was due to synchronistic actions between two or more of the pollutant factors.