A Histological Study of the Effects of Weather Fleck on Leaf Tissues of Flue-Cured Tobacco

Abstract

Microscopic studies were conducted on weather fleck, a non-parasitic plant disorder of flue-cured tobacco in Ontario. Within a few hours of the initial appearance of the flecks, damage was restricted to the palisade parenchyma cells; after about one day some of the spongy parenchyma cells also were affected. There was no apparent difference between flecked and healthy tissue with respect to thickness or structure of the cell wall. In affected tissue, the nuclei of the palisade cells were shrunken to spherical, ellipsoid, or irregularly ellipsoid shapes with average measurements of 6.0 × 5.1 μ; in adjacent undamaged cells, they averaged 9.1 × 7.3 μ. The nuclei of damaged cells stained more heavily than nuclei of normal cells and their structural details were lost. Chloroplasts of flecked and healthy palisade cells were almost of an equivalent size, the average lengths being 4.3 μ and 4.4 μ, respectively. However, chloroplasts in flecked cells had often been disrupted. Evidence is presented to support the hypothesis that the unknown agent or agents causing weather fleck first affect the cytoplasm and nucleus, then the chloroplasts.

The effect of ozone on leaf cell membranes

“…In our ultrastructural studies, the palisade parenchyma cells were the most extensively disrupted with massing of the cytoplasm in the center of extensively damaged cells; but less drastic alteration in spongy mesophyll cells was seen as well. This is in agreement with light-microscope studies which indicate severe disruption of palisade cells (2, 3, 7, ll), and some change in spongy mesophyll in advanced stages of leaf injury (21).…”

Varietal Tolerance of Tobacco to Ozone Dose Rate

“…Avery (1) found that cells of the apical sector of tobacco leaves were first to reach physiologic maturity. The relationship between ozone susceptibility and the development of leaf maturity has been observed by many investigators (6,8,13). Beyond the initial display of symptoms at the leaf tip, Bel-B and Bel-W3 exhibited quite a different response to ozone doses.…”

Illustration of the Genetic Differences in Ozone Sensitivity Between the Varieties Nicotiana tabacum Bel W3 and Bel B Using Various Plant Systems

“…Different stomatal reactions between the genotypes may exist and contribute somewhat to whole plant behaviour: ozone-caused cytological alterations of guard cells were found after threshold-exceeding fumigation. The disappearance of the nucleus structures was an indication of approaching cell death (Povilaitis, 1962) which was seen in necrotic leaves (Fig.6 E). During fumigation a significant difference between Bel J1.j and more tolerant varieties was found in stomatal resistance, but not if the plants had plenty of water (Rich and Turner, 1972).…”

“…a) The exposed surface plays an important role in tissue sensitivities and was high in the mesophyll of sterile plants containing many intercellular spaces. It has been known for a long time that ozone damages the palisade parenchyma, not the spongy parenchyma, in spite of the fact that ozone enters through the stomates of the lower leaf side, which was histologically documented by Povilaitis (1962). Evans and Ting (1974) suggested that this was due to the surface-to-volume quotient, which was found to be 1.6-3.5-fold higher in dicotyledonous palisade parenchyma cells than in spongy parenchyma cells (Turrell, 1936).…”