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The density of excised 2-cm celery (Apium graveolens L.) petiole segments was highly correlated with a subjective evaluation of pithiness. Loss of density and the appearance of pithiness was stimulated by lengthening the duration of storage, raising the storage temperatures above 0C, and excising petiole segments. Segments excised from the upper two- thirds of the petiole lost less density during storage than segments excised from the bottom third of the petiole. Segments with initial high densities lost slightly less density during storage at 5C for 5 weeks than segments that were initially less dense. The extent of pithiness development varied significantly among six cultivars held at 5C for 2 weeks. Treating whole petioles with 1 µM abscisic acid for 4 days significantly increased density loss. Exposing petiole segments to up to 100 µl·liter -1 ethylene in humidified air for up to 2 weeks at 5C did not significantly change density over air controls. The loss of density and the development of pithiness in lightly processed celery petioles could be reduced by selecting resistant cultivars, monitoring water stress during growth, using only segments excised from the upper two-thirds of the petiole, and selecting segments with initial high densities. Air spaces that form in the cortex of many plant tissues such as celery petioles, corn roots, tomato stems, and roots of wetland species are collectively called aerenchyma. Aerenchyma develops through the lysis of parenchymatou s cells of the cortex and is stimulated by a variety of stresses, such as deficiencies in water, oxygen, and minerals, and is thought to enhance oxygen transport within the plant since it is associated with the tolerance of some crops to anaerobic soils (Drew et al., 1979). When aerenchyma forms in tomato stems and celery petioles, it is often referred to as pithiness. Pithiness is characterized by the appearance of whitish regions and air spaces within the tissue, and is a major source of quality loss and decreased shelf life in celery. Several climatic and storage factors have been associated with the development of pithiness in field-grown celery plants, in harvested stalks and processed petioles. Abscisic acid (ABA) may have a role in aerenchyma development since an increase in endogenous levels of ABA followed water stress and the develop- ment of aerenchyma (Livne and Vaadia, 1972). Water stress during growth favors development of pithiness in celery (Aloni and Pressman, 1979). They found that the level of free ABA in water stressed celery increased before the onset of pithiness and was related to the development of pithiness in celery petioles. Withholding water increased the level of ABA 7-fold after 24 h, with a rapid increase in pithiness occurring after 3 days. Also, exogenously applied ABA induced pithiness in detached petioles, with the degree of pithiness increasing with ABA concentration. They hypothesized that ABA production induced by water stress stimulated the development of pithiness. Aloni and Pressman (198 1) subsequently investigated the relationship between water stress in the field and ABA content on the development of pithiness in tomato stems. The degree of pithiness was greater in drought- stressed plants treated with 20 µ M ABA than in water-stresse d plants alone. Nonstressed plants treated with ABA exhibited more pithiness than controls, but less than water-stressed plants. Aloni

Using Density Measurements to Study the Effect of Excision, Storage, Abscisic Acid, and Ethylene on Pithiness in Celery Petioles