2014 ASHS Annual Conference

Many plant species are prone to physiological disorders in which lesions develop on the leaf tissue. Nomenclature for such lesions has included intumescences, excrescences, neoplasms, galls, genetic tumours, enations and oedemata. Interchangeably used, these terms cause confusion as to whether they refer to the same or different disorders. Two of the most commonly used names are oedema and intumescence. The objective of this study was to characterize the development of lesions on ornamental sweet potato (Ipomoea batatas ‘Blackie’), tomato (Solanum lycopersicum var. hirsutum ‘Maxifort’) and interspecific geranium (Pelargonium  x ‘Caliente Coral’) to determine similarities and differences between them. Light microscopy was used to characterize differences in height, width, and area of lesions on each species. Leaf tissue samples were embedded in paraffin, and 10 µm sections were obtained and stained with toluidine blue to allow identification of specific cell layers involved with lesion development. Field emission scanning electron microscopy (FESEM) and digital photography were used to observe the micro and macroscopic stages of lesion development, respectively, on each species. The lesions found on sweet potato were significantly greater in height (629 µm) and area (240 x 10³ µm²) than on tomato (height: 157 µm; area: 119 x 10³ µm²) and geranium (height: 187 µm; area: 104 x 10³ µm²). Tomato lesions had the greatest width (1137 µm), followed by geranium (808 µm) and sweet potato (383 µm), respectively. Lesions on sweet potato occurred on the adaxial surface of the leaf, with hypertrophy and hyperplasia of the epidermal and palisade parenchyma cell layers.  The lesions on geranium and tomato predominantly occurred on the abaxial surface of the leaf. Lesions on geranium involved hypertrophy of the spongy parenchyma cells, while lesions on tomato involved hypertrophy and hyperplasia of epidermal and spongy parenchyma cells. The involvement of epidermal cells in the lesions on sweet potato and tomato allowed for greater cell expansion, while the restriction of spongy mesophyll cell expansion by the lower epidermis on geranium ultimately resulted in epidermal tearing. Previous research indicates that light quality may be a significant factor in lesion development in tomato and sweet potato, while plant water status is a more significant factor in geranium. Based on our observations, the two disorders are different.  The term ‘intumescence’ should be used when referring to abnormal lesions on tomato and sweet potato and the term ‘oedema’ should be used when referring to lesions on geranium.