Effect of desiccation and scarification on the permeability and structure of the seed coat of Cuscuta campestris

Changes occurring within the seed coat of Cuscuta campestris Yunkr. during desiccation were correlated with a decrease in germinability. Germination initially increased as the embryo matured, but then decreased as the testa dried. Completion of macrosclereid development signaled the onset of dormancy. Mechanical scarification with sandpaper or immersion in concentrated H2S04 broke dormancy. The impermeable layer is a region above the light line and at the junction of the hypodermis and the palisade cell layers. This region appeared to be deposited during desiccation of the hypodermis. Overwintering or a period of cold storage also broke dormancy. The change in permeability was found throughout the testa rather than in a specific area. THE MATURE SEEDS of field dodder, Cuscuta campestris Yunker, do not germinate readily and are termed hard-seeded. The maintenance of the dormant state has been attributed to exclusion of water by the testa (seed coat) (Gaertner, 1950). The location of the impermeable layer(s) was not determined. Yunker (1942) observed that immature seeds of Cuscuta nevadensis could germinate while still in the capsule. Gaertner (1950) confirmed this report and concluded that desiccation was, in part, responsible for dormancy. Various methods have been employed to germinate dormant dodder seeds. Drummit (1946) pierced the hilum region with a pin; Kinzel (quoted by Gaertner) was the first to soak the seeds in concentrated H2S04; Walzel (1952) ground the seeds with glass particles; while Tingey and Allred (1961) scarified the seeds with sandpaper. It has not been shown which regions of the seed coat are altered by these treatments. Further1 Received for publication 10 May 1978; revision received 19 July 1978. The authors thank Dr. Thomas L. Rost for reading this manuscript, Naomi Fuginomi for Fig. 16, and the California Tomato Growers Advisory Board for financial assistance. more, the mechanism by which dormancy is overcome under field conditions is unknown. Many leguminous seeds also have hard seed coats. A large literature dealing with the anatomy and function of the testa of these species has accumulated (see reviews by Hamly, 1932; Watson, 1948; Porter, 1949). Workers have disagreed as to the location of the barrier to water. Watson (1948) regarded the cuticle as the impermeable layer, whereas Coe and Martin (1920) reported that the cuticle was permeable and that portions of the epidermal cells in the region of the light line excluded water. Reeve (1946), however, concluded that the light line in Pisum sativum was an optical phenomenon caused by light refraction in the upper portions of the macrosclereids and should not be assigned special functions of texture and permeability. Scott et al. (1962) have observed a marked difference in the microfibrillar pattern in the light line of the seed coat of Cercidium floridum. Marbach and Mayer (1974, 1975) have implicated the color changes and increase in phenolics in seed coats of Pisum with changes in permeability. In this study the anatomical changes occurring KEY TO LABELING: E, epidermis; H, hypodermis; P, palisade; L, light line. Fig. 1-8. Seeds of Cuscuta campestris. 1. Comparison of three stages of seed maturation; A, immature; B, intermediate; C, mature. x7. 2, 3. Median section of immature testa. Note abundant organelles in palisade cell layer and starch-containing parenchyma beneath. Fig. 2 x200; Fig. 3 x900. 4. Scanning electron micrograph of a mature seed. x50. 5. Higher magnification showing surface reticulation. x 1,200. 6. Portion of testa of a seed dried for 7 days. The cytoplasm of the hypodermal cells has coagulated and begun to precipitate. Secondary wall formation of the developing macrosclereids has begun. x 1,500. 7. Cross-section of the mature seed coat showing the epidermis, hypodermis, and palisade cell layers. The light line is distinct. x 1,000. 8. Scanning electron micrograph of a germinating seed. Arrow indicates the intact hilum region. x50.