Origin of Fragaria polyploids. I. Cytological analysis

Leaves of resistant and susceptible breeding lines of two closely related varieties of cabbage, 'Glory' and 'Globe,' showing internal tipbum were examined anatomically. Since both resistant and susceptible lnes developed tipburn, the type of injury rather than the amount was considered. Regardless of the degree of resistance, cellular disorders in the tipburned areas were the same. Early symptoms showed small gray areas at the lobes of the leaves. The outer parenchymatous cells of these areas remained normal. Some of the xylem vessels became occluded with a granular substance; surrounding cells, including the idioblasts, enlarged. The idioblasts, reported to contain the enzyme myrosin, contained a particulate substance. Some idioblasts hypertrophied and also divided resulting in the distribution of the particles to additional cells. The conditions causing tipburn also affected cell walls, causing thickening and the production of pectic warts between cells and on walls facing the intercellular spaces. As more xylem vessels became occluded, and as the number of idioblasts with particles increased, the cells at the periphery of the leaf collapsed. INTERNAL TIPBURN of cabbage has plagued growers for some time, but the exact cause of this disorder has not been established. Investigators agree that the cause is not a pathogen. As reviewed by Natti and Atkin (1962), conditions that have been reported as causes of tipburn include high and low amounts of certain elements, environmental conditions, and cultural practices such as time of planting and harvest. Walker, Edgington, and Nayudu (1961) showed that compared to normal heads, tipburned cabbage heads had a lower Ca content and that the amount of Ca in interior leaves was much lower than in the outer leaves, and leaf marginal tissue contained the least. High soil moisture and poor drainage contributed to the severity of the disorder. Maynard, Gersten, and Vernell (1965) studied the effect of levels of Ca on a resistant and a susceptible variety of cabbage and found that tipburn was related to low Ca content in the tissue. Under stress conditions the resistant variety was able to transport and accumulate more Ca than the susceptible one. They suggested that this possibly resulted from genetic variability. The selection of varieties resistant to tipbum has been Received for publication February 23, 1966. Published with the approval of the director of the Agricultural Experiment Station. reported and is in progress by Walker, Williams, and Pound (1965). Natti and Atkin (1962) were planning a breeding program designed to select F1 hybrids of cabbage that would be resistant to internal tipburn and other diseases. Considerable attention has been focused on the causes of internal tipburn, but the internal cellular responses have not been investigated. Consequently, this i.nvestigation was conducted to determine what abnormal cellular responses occurred. MATERIALS AND METHODS-In 1961 samples of marginal leaf tissue showing mild and severe tipburn symptoms were taken from eight fieldgrown mature heads of resistant and susceptible lines derived from the variety 'Glory.' In each of the following 3 years samples were taken from resistant and susceptible lines derived from the variety 'Globe.' The extent of internal tipburn was not considered in the resistant and susceptible varieties; only the anatomical responses were studied. The leaves were fixed in formalin-aceticalcohol or Craf (Nawaschin type III) solutions. Marginal areas of healthy tissue were used for comparison. The fixed leaves were dehydrated with n-butyl alcohol, infiltrated and embedded in Paraplast (Aloe Scientific, Division of Brunswick, St. Louis 3, Mo.). Sections were cut 10 ,u thick on a This content downloaded from 207.46.13.153 on Fri, 05 Aug 2016 05:44:13 UTC All use subject to http://about.jstor.org/terms