Milford S. Brown

Freezing of nonwoody plant tissues: III. Videotape micrography and the correlation between individual cellular freezing events and temperature changes in the surrounding tissue

Abstract A new technique was developed for the observation and recording on videotape of thermal and microscopic changes that occur simultaneously during the freezing of cucumber tissue. The freezing process occurs in two steps. Nucleation and growth of ice crystals in the continuous extracellular liquid phase is followed by nucleation and growth of ice crystals in individual supercooled cells. The freezing of cells in rapid succession causes the average temperature to remain constant for a short time. This mechanism explains the second freezing plateau found in most plant tissue freezing curves.

Effects of Water and Mineral Nutrient Deficiencies on Pyrrolizidine Alkaloid Content of Senecio vulgaris Flowers

The flowers of Senecio vulgaris contain the alkaloids senecionine, sene- ciphylline, and their N-oxides, compounds which are toxic to some grazing animals. Deficiencies of water, mineral nutrients, or both did not significantly alter concentrations of the individual alkaloids. However, more favourable growing conditions resulted in a greater quantity of flowers, which animals find quite palatable, so that each of these plants was potentially more toxic because it contained a larger total amount of the alkaloids.

Frozen Fruits and Vegetables: Their Chemistry, Physics, and Cryobiology

Publisher Summary This chapter discusses that freezing has become an important method of food preservation during the last 30 years. If the most desirable food quality is that of the freshly harvested fruit or vegetable, then the ultimate in frozen food would be perfect preservation of the living state. Water is the major component of plants and most plant parts (except seeds and woody stems) and the major component undergoing a phase change during freezing. Water, the major component of plants, undergoes a phase change during freezing. When solutions freeze, loss of water to the solid phase leaves the remaining solution more concentrated. Plants differ widely in their responses to low temperatures. Seeds are a means of reproducing and increasing the number of plants. In plants, beyond the young seedling stage, a number of responses to chilling have been observed. Most plants that survive freezing temperatures during winter experience a frost-free season during which their major growth takes place. Dehardening (loss of the ability to survive low temperatures) is the response of hardened plants to temperature increases. Fruits and vegetables include a wide range of plant parts harvested and eaten at various stages of maturity. Fresh fruits and vegetables are living organs that continue their metabolic processes after they have been harvested. The chapter discusses that fruits and vegetables are obtained from a variety of plants whose edible parts are harvested at times ranging from the extreme immaturity of germinating seedling to the senescence of mature fruit.

Some structural and histochemical changes related to frozen fruits and vegetables.

Summary Freezing damage and freeze killing of living plant tissues contrast sharply with problems of commercial freezing preservation of fruits and vegetables. Enzymes of most vegetables and some fruits are inactivated, usually by heat, prior to freezing. Chemical treatments are also used, including sulfiting and freezing in sugar or heavy syrup. Thus the life processes of the ceils are either destroyed or greatly changed, and ice crystal formation is accordingly affected. The senescent condition of ripe fruits contrasts both structurally and physiologically with the immature stage at which most vegetables are harvested for prime quality. Compositional changes in growth and ripening of fruits have additional bearing on some of the problems encountered in freezing preservation. Included are changes in phenolic compound content, methyl esterification of pectins, and crystallinity of cellulose. In addition, the dehydration of cellulose during freezing may account for changes in textural qualities of frozen fruits and vegetables; cell wall toughening is superimposed on the loss of succulence when membrane semipermeability is destroyed. When cooked potatoes are frozen, gelled starch undergoes marked physical changes which result in a lowered capacity to hold water. This has formed a basis for the control of texture in potato products. Green beans have tissues at different stages of maturity when they are harvested. The cells of the embryonic parchment layer or fiber sheath of the young pod are very thin walled, and much smaller than the other young parenchyma cells. These cells are the most susceptible to freezing damage, followed by the parenchyma cells of the center of the pod. Cells of the outer parenchyma have relatively thick walls, and often separate from each other with no breakage of the individual cell walls. Thus differences in maturity and structure between plant tissues are highly significant to the pattern of damage produced by freezing.

The glycine‐glomus‐rhizobium symbiosis III. Endophyte effects on leaf carbon, nitrogen, and phosphorus nutrition

Abstract Soybean plants [Glycine max (L.) Merr.] were grown in pots and inoculated with Rhizobium japonicum and/or Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe, either at planting or 20 days later. Nitrogen was supplied in the nutrient solution to plants without nitrogen‐fixing bacteria, and P was added to those without the mycorrhizal fungus. At harvest, 50 days after planting, all plants had leaves of similar dry mass. Each root symbiont grew best in the absence of the other. Growth of Glomus reflected the duration of its growing time and the presence and duration of competition from Rhizobium. Nodule weight in the tripartite associations, on the other hand, was inhibited only by the earlier introduction of Glomus. Dipartite associations and the plants inoculated with both root symbionts at planting had the highest concentration of leaf N, and the lowest was in those inoculated with both organisms at d 20. Leaf P was highest in plants inoculated only with Rhizobium, and lowest in those tripartite asso...