Role of the Testa in Preventing Cellular Rupture During Imbibition of Legume Seeds

Abstract

Studies with the seeds of soybean, navy bean, pea, and peanut were made to determine the extent of leakage of intracellular enzymes during imbibition. Embryos with intact testae from all four species were found to leak detectable activities of either intracellular enzymes of the cytosol (glucose--phosphate dehydrogenase) or and that, with removal of the testa from seeds, the "leakage phenomenon" is enhanced (14,32). The amount of leakage during imbibition has been shown to correlate negatively with viability in studies with seeds of soybean (6, 39), pea (14,24), bean (17), and peanut (1) and has suggested to some that the leaked substances may, in some way, decrease viability. Another study has shown that removal of the testa of pea seeds results in death of the outer layers of cotyledonary cells during imbibition (25). The question arises: does the testa protect against leakage or is the leakage only a symptom of a fundamental dysfunction which can occur in imbibition?Two hypotheses have been promoted to explain the mechanism(s) ofleakage of solutes during the imbibition ofseeds. Larson (14) has suggested that cell membranes are ruptured during the initial phases of imbibition. Simon (31, 33) has proposed that the membranes of dry seeds are formed into hexagonal plates with pores formed in the areas of the phospholipid heads through which low-molecular weight solutes can leak from cells by passive diffusion during initial stages of membrane hydration (e.g. before phospholipids form typical bilayer membranes). Recently Powell and Matthews (25) have suggested that, in peas, cellular rupture and leakage through membranes may both occur when the testae are removed from seeds. To date, there has been a paucity of data that any macromolecules could move through the cell membrane during imbibition. Here, we have examined the leakage of imbibing seeds with and without testae for the presence of cytoplasmic, organelle, and organelle marker enzymes which would not pass through small membrane pores but which could only pass through very large membrane discontinuities or which would be the result of membrane rupture. In this way, we have tested both of the aforementioned hypotheses in a more definitive manner than has been hitherto reported.In the development of the legume seed, the testa appears to function in interconverting amino acids and sugars supplied by the phloem to the developing embryo (19,35,36) and in preventing injury by differentiating into a sclerified integument as the embryo matures (27). It has also been proposed that the testa protects seeds against "leakage" of intracellular substances during imbibition (32). This function has been suggested to be of great importance in the initial stages of germination of legume seeds in that many substances which leak from seeds may offer a substrate for potential pathogens (32). Past studies have demonstrated that electrolytes, sugars, amino acids, organic acids, and proteins are released from seeds during imbibition (1,6, 14,18,23,29,33,34) Mammoth Virgi...

The Role of Seed Structure and Oxygen Responsiveness in Pre-Germination Flooding Tolerance of Soybean Cultivars

“…Nordin (1984) demonstrated that the seed coat acted as a semi-permeable membrane, permitting the entry of water and certain solutes while restricting others. Duke and Kakefuda (1981) compared the seeds of four legumes with or without seed coats and found that the embryonic cells of soybean and navy bean seeds without seed coats ruptured when submerged. Matsui et al (1996) demonstrated, using seeds without a seed coat or aleurone layer, that the aleurone layer signifi cantly alleviated the extent of fl ooding damage in soybean.…”

Differential Leakage of Intracellular Substances from Imbibing Soybean Seeds

“…It has been proposed that the reason for this correlation is that seed pathogen growth may be enhanced by the presence of seed leachates (23). However, recent studies with imbibing legume seeds suggest that some loss of vigor may be due to the destruction of seed tissues and the creation of infection sites by massive cellular rupture which can occur during imbibition (7,9,11).…”

“…Seeds with various degrees of testa integrity were used because testa integrity is a primary factor in controlling legume seed leakage and cellular damage during germination (7,11,12,21,24). Effects of temperature were studied to gain insight into the energetics of the leakage of various seed components during imbibition and effects of 4,2 were studied because of previous studies which indicate a 4iw effect on soybean seed leakage and seedling vigor (28,29).…”

“…The most common measurements of seed leakage in steep water are conductivity of electrolytes (1,3,7,14,16,19,22,24,25,27,30) and UV light absorbance at various wavelengths (5,7,17,19).…”

Electrical conductivity testing of corn seeds as influenced by temperature and period of storage

“…Although EC test results are reproducible among laboratories, some factors affect the results, such as seed size (Tao, 1978;Deswal & Sheoran, 1993), soaking temperature (Murphy & Noland, 1982), soaking period (Loeffler et al, 1988;Schmidt & Tracy, 1989), initial water content (Tao, 1978;Loeffler et al, 1988;Vieira et al, 2002), and the presence of physical damaged seeds (Tao, 1978;Duke & Kakefuda, 1981). However, all these factors can be controlled to minimize their effects.…”