Biology of over-wintering of perennial root nodules and seed development in beach pea (Lathyrus maritimus L.)

Chinnasamy, Gurusamy (2000) Biology of over-wintering of perennial root nodules and seed development in beach pea (Lathyrus maritimus L.). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Beach pea (Lathyrus maritimus L.) is a potential cold-climate circumpolar legume crop that naturally grows along the shorelines of Newfoundland, Canada. The present study was undertaken to determine the over-wintering (winter survival) strategies of perennial root nodules of beach pea on the sandy beach of Salmon Cove, Newfoundland, with emphasis on oleosomes (lipid bodies). Seasonal changes in other biochemical constituents of nodules such as carbohydrates, proteins and elements were also studied. In addition, attempts were made to screen the suitable rhizobial inoculum for beach pea and also to trace the patterns of seed development, maturation and abortion in beach pea. -- The native rhizobial strain (ACCCRC) isolated from beach pea nodules was found to be the most effective inoculum. It was superior in nitrogen-fixing traits compared to other bacterial strains. Studies on seasonal changes in nodule structure revealed that perennial nodules undergo winter dormancy when the aerial parts of the plant dry out. With the advent of spring, the nodule meristem is activated to regenerate effective nodule tissues that perform normal functions of the nodule in the following summer. A higher number of oleosomes was seen in histological preparations of winter nodules compared to summer nodules. As winter approached, nodules stored higher amounts of lipids, which decreased gradually throughout winter and spring. The degradation of oleosomes could be localized at the electron microscopic level. -- Phospholipids and monoglycerides were identified as the major oleosomic lipids in perennial root nodules. C16:0, C18:0 and C18:1 were the dominant fatty acids. The fatty acid composition and double bond index differed among lipid classes depending upon the season. Overall, the level of many unsaturated fatty acids increased and many saturated fatty acids decreased in oleosomes of winter nodules. Nodules selectively utilized fatty acids depending upon the season to overcome environmental influence. -- Microscopical studies showed that nodule cells accumulated large amounts of amyloplasts with multiple starch grains in summer to be used in winter, and the degraded products could be visualized by electron microscopy. This was confirmed by quantitative data that starch content of nodules increased during the summer and decreased during the winter. Soluble sugars and non-reducing sugars increased in the winter, whereas reducing sugars increased in the fall. These results indicate that perennial nodules act as temporary storage organs. Nodules have large amounts of storage organelles such as oleosomes and amyloplasts during pre-winter months. These storage organelles may be catabolized to protect the nodule tissues from cold temperatures and also mobilized just before the growing season to support re-growth. Accumulation of more sugars in winter may help to maintain high osmolarity of cells that prevents freezing of dormant nodule tissues. Ultrastructural morphology of winter nodules confirms the lack of freezing damage. -- Protein contents of nodules increased as the winter approached. A wide variation was observed in season-specific accumulation of amino acids and elements. Amino acids such as arginine, cystathionine, ethanolamine, histidine, hydroxyproline, ornithine and proline increased in winter nodules, whereas the level of y-aminobutyric acid declined at the same time. Nodules collected in winter contained higher amounts of P, K, Ca, Mn, Cu and Zn, while summer nodules showed more Cl, Na, S, Mg, Al, Si, Mo and Fe. The possible involvement of some amino acids and elements in cold adaptation was discussed. This study shows that perennial nodules of beach pea have complex winter survival strategies that involve modifications of anatomy, physiology and biochemistry of the nodules. -- Generally, beach pea seeds showed almost a sigmoidal pattern of development. The stage 6 (S6) was identified as the physiological maturity stage in both beach pea and grass pea (Lathyrus sativus L.). This study suggests that harvesting of a seed crop when physiologically mature will ensure seeds of the best quality. Beach pea seeds exhibited precocious germination as well as hardseededness at S4. Hard seed coats completely prevented water absorption and germination at S5 and S6. In grass pea, precocious germination started at S3 and increased with seed maturity. There was no hardseededness in grass pea. Studies on seed abortion revealed that cryptic seed abortion was observed early in pod ontogeny in both beach pea and grass pea. Only 68% and 82% of ovules produced mature seeds in beach pea and grass pea, respectively. The patterns of seed abortion and development at different ovule positions within developing pods were random in beach pea and nonrandom in grass pea. Ovules in position 1 and 2 at the basal end of pods showed a greater probability of seed abortion in beach pea. Grass pea pods showed increased seed abortion towards the basal region of the pod.

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