NON-PHOTOSYNTHETIC FIXATION OF CARBON DIOXIDE AND POSSIBLE BIOLOGICAL ROLES IN HIGHER PLANTS

1. Non-photosynthetic fixation of CO₂/HCO₃^- occurs both under light and dark conditions and involve the addition of carbon to substrates which in higher plants are derived originally from carbon reduced to carbohydrates during photosynthesis. Despite the endergonic nature of these carboxylations, the advantages offered seem to be sufficient to outweigh the disadvantages of energy loss. 2. Non-photosynthetic carbon incorporation into metabolism is dealt mainly in relation to PEP carboxylase, acetyl-CoA carboxylase, carbamoyl phosphate synthetase and phosphoribosylaminoimidazole carboxylase while other carboxylases await further characterization or discovery. The extent to which a carboxylase participates depends upon the need for products of its activity in metabolism. 3. Non-photosynthetic carbon fixation is intricately involved in several pathways of metabolism throughout the ontogeny of plants. The roles in relation to leaf carbon metabolism, respiratory metabolism, nitrogen metabolism, lipid and isoprenoid biosynthesis, purine and pyrimidine metabolism and metabolism associated with the action of growth regulators have been described. The fixation reactions appear to be largely concerned with the production of intermediary metabolites, circumvention of energy barriers in metabolism and regulation of plant metabolism. In addition, the activity of PEP carboxylase is involved in ionic balance and pH-stat. 4. Malate derived by way of PEP carboxylase and NAD-malate dehydrogenase acts as an effective osmoticum and a counter-ion for K⁺ accumulation in actively growing plant cells. In addition, malate may enter the TCA cycle or can be decarboxylated by cytoplasmic NADP-malic enzyme converting NADH to NADPH. Wherever it has been sought in different plant tissues, some evidence for PEP carboxylase and metabolism of malate has always been found. 5. Almost every plant process spanning from seed development and germination to flowering and fruit-set requires the essential participation of non-photosynthetic carbon fixation in regulating certain metabolic and cellular functions but it does not contribute in a major way to the carbon nutrition of plants. It is largely the tissue type that appears to determine which of the roles is predominant at any one time.     

Accelerated Germination of Maize Seeds Under Chilling Stress by Osmotic Priming and Associated Changes in Embryo Phospholipids

Osmotic priming of maize seeds (Zea mays L. cv. Partap) usingpolyethylene glycol or potassium salts (K₂HPO₄, KH₂PO₄, KNO₃and K₂HPO₄+KNO₃) resulted in accelerated germination at a chillingtemperature (10 °C). The response of seeds primed in solutionsof either 2.5% K₂HPO₄ or 2.5% K₂HPO₄+ KNO₃ was particularlymarked compared with the untreated seeds, and the effect ofpriming was largely retained after seeds had been dried back.All embryo phospholipid fractions and sterols increased duringsalt-priming and the proportion of phospholipid which was diphosphotidylglycerol(DPG) also increased. It is suggested that the marked increasein the DPG content of primed embryos may be due to enhancedinternal organization of their mitochondrial membranes, andthat the benefit of osmotic priming may be at least partly dueto an increased potential for ATP accumulation. Germination, Zea mays L., osmotic priming, phospholipid changes     

Thermal Hardening： A New Seed Vigor Enhancement Tool in Rice

In a laboratory study, indica and japonica rice (Oryza sativa L.) seeds were exposed to thermal hardening (heating followed by chilling followed by heating; chilling followed by heating followed by chilling; heating followed by chilling or chilling followed by heating). In indica rice, heating followed by chilling followed by heating resulted in decreased mean germination time, time to start germination, electrical conductivity of seed leachates, and time to 50% germination, as well as increased germination index, energy of germination, radicle and plumule length, root length, root/shoot ratio, root fresh and dry weight, radicle and plumule growth rate, and shoot fresh weight. In japonica rice, chilling followed by heating followed by chilling performed better than all other treatments, including control.     

Thermal Hardening: A New Seed Vigor Enhancement Tool in Rice

In a laboratory study, indica and japonica rice (Oryza sativa L.) seeds were exposed to thermal hardening (heating followed by chilling followed by heating; chilling followed by heating followed by chilling; heating followed by chilling or chilling followed by heating). In indica rice, heating followed by chilling followed by heating resulted in decreased mean germination time, time to start germination, electrical conductivity of seed leachates, and time to 50% germination, as well as increased germination index, energy of germination, radicle and plumule length, root length, root/shoot ratio, root fresh and dry weight, radicle and plumule growth rate, and shoot fresh weight. In japonica rice, chilling followed by heating followed by chilling performed better than all other treatments, including control.     

Dark Metabolism of CO2 during Fibre Elongation of Two Cottons Differing in Fibre Lengths

A study has been made of the dark metabolism of CO₂ by elongatingfibres of Gossypium arboreum L. cv. LD 133 (a short staple type)and Gossypium hirsutum L. cv. LH 372 (a long staple type) atdifferent fibre ages. In both cultivars, phosphoenolpyruvatecarboxylasc, glutamate-oxalacetate transaminase and malate dehydrogenaseshow elevated activities during the period of rapid fibre growthand lowered activity with ageing. Malic enzyme activity increasesas extension growth levels off. Levels of K⁺ and malate riseduring rapid extension growth and fall as the rate of elongationdecreases. The results indicate that malate may act as an osmoticumand a counterion for K⁺ accumulation during rapid expansionof the fibres. Amounts of enzymes, K⁺ and malate are higherin the fibres of the long staple cultivar than the short staple.During the period of active elongation, K⁺/malate ratio is higherin the short staple cultivar. Key words: Gossypium hirsutum, CO₂ metabolism, Fibre extension     

Influence of Seed Pre-treatments with Plant Growth Regulators on Metabolic Alterations of Germinating Maize Embryos Under Stressing Temperature Regimes

The germination performance of maize seeds (Zea mays L. cv.Partap-1) pre-treated individually with the substituted phthalimideAC 94,377 (1-(3-chlorophthalimido)-cyclohexanecarboxamide),GA₄₊₇ and ABA was markedly improved under sub- and supra-optimaltemperature regimes. ABA was especially stimulatory at the sub-optimaltemperature. Metabolic alterations in the germinating embryosof treated seeds revealed an increased accumulation of solublesugars and proteins compared with the controls under stressingtemperatures. The activities of acid phosphatase, invertase,catalase and peroxidase were seemingly related to the alleviationmetabolism. It is suggested that gibberellins and abscisic acidmay have positive regulatory effects in triggering the systemfor stress alleviation. Germination, Zea mays, temperature, growth regulators, embryos, metabolism