E, 1996, Effects of nitrogen deficiency on accumulation of fructan and fructan metabolizing enzj-me activities in sink and source leaves of barley (Hordeum vatgare).-PhysM. Piant, 97: 339-345.Seedlings of barley (Hordeum vulgare L. cv, Agneta) were grown hydroponicaliy under continuous light, constant temperatare and relative humidity. During the first two weeks, the relative growth rate (RGR) was kept at 25% by limiting only the supply of nitrogen. The cultures were then transferred to nitrogen-free media and the amounts of fructan, starch, sucrose, glucose and fructose in sink and source leaves were measured at 0, 12, 24, 48, 72, 120 and 156 h. The activities of two key enzjmes in fructan metabolism, sucrose:sucrose fructosyltransferase (SST), fructan exohydrolase (FEH), as well as acid invertase were aiso measured in the two types of leaves. The fructan and starch levels in both sink and source leaves increased during nitrogen deficiency. The highest increase in starch was 200% of the control, while for fructans a 700% increase was recorded. The activity of SST increased parallel to fructan accumulation in sink leaves. However, the FEH activitj' was constant and not affected by nitrogen deficiency. The invertase activity both in sink and source leaves was reduced by nitrogen deficiency More fructans, as well as sucrose and fructose, accumulated in source leaves compared to sink leaves both before and after nitrogen starvation. The results show that fructan is the major carbohydrate reserve accumulating under nitrogen deficiency both in sink and source leaves in barley plants. The induction of fructan accumulation in sink leaves caused by nitrogen deficiency is intimately connected with the regulation of SST,

Hydroponically cultivated barley plants were exposed to nitrogen (N)-deficiency followed by N-resupply. Metabolic and genetic regulation of fructan accumulation in the leaves were investigated. Fructan accumulated in barley leaves under N-deficiency was mobilized during N-resupply. The enhanced total activity of fructan-synthesizing enzymes, sucrose:sucrose 1-fructosyltransferase (EC 2.4.1.99) and sucrose:fructan 6-fructosyltransferase (6-SFT; EC 2.4. 1.10) caused by N-deficiency decreased with the mobilization of fructan during N-resupply. The activity of the barley fructan-degrading enzyme, fructan exohydrolyase (EC 3.2.1.80) was less affected by the N status. The low level of foliar soluble acid invertase activity under N-deficiency conditions was maintained during the commencement of N-resupply but increased subsequently. Further analyses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western blot and northern blot demonstrated that the fructan accumulation and the total activity of fructan-synthesizing enzymes correlated with the 6-SFT mRNA level. We suggest that the changes in fructan levels under N stress are intimately connected with the regulation of fructan synthetic rate which is mostly controlled by 6-SFT.

The effects of phosphorus starvation on morphology and intracellular structure and on reactions related to the energy metabolism of the unicellular green alga Scenedesmus obtusiusculus (Chod.) were studied over a period of 96 h by employing transmission electron microscopy and various methods for measurement of physiological reactions. Increase in cell size and shape and in cell wall thickness are dominating features of phosphorus starvation. There is also an increase in number and size of starch granules and lipid globules and the internal structure of the cells appears successively disorganized. Shortage of phosphorus in the medium initially induces an increase of the adenylate pool whereas the energy charge value remains the same as for the controls. The photosynthetic and respiratory activities are high during incipient phosphorus starvation. After 24 h, as shortage of phosphorus becomes critical, the internal phosphorus reaches a low steady‐state value, and this is also true for the adenylate energy charge. The total content of adenylates, however, peaks after 24 h of starvation and then decreases with increasing length of phosphorus starvation. Light‐induced oxygen evolution appears not to be as much inhibited by a low phosphorus content in the cells as by the concomitant starch accumulation. The data indicate that the strategy for survival of the cells in a phosphorus‐poor environment includes morphological and physiological changes that facilitate the transfer and adaption of the cells to environments with a more favourable supply of phosphorus, such as the often oxygen‐poor but phosphorus‐rich bottom zones.

Mellvig, S. and Tillberg, J-E. 1986. Transient peaks in the delayed luminescence from Scenedesmus obtusiusctilus induced by phosphorus starvation and carbon dioxide Cells of the unicellular green alga Scenedesmus obtusiusculus (Chod.) were starved of phosphorus for 24, 48, 72 and 96 h, and the decay kinetics of the delayed luminescence from the differently starved cells was monitored for several minutes. Cells starved for 24 h showed similar delayed luminescence decay kinetics and accumulated output of photons as control cells after excitation with white light. Two transient peaks (with several components) in the decay kinetics of delayed luminescence were observed after 48 h of phosphorus starvation but not after 72 or 96 h. The amplitude of the transient peaks varied depending on the length of the excitation period with white light and on the length of the dark period preceding light excitation. High CO, availability induced no transient peak, whereas low CO, availability induced a high transient peak. Transient peaks could not be induced by excitation with light of 660 or 680 nm and only a single transient peak developed using 700 nm light. The kinetics of the delayed luminescence was changed, and the accumulated output of photons was decreased when the pH of the medium was changed from 7.2 to 9.5, both in ceils starved for phosphorus for 96 h and in controls. The data indicate that a complicated metabolic pattem is involved in the mechanisms giving rise to the observed transient peaks in the delayed luminescence. The main factors may be a reduction in the translocation of trioses from chloroplasts, a concomitant reduction in Calvin cycle activities and changes in the amount of ATP and reducing agents available Additional key words of S-states.Calvin cycle activity, charge separation, pH, recombinations S. Meiivig (reprint requests) and

E, 1987. Carbohydrate partitioning, photosynthesis and growth in Lemna gibba G3. 1. Effects of nitrogen limitation -Physiol. Plant. 71: 264-270.The growth rate of Lemna gibba L. G3 was varied by limiting the supply of nitrogen (N) under otherwise constant conditions. Two experimental approaches were used. 1) A series of suboptimaiiy growing cultures were supplied daily with exponentially increasing doses of N. 2) Optimally growing cultures were transferred to a N-free medium and cuitivated in it for 10 days. Levels of starch, sucrose, glucose, fructose, and rates of COj assimilation, dark respiration and growth (RGR) were measured in both systems. At RGR ranging from optimal to 50% of optimal all measured carbohydrates accumulated. RGR below 50% of optimal caused decreased levels of soluble sugars, but increased levels of starch. Starch accumulation showed a strong negative correlation with the CO2 assimilation rate, indicating increased triose phosphate/inorganic phosphate (TP/PJ ratio in the chloroplast causing end product inhibition of photosynthesis. The data indicate that the quantitative relationship between the photosyathetic activity and the carbon utilization rate influences the activity of the sucrose synthesis pathway and thus the rate of the triose phosphate/P, exchange at the chloroplast membrane, which in turn regulates the activity of starch synthesis and the Calvin cycle.