Edna Ben-Izhak Monselise and Daniel Kost

Nitrogen assimilation and amino acid production in Spirodela oligorrhiza plants exposed to 30 mM 15NH4Cl was studied using l5N NMR spectroscopy. Green and etiolated plants were studied under different light regimes and in the presence of added carbon, either as sucrose or as α-ketoglutarate. Etiolated plants are capable of ammonium assimilation and, as in green plants, this occurs via the glutamine synthetase/glutamine oxoglutarate amine transferase (GS/GOGAT) and the aspartate aminotransferase/asparagine synthetase pathways. The major assimilation products in both etiolated and green plants were glutamine and asparagine. Thus our results confirm that N-amides are key detoxification products when plants are exposed to external ammonium ion, and act as storage reservoirs or sinks for assimilated ammonium. In plants grown under continuous light, ammonium ion was taken up and assimilated to completion. L-methionine DL-sulfoximine, a GS inhibitor, inhibited ammonium ion assimilation but not its uptake. Addition of azaserine, a GOGAT inhibitor, resulted in the disappearance of α-amino signals, and l5N incorporation into the glutamine amide-N position only. This is evidence for the operation of the GS/GOGAT pathway, as opposed to the glutamate dehydrogenase (GDH) pathway, in both green and etiolated plants. Even in the dark and under various stress conditions, no sign of ammonium ion assimilation via the GDH pathway could be detected.

The amount of amino acid metabolites strongly depended on the light regime and the extent of external carbon supply. Supply of α-ketoglutarate to the etiolated plants increased ammonium ion uptake and assimilation. Ornithine and arginine were also formed, consistent with the operation of the ornithine cycle.

Ella Werker

780,000-year-old wood of the Early/Middle Pleistocene, from excavations at Gesher Benot Ya'aqov in the Dead Sea Rift, was examined anatomically. Twenty-eight genera of trees, shrubs, and climbers have been botanically identified. The most common among them were Fraxinus, Quercus, Olea, Salix, Pistacia, and Ulmus. Most of the identified plants represent typical Mediterranean vegetation. Remnants of almost all the identified species are still found in the region today, in spite of the dramatic changes that took place in the Holocene through the intervention of man. One species, however, could not be identified with wood anatomical data of European, Middle Eastern, or North African species. It appears to be a species that is extinct in this region.

Simcha Lev-Yadun

Firewood is a vital energy source for cooking and heating in traditional societies worldwide. During the past century, increasing human populations have depleted many previously available resources, resulting in severe shortages of firewood in many regions, especially in arid zones. Here, I describe the use as a source of firewood for a local semi-nomadic Bedouin village of several dozen families and the fate of a 0.3 km2 abandoned fiber-crop plantation of Agave sisalana Perr. which is more than 50 years old in the Negev Desert, Israel. The amount of firewood extracted in the last decade equals several thousand local wild shrubs, which, in the current vegetation density, grow in a radius of several square kilometers. Harvesting the plantation for about 20 years almost fully exploited the plantation. Because the Bedouins do not replant plants used only for firewood, this source is not sustainable despite its biological potential to be so. However, because Agave are CAM (Crassulacean acid metabolism) plants, they can grow under very arid conditions, and because they are well defended from grazing by thorns and poisons, they may be planted as a source of firewood in various arid regions where other plants will not sustain.

Eliezer Goldschmidt, Raffi Goren and David Weiss

Serpil ÜNyayar, Ş. Fatih Topcuoglu and Suna Bozcuk

The levels of endogenous free, bound, and total abscisic acid (ABA) and the changes in dry weight of the mycelium depending on the culture periods were examined in Pleurotus florida (Basidiomycetes) cultured both in shaking and static media. The relationship between ABA production and the growth rate of the fungus was determined. Our findings show that this fungus synthesizes ABA as a secondary metabolite and the maximum total ABA is produced on the 24th day of the growth period in both shaking and static conditions. It was observed that, depending on the culture period, the dry weight of mycelium was enhanced in the primary metabolic phase, while it was constant in the secondary metabolic phase.

These results show that there is a negative relationship between growth rate of the fungus and ABA synthesis.