Primary production and nitrogen allocation of field grown sugar maples in relation to nitrogen availability

Above ground net primary production (NPP), nitrogen (N) allocation, and retranslocation from senescing leaves were measured in 7 sugar-maple dominated sites having annual net N mineralization rates ranging from 26 to 94 kg · ha^–1 · yr^–1. The following responses were observed: (1) Green sun leaves on richer sites had higher N mass per unit leaf area than sun leaves on poorer sites; (2) Total canopy N varied much less than annual net mineralization, ranging from 81 to 111 kg · ha^–1; (3) This was due to the existence of a large and relatively constant pool of N which was retranslocated from senescing leaves for use the following year (54 to 80 kg · ha^–1); (4) The percentage of canopy N retranslocated by sugar maple was also relatively constant, but was slightly higher on the richer sites. Percent N in leaf litter did not change across the gradient; (5) Above ground NPP increased linearly in relation to N allocated above ground. Therefore, N use efficiency, expressed as above ground NPP divided by N allocated above ground was constant; (6) N use efficiency expressed as (NPP above ground/total N availability) was a curvilinear function of N availability; and (7) This pattern reflected a decreasing apparent allocation of N below ground with decreasing N availability.     

Analysis of the inducible MEL1 gene of Saccharomyces carlsbergensis and its secreted product, alpha-galactosidase (melibiase)

We have determined both the nucleotide sequence of the MEL1 gene of Saccharomyces carlsbergensis and the N-terminal amino acid (aa) sequence of its extracellular gene product, alpha-galactosidase (melibiase) (alpha-Gal). The predicted translation product of MEL1 is a pre-alpha-Gal protein containing an 18 aa N-terminal signal sequence for secretion. The purified enzyme is a dimer consisting of two 50-kDal polypeptides, each of which is glycosylated with no more than eight side chains. The 5'-flank of the MEL1 gene contains a region (UASm) having certain areas of sequence homology to similar sites found upstream of the structural genes GAL1, GAL7 and GAL10, which are also regulated by the action of the products of genes GAL4 and GAL80. There are three TATA boxes between UASm and the initiation codon of pre-alpha-Gal, as well as a typical yeast cleavage/polyadenylation sequence in the 3'-flank of the gene.     

The regulation of ferredoxin-dependent nitrogenase activity in Rhodospirillum rubrum and Rhodopseudomonas capsulata

Abstract The regulatory properties of Rhodospirillum rubrum nitrogenase reduced by either the endogenous electron donor (ferredoxin) or an artificial donor (dithionite) were examined. The nitrogenase obtained from glutamate-grown cells required activating enzyme for maximum activity with either reductant. The activating enzyme requirement of ferredoxin-dependent nitrogenase activity implies a physiological significance of the activating enzyme in R. rubrum. Rhodopseudomonas capsulata nitrogenase also required activating enzyme when dithionite was the reductant, but there appeared to be no activating enzyme requirement with ferredoxin as the reductant. Because the catalytic activity of the enzyme was very low under these conditions, the physiological significance of activating enzyme in this organism remains in question.     

Heterologous hybridization of Azospirillum DNA to Rhizobium nod and fix genes

Abstract Probes containing the nod and hsn regions of Rhizobium meliloti and the fixABC genes of Rhizobium japonicum were used to perform hybridization experiments with endonuclease-restricted DNA from Azospirillum brasilense strains and 2 Azospirillum lipoferum strains. Homology to nod, hsn and fixA was found in the 4 Azospirillum strains.     

脯氨酸对缺钾植株的保护作用

在缺钾培养条件下,高粱苗地上部单位鲜重含钾量由于地上部鲜重的增加而下降。从高梁苗钾缺乏症出现的时间、频率和程度来看,Pro培养似乎有利于缓和或减轻缺钾对植物造成的伤害。在同样含钾量情况下,以Ca(NO_3)_2为氮源的苗受到缺钾伤害要重于以Pro为氮源的苗受到的伤害,其钾缺乏症植株的百分数增加值与含钾量降低值之比为0.26,以Pro为氮源的缺钾营养液培养的苗为0.09,即下降同样量的钾前者苗的钾缺乏症的增加要大大地超过后者。Pro并不减轻在缺钾情况下叶绿素含量的下降。     

Effects of phosphorus and nitrogen enrichment on the phytoplankton in a tropical reservoir (Lobo Reservoir,Brazil)

The effects of enrichment with phosphate (0–500 µg. 1^–1) and forms of nitrogen (nitrate, nitrite, ammonia an and urea) (0–3500 µgg. ^–1) on the phytoplankton growth of Lobo Reservoir (Brazil) were studied in July, 1979. Suspended matter, chlorophyll a, cell concentrations and the carotenoid:cchlorophyll ratio were estimated following 14 days of in situ incubation. Phosphate alone caused no significant effects, but enrichment with nitrogen caused a substantial increase on the growth of phytoplankton. Comparison between the different forms of nitrogen showed insignificant effects after their additions with 350 µg. ^–1 and in combination with phosphate. However, when nitrogen was added in large quantities (3 500 µg. ^–1), significant differences between the nitrogeneous forms were found, with urea causing the strongest effect. In July, nitrogen is mhe main limiting nutrient to phytoplankton growth of Lobo Reservoir.Supported by CNPq and FAPESP.     

Nutrient requirements and stress response of Populus simonii and Paulownia tomentosa

The aim of this investigation was to estimate the optimum nutrient requirements and responses to low relative nutrient addition rates of seedlings of two important broadleaf tree species in China, Populus simonii Carr. and Paulownia tomentosa (Thunb.) Steud. In preliminary experiments the optimum nutrient proportions were estimated under high concentration conditions. The nutrients consumed were replaced by means of daily additions determined by pH and conductivity titrations without changing the nutrient solutions. A relatively high K level was needed in relation to nitrogen; higher than in birch or grey alder seedlings. To obtain a high relative growth rate, suitable proportions by weight were 100 N:70 K:14 P:7 Ca:7 Mg for the Populus seedlings and 100 N:75 K:20 P:8 Ca:9 Mg for the Paulownia seedlings.
In studies of nutrient stress responses the relative nutrient addition rate was used as the treatment variable under low conductivity conditions. The responses and relationships were similar to those described for birch, grey alder and Salix . The relative addition rate, and there was also a strong linear regression between relative growth rate and nitrogen status. Relative growth rates were high and the maximum weight increase was about 19% day⁻¹ in Populus and over 25% day⁻¹ in Paulownia . The nitrogen productivity of Paulownia was very high, 0.26 g dry weight (g N)⁻¹ h⁻¹, and for Populus it was 0.16 g dry weight (g N)⁻¹ h⁻¹.     

Possible evidence for novel metabolism of nitrogen gas by a green alga

We report the isolation of a cukaryotic green alga ( Chlorella , strain WPI-2) which accumulates large stores of nitrogen (N) during growth in N-free medium and seems to incorporate¹⁴N₂, yet does not reduce acetylene to ethylene. Total N accumulation during growth on N-free medium and in gases free of combined N was measured by three methods: Kjeldahl, oxidative pyrolysis via chemiluminescence (Antek N analyzer), and Dumas (Coleman N analyzer). Increases in N ranging from 22–64%± 1% were observed. Isotope dilution studies using cells labelled with ¹⁵NO ₃- and then shifted to ¹⁴N₂ in N-free medium showed dilution of the ¹⁵N isotope by ¹⁴N from 5.67 to 5.32%± 0.05%. Using a variety of conditions, we were unable to demonstrate the reduction of acctylene to ethylene by WPI-2, although diazotrophic cyanobacteria gave positive results. Although the data on WPI-2 are not conclusive in establishing this alga as a diazotroph, the data do suggest that within the Chlorophyceae there may exist a novel form of nitrogen gas metabolism.