Canopy structure,nitrogen distribution and whole canopy photosynthetic carbon gain in growing and flowering stands of tall herbs

Using a combination of mathematical modeling and field studies we showed that in dense stands of growing herbaceous plants the vertical pattern of leaf nitrogen distribution resembles the pattern of mean light attenuation in the stand and hence tends to maximize total daily photosynthetic carbon gain of the whole stand. Flowering represents a strong sink of nitrogen away from the photosynthetic apparatus and in herbs like Solidago altissima it induces leaf shedding. We studied both the effect of nitrogen reallocation and leaf shedding on the whole canopy photosynthesis and changes in leaf nitrogen distributions in stands moving from the growing to the flowering stage. Despite a decrease in leaf area index and total nitrogen available for photosynthesis in the flowering stand, the leaf nitrogen distribution here also leads to an almost maximum canopy photosynthesis. In both the growing and the flowering stands the leaf area index was higher than calculated optimum values. It is pointed out that this should not necessarily be interpreted as non-adaptive.     

Toxoplasma gondii: Purine synthesis and salvage in mutant host cells and parasites

Toxoplasma gondii, growing exponentially in heavily infected mutant Chinese hamster ovary cells that had a defined defect in purine biosynthesis, did not incorporate [U-¹⁴C]glucose or [¹⁴C]formate into the guanine or adenine of nucleic acids. Intracellular parasites therefore must be incapable of synthesizing purines and depend on their host cells for them. Extracellular parasites, which are capable of limited DNA and RNA synthesis, efficiently incorporated adenosine nucleotides, adenosine, inosine, and hypoxanthine into their nucleic acids; adenosine 5′-monophosphate was the best utilized precursor. Extracellular parasites incubated with ATP labeled with ³H in the purine base and ³²P in the α-phosphate incorporated the purine ring 50-fold more efficiently than they did the α-phosphate. Thus, ATP is largely degraded to adenosine before it can be used by T. gondii for nucleic acid synthesis. Two pathways for the conversion of adenosine to nucleotides appear to exist, one involving adenosine kinase, the other hypoxanthine—guanine phosphoribosyl transferase. In adenosine kinase-less mutant parasites, the efficiency of incorporation of ATP or adenosine was reduced by 75%, which indicates the adenosine kinase pathway was predominant. Extracellular parasites incorporated ATP into both the adenine and the guanine of their nucleic acids, so ATP from the host cell could supply the entire purine requirement of T. gondii. However, ATP generated by oxidative phosphorylation in the host cell is not essential for parasites because they grew normally in a cell mutant that was deficient in aerobic respiration and almost completely dependent upon glycolysis.     

Different substitution tendencies of leaf flavones in theVeronica hederifolia group (Scrophulariaceae)

Flavonoid patterns of several provenances ofVeronica triloba, V. sublobata, andV. hederifolia s. str. have been investigated. 7-0-glucuronides of apigenin and luteolin as well as of 6-substituted (6-hydroxyluteolin and hispidulin) and 3′-methylated (chrysoeriol) derivatives represent characteristic accumulation tendencies in theV. hederifolia group. The three species show slightly different flavonoid profiles.V. hederifolia exhibits a cumulative pattern of all compounds occurring either inV. triloba orV. sublobata, thus supporting the hypothesis of an allopolyploid origin ofV. hederifolia.     

Quantification and analysis of reverse mutations at the hgprt locus in Chinese hamster ovary cells

We describe an assay for the quantification of reverse mutations at the hypoxanthine-guanine phosphoribosyltransferase (hgprt) locus in Chinese hamster ovary cells utilizing the selective agent L-azaserine (AS). Conditions are defined in terms of optimal AS concentration, cell density, and phenotypic expression time. After treatment, replicate cultures of 10⁶ cells are allowed a 48-h phenotypic expression time in 100-mm plates. AS (10 μM) is then added directly to the growing culture and AS-resistant (AS^r) cells form visible colonies. This assay is used to quantify ICR-191-, ICR-170-, and N-ethyl-N-nitrosourea-induced reversion of independently isolated HGPRT^? clones. The AS^r phenotype is characterized both physiologically and biochemically. All AS^r clones isolated are stably resistant to AS and aminopterin but sensitive to 6-thioguanine. They also have re-expressed HGPRT enzyme. In addition, several revertants are shown to contain altered HGPRT. The data provide further evidence that ICR-191 and ICR-170 cause structural gene mutations in mammalian cells and also suggest that ICR-191, ICR-170, and N-ethyl-N-nitrosourea induce similar types of mutations in Chinese hamster ovary cells.     

Retardation of leaf senescence by inhibitors of RNA and protein synthesis

Effects of actinomycin-D (ACT), cycloheximide (CH), rifampicin (RIF) and chloramphenicol (CAP) on senescence of soybean leaf discs were investigated. All inhibitors tested are effective in retarding senescence of soybean leaf discs. However, CH is more effective than ACT, RIF and CAP, suggesting that activation of preexisting, latent metabolic systems present in the cytoplasm piays predominant role in the initiating of leaf senescence. However, the possibility that events taking place in the nucleus or chloroplast are essential for the initiation of leaf senescence cannot be excluded.     

Comparative phytotoxicity of nitrapyrin and ATC to several leguminous species

Summary The relative toxicity of nitrapyrin 2-chloro-6-(trichloromethyl) pyridine and ATC (4-amino-1, 2, 4-triazole) on the growth of chick peas (Cicer arietinum L.) cow peas (Vigna sinensis L.), green beans (Phaseolus vulgaris L.), green peas (Pisum sativum L.) and mung beans (Phaseolus aureus Roxb.) and their effectiveness as nitrification inhibitor were studied under greenhouse conditions. ATC produced no toxicity symptoms in green peas, whereas resulted in leaf chlorosis in cow peas, chick peas and green beans. However, nitrapyrin toxicity appeared as leaf chlorosis in cow peas, and interveinal chlorosis in chick peas. Moreover, nitrapyrin-treated green beans and peas developed leaf curling and cupping. Although ATC had no significant effect on growth, a suppression in plant growth was associated with nitrapyrin application. Furthermore, green beans was the most resistant and chick peas the most sensitive to nitrapyrin. Nitrapyrin was more effective nitrification inhibitor than ACT, especially at the lower rates.     

Pigment distribution and nitrogen fixation inAnabaena azollae

Summary The symbiotic heterocystous cyanobacteriumAnabaena azollae present in the leaf cavities of the water fernAzolla spp. was studied. The cyanobacteria extracted from the leaf cavities showed differences in pigment composition in three species ofAzolla, i.e A.pinnata var.pinnata, A.caroliniana and A.filiculoides, as observed by pigment absorption and epifluorescence tests. These differences suggest that of these species the cyanobiont ofA. pinnata is the most actively nitrogenfixing form. This has been confirmed by nitrogen fixation (acetylene reduction) tests. Heterocysts of the symbiont ofA. pinnata were characterized by high chlorophylla and low phycocyanin content, a low fluorescence yield of chlorophyll in the heterocysts compared to vegetative cells and a gradient of phycocyanin concentration in the vegetative cells adjacent to heterocysts. This indicates that only photosystem I is present in the heterocyst. In the two otherAzolla species quantitative shifts in the pigment composition occurred suggesting a lower nitrogen fixation activity.In the cyanobiontAnabaena azollae the heterocyst frequency could reach a value of 44–45%. It is argued that there are two generations of heterocysts in a matureAzolla plant, which are concomitant with two peaks of nitrogen fixation activity correlated with leaf age,i.e. leaf number along the main axis of the plant. At both peaks of maximal N₂-ase activity, only 20–25% of the heterocysts present are metabolically active as demonstrated by the reduction of Neotetrazolium chloride (NTC) in the heterocysts and darkening of nuclear emulsions by silver salt reduction. Vegetative cells of the cyanobiont reduce Neotetrazolium chloride (NTC) to formazan more rapidly than has been observed in the free-living heterocystous cyanobacteriumAnabaena cylindrica tested in parallel experiments. This feature may be due to a more permeable cell wall of the vegetative cells of the cyanobiont compared to the free-living form, since the vegetative cells of the symbiont play a role in cross-feeding of the host (Azolla).Evidence is obtained that only the heterocysts of the cyanobiont ofAzolla are involved in the nitrogen fixation process as in free-living heterocystous cyanobacterium species. This situation is different from other cyanobacterial symbioses such as inGunnera, Blasia andAnthoceros, where physiological modifications are reported in the symbiosis with another photosynthetic partner such as the absence of O₂ evolution and the absence of photo-fixation of CO₂ in the cyanobionts.Pigment composition and N₂-ase activity in the symbiotic cyanobacteria of three Azolla species have indicated the superiority of theA. pinnata symbiont.A. pinnata var.pinnata is a semidomesticated form used in S.E. Asia for agricultural purposes (irrigated rice culture) to increase soil fertility.It is suggested that by selection (domestication) more efficient strains (clones) can be obtained, and further that with more advanced techniques such as gene mutation and genetic manipulation even more efficient and for agriculture more beneficial clones can be obtained.     

Structure and seasonal distribution of litterfall in young plantations of Populus ‘Tristis#1’

Litterfall was sampled in 3- to 7-year-old irrigated and fertilized plantings of Populus Tristis#1 in northern Wisconsin. Leaves accounted for more than 90 per cent of the total litter and reached an asymptotic value of about 4,000 to 4,200 kg/ha. Leaf litterfall culminated in late September or first half of October. Inadequate light appeared to be responsible for an early (June) leaf fall in dense plantings. The maximum cumulative leaf area index ranged from 8.4 to 8.7. The pattern of foliation and defoliation was similar: it started with small leaves in the lower canopy and ended with large leaves of the upper canopy. Longevity of the small leaves was shorter than that of the large leaves. Leaf size was positively affected by fertilization but was not affected by a discontinuation of irrigation by mid-July during the droughty 1976. The specific leaf weight ranged from 2.6 mg/cm² in the lower canopy to 10.2 mg/cm² in the upper canopy. The specific leaf weight of litter leaves was 4 to 38 per cent lower than that of green leaves of the same size.