New Phytologist

doi: 10.1111/j.1469-8137.1992.tb01086.xpmid: N/A

RAVEN, JOHN A.; LINDA, BERND WOLLENWEBER; HANDLEY, L.

doi: 10.1111/j.1469-8137.1992.tb01087.xpmid: N/A

Ammonium (used here to signify NH4+ plus NH3) is the immediate inorganic precursor of organic nitrogen in photolithotrophs. In marine habitats ammonium is also the major exogenous nitrogen source, and the same is probably true of terrestrial habitats. In addition to this major role of ammonium as exogenous nitrogen source, it is also quantitatively very important as an endogenous nitrogen source with nitrate or N2 as exogenous nitrogen source, and as a recycled nitrogen compound in photorespiration and in phenylpropanoid synthesis. The shoots of terrestrial plants have higher NH3 compensation partial pressures than most natural soils, and especially higher than that of the sea. However, long‐distance transport of gaseous NH3 (e.g. from continents to oceans) is a negligible component of the ‘natural’ global nitrogen cycle. Current concepts of evolution of the atmosphere and biosphere do not involve high partial pressure of NH3 in the atmosphere: any ammonium produced in inorganic or biological processes is removed from the atmosphere by rain‐out, or, over larger time‐scales, by the action of ultraviolet radiation (especially before the ozone screen came into being) and of hydroxyl radicals (especially after oxygenation of the atmosphere). In addition to posing problems for the origin of life and arguing for an early rather than a late evolution of diazotrophy, a low NH3 partial pressure renders implausible arguments that phenylpropanoid synthesis was restricted early in evolution by the effect of high ammonia in reversing phenylalanine ammonia‐lyase.

RAVEN, JOHN A.; WOLLENWEBER, BERND; HANDLEY, LINDA L.

doi: 10.1111/j.1469-8137.1992.tb01088.xpmid: N/A

Potential advantages of ammonium relative to nitrate assimilation (assuming equal nitrogen supply in the two forms), deduced from known biochemical pathways and the site of nitrate assimilation in vascular land plants, include (i) a greater maximum specific growth rate, (ii) lower costs of photons and (in transpiring plants) water per unit carbon assimilated, and (iii) lower costs of iron, manganese and molybdenum per unit carbon assimilated per unit time. Actual measurements show that the growth rate and photon cost predictions are often, but not invariably, borne out; while data on the water cost of growth almost invariably contradict the prediction by showing a lower water cost from nitrate‐supplied than ammonium‐supplied plants. Few data seem to be available that test the predictions as to metal costs of growth; some of the predictions are borne out. These possible or realised advantages to the photolithotroph of using ammonium must be considered in the context of the relative availability of the two sources of combined nitrogen. In the oceans nitrification of ammonium produced in mineralization does not compete well with photolithotrophic assimilation of ammonium in the euphotic zone, so ammonium is the major combined nitrogen source for ‘recycled’ primary production, while ‘new’ production largely uses nitrate produced in deep, dark water. When both nitrogen sources are available to phytoplankton organisms ammonium is almost invariably preferred, with complete suppression of nitrate use with as little as 1–2 mmol m−3 ammonium. In terrestrial habitats the nitrification of ammonium produced in mineralization is not photoinhibited (as can occur in the surface water of the ocean) but is subject to inhibition by anoxia, low pH and (possibly) plant‐produced defence compounds. However, the lower diffusion coefficient for ammonium than for nitrate in soils means that nitrogen‐limited plant growth as a given rate needs a higher mean dissolved ammonium concentration in the soil than is the case for nitrate when the soil contains only one of these nitrogen sources and root distribution and morphology are unaffected by the nitrogen source. With equal mean ammonium and nitrate concentrations, a nitrogen‐limited plant supplied solely with ammonium would need a more extensive root/root hair/mycorrhiza system to attain the same nitrogen uptake rate on a per plant basis as would a nitrate‐supplied plant, with consequences for resource allocation by, and growth rate of, the ammonium‐grown plant. In addition to the larger mean area‐based ammonium assimilation rate by photolithotrophs in the oceans, consideration of the interactions among ammonium diffusion coefficient, ammonium diffusion distance, organism surface area per unit biomass and the organism maximum specific growth rate in the ocean relative to soils can plausibly account for the lower mean ammonium concentrations in the ocean than in soils.

Page, T.S.; Gallon, J. R.

doi: 10.1111/j.1469-8137.1992.tb01089.xpmid: N/A

Extracts from aerobically grown cultures of the unicellular cyanobacterium Gloeothece (Nägeli) sp. ATCC 27152 converted relaxed ColE1 plasmid cccDNA into a supercoiled form. They therefore contained a DNA gyrase. However, when DNA gyrase was inhibited, these same extracts catalyzed a net relaxation of ColE1 DNA, implying that they also contained an enzyme similar to DNA topoisomerase I. During the first 2 h after transfer of aerobically grown cultures of Gloeothece to an atmosphere of O2, the ability of extracts to generate supercoiled DNA was temporarily lost. Instead, extracts catalyzed net relaxation of ColE1 DNA during this period, which coincided with a transient inhibition of nitrogenase synthesis. O2 and some related compounds also inhibited DNA supercoiling by extracts of Gloeothece in vitro. As in other diazotrophs therefore, O2 may inhibit nitrogenase synthesis in Gloeothece by altering the relative activities of DNA gyrase and DNA topoisomerase I in such a way that relaxation of one or more of the nif genes may occur, with consequent inhibition of transcription. However, this effect is only transient. In contrast, addition of NH4+ to cultures of Gloeothece permanently inhibited nitrogenase synthesis and did not abolish the ability of extracts to catalyze net supercoiling of DNA. Moreover, NH4+ did not abolish DNA supercoiling in vitro. NH4+ therefore appears to inhibit nitrogenase synthesis in Gloeothece by a mechanism different from that of O2.

HARRIS, S. L.; SILVESTER, W. B.

doi: 10.1111/j.1469-8137.1992.tb01090.xpmid: N/A

Daily addition of sterile media at a dilution rate of 0.100–0.125 d−1 allowed Frankia to be maintained in continuous, derepressed (N2‐fixing) culture for periods of more than 30 d. Continuous cultures yielded Frankia populations with stable levels of growth and nitrogenase activity as well as constant vesicle morphology. These were then used to investigate the influence of oxygen on vesicle development.

Claassen, V. P.; Zasoski, R. J.

doi: 10.1111/j.1469-8137.1992.tb01091.xpmid: N/A

A useful device for manipulating root segments during clearing and staining of VA mycorrhizal roots is described. The device is constructed by cutting the tip off of a disposable syringe and welding a polypropylene screen on to the barrel. With this system, the plunger is used to draw in or expel reagents rapidly with little or no loss of root material during the repeated changing of reagents. The system speeds the process of clearing and staining and reduces the potential for operator contact with reagents.

WARWICK, N. W. M.; HALLORAN, G. M.

doi: 10.1111/j.1469-8137.1992.tb01092.xpmid: N/A

When two accessions of brown beetle grass [Diplachne fusca (L.) Beauv. syn. Leptochloa fusca (L.) Kunth], differing in tolerance to salinity, were exposed to NaCl over 15 d, Na concentrations were constant in the shoot. However, there were differences in shoot Na concentration between the two accessions which were manifest after 5 d exposure to 100 mol m−3 NaCl. Na and Cl concentrations in the shoot were found to be controlled within a narrow range and did not increase any further after 5 d exposure to NaCl. Leaves of differing maturity and growth rate of both accessions did not differ in their Na concentrations. Na concentrations remained stable in leaves of differing ages after the first 6 d of exposure to NaCl whereas K declined with time, possibly due to remobilization to younger leaves. Avoidance of high concentrations of Na in younger leaves by its sequestration in older tissue does not appear to be a salt‐tolerance mechanism in D. fusca. The primary mechanism of exclusion of Na and Cl from the plant is in the root, with secondary control of leaf Na and Cl concentration by excretion of excess Na and Cl through leaf salt‐glands. Total Na uptake for the relatively salt‐tolerant accession of D. fusca. M1, was similar to that for the relatively intolerant accession F3, although M1 had a lower Na concentration in its leaves than F3 after 5d exposure to NaCl. This suggests a possible mechanism for the higher tolerance of M1 to NaCl. Leaf Na concentration was significantly higher in F3 than M1, but M1 had significantly higher Na excretion rates than F3. The leaf salt‐glands of both accessions of D. fusca were found to be capable of excreting approximately 50–80 % of the Na entering the leaf.

CUARTERO, J.; YEO, A. R.; FLOWERS, T. J.

doi: 10.1111/j.1469-8137.1992.tb01093.xpmid: N/A

Attempts to enhance the salt‐tolerance of the cultivated tomato using the tolerance of related wild species of Lycopersicon Mill, have been unsuccessful commercially. An alternative approach is to attempt to accumulate physiological characters that contribute to tolerance within a genotype. We have investigated the relationships between tolerance and certain physiological characteristics in accessions of L. esculentum Mill., L. cheesmanii (Hook) C. H. Mull., L. pennellii (Correll) D'Arcy, L. peruvianum (L.) Mill, and L. pimpinellifolium Mill, with this aim in view.

SNAPP, S. S.; Shennan, C.

doi: 10.1111/j.1469-8137.1992.tb01094.xpmid: N/A

Numerous hydroponic studies have shown that root growth of tomato (Lycopersicon esculentum Mill.) is little affected by salinity. In contrast, data from soil‐grown plants show salinity may induce reductions of up to 50% in root length density. In this study, root growth of two tomato cultivars exposed to salinity (NaCl and CaCl2, 4:1) was examined in the field, in large soil‐filled containers, and in hydroponics. The two cultivars, UC82B and CX8303, differed in susceptibility to a common root rot organism (Phytophthora parasitica Dast.), and were hypothesized to differ in root growth response to environmental stress. In agreement with the literature, root weight of young, hydroponically‐grown plants (as determined by multiple, destructive harvests) was unaffected by salinity in both cultivars. In contrast, root length density of cv. UC82B was reduced 40% by salinity in the field, whereas root length density of root rot‐tolerant cv. CX8303 was unaffected. Similar responses to salinity were observed in the containers, with root counts from horizontal minirhizotron tubes reduced in cv. UC82B, but not in cv. CX8303. Observations from windows in the sides of the containers showed that reduction in net root growth in cv. UC82B was primarily due to an increased rate of root death at high salinity. Root turnover remained low in cv. CX8303 under both low and high salinity. Differential effects of salinity on root growth of the two cultivars in containers were not evident until about 60 d after transplanting. This suggests that the discrepancy between salinity responses of hydroponic and soil‐grown plants was primarily due to differences in phenology. Enhanced rates of root death during the reproductive growth stage may represent an important, previously undocumented carbon cost to some genotypes exposed to salinity.

MacLEAN, D. C.; HANSEN, K. S.; SCHNEIDER, R. E.

doi: 10.1111/j.1469-8137.1992.tb01095.xpmid: N/A

The deleterious effects of aluminium in the rhizosphere on growth, root morphology, ion uptake, and stomatal conductance in wheat (Triticum aestivum L.) were prevented or attenuated by the inclusion of fluoride in the growth medium; the more fluoride present relative to the concentration of aluminium, the greater the alleviation. Results support the conclusion that the ameliorative effects of fluoride were due to reductions in the concentration of the rhizotoxic trivalent cation (Al3+) through the formation of aluminium–fluoride complexes that are neither phytotoxic nor readily translocated from roots to shoots.