ALBERT

Notes: Abstract Turgor, and osmotic and water potentials of subsidiary cells, epidermal cells and mesophyll cells were measured with a pressure probe and a nanoliter osmometer in intact transpiring leaves of Tradescantia virginiana L. Xylem water potential was manipulated by changing air humidity, light, and water supply. In a transpiring leaf the water potential of mesophyll cells was lower, but turgor was higher, than in cells surrounding the stomatal cavity owing to the presence of a cuticle layer which covers the internal surface of subsidiary and guard cells. Cuticular transpiration from the outer leaf surface was negligibly small. When stomata closed in dry air, transpiration decreased despite an increasing vapor-pressure difference between leaf and air, and the water potential of subsidiary cells dropped to the level of the water potential in mesophyll cells. We suggest that the observed decrease of transpiration at increasing vapor-pressure difference can be attributed to a shortage of water supply to the guard cells from subsidiary cells, causing turgor to decrease in the former more than in the latter. The leafs internal cuticle appears to play a special role in channelling the internal water flow during a water shortage.

Notes: Abstract. Although nitrate reductase (NR, EC 1.6.6.1) is thought to control the rate of nitrate assimilation, mutants with 40–45% of wildtype (WT) NR activity (NRA) grow as fast as the WT. We have investigated how tobacco (Nicotiana tabacum L. cv. Gatersleben) mutants with one or two instead of four functional nia genes compensate. (i) The nia transcript was higher in the leaves of the mutants. However, the diurnal rhythm was retained in the mutants, with a maximum at the end of the night and a strong decline during the photoperiod. (ii) Nitrate reductase protein and NRA rose to a maximum after 3–4 h light in WT leaves, and then decreased by 50–60% during the second part of the photoperiod and the first part of the night. Leaves of mutants contained 40–60% less NR protein and NRA after 3–4 h illumination, but NR did not decrease during the photoperiod. At the end of the photoperiod the WT and the mutants contained similar levels of NR protein and NRA. (iii) Darkening led to a rapid inactivation of NR in the WT and the mutants. However, in the mutants, this inactivation was reversed after 1–3 h darkness. Calyculin A prevented this reversal. When magnesium was included in the assay to distinguish between the active and inactive forms of NR, mutants contained 50% more activity than the WT during the night. Conversion of [15N]-nitrate to organic compounds in leaves in the first 6 h of the night was 60% faster in the mutants than in the WT. (iv) Growth of WT plants in enhanced carbon dioxide prevented the decline of NRA during the second part of the photoperiod, and led to reactivation of NR in the dark. (v) Increased stability of NR in the light and reversal of dark-inactivation correlated with decreased levels of glutamine in the leaves. When glutamine was supplied to detached leaves it accelerated the breakdown of NR, and led to inactivation of NR, even in the light. (vi) Diurnal changes were also investigated in roots. In the WT, the amount of nia transcript rose to a maximum after 4 h illumination and then gradually decreased. The amplitude of the changes in transcript amount was smaller in roots than in leaves, and there were no diurnal changes in NRA. In mutants, nia transcript levels were high through the photoperiod and the first part of the night. The NRA was 50% lower during the day but rose during the night to an activity almost as high as in the WT. The rate of [15N]-nitrate assimilation in the roots of the mutants resembled that in the WT during the first 6 h of the night. (vii) Diurnal changes were also compared in Nia30(145) transformants with very low NRA, and in nitrate-deficient WT plants. Both sets of plants had similar low growth rates. Nitrate reductase did not show a diurnal rhythm in leaves or roots of Nia30(145), the leaves contained very low glutamine, and NR did not inactivate in the dark. Nitrate-deficient WT plants were watered each day with 0.2 mM nitrate. After watering, there was a small peak of nia transcript, NR protein and NRA and, slightly later, a transient increase of glutamine and other amino acids in the leaves. During the night glutamine was low, and NR did not inactivate. In the roots, there was a very marked increase of nitrate, nia transcript and NRA 2–3 h after the daily watering with 0.2 mM nitrate. (viii) It is concluded that WT plants have excess capacity for nitrate assimilation. They only utilise this potential capacity for a short time each day, and then down-regulate nitrate assimilation in response, depending on the conditions, to accumulation of the products of nitrate assimilation or exhaustion of external nitrate. Genotypes with a lower capacity for nitrate assimilation compensate by increasing expression of NR and weakening the feedback regulation, to allow assimilation to continue for a longer period each day.

Notes: Abstract The water potentials of frozen leaves of Afroalpine plants were measured psychrometrically in the field. Comparison of these potentials with the osmotic potentials of an expressed cellular sap and the water potentials of ice indicated almost ideal freezing behaviour and suggested equilibrium freezing. On the basis of the osmotic potentials of expressed cellular sap, the fractions of frozen cellular water which correspond to the measured water potentials of the frozen leaves could be determined (e.g. 74% at -3.0° C). The freezing points of leaves were found to be in the range between 0° C and -0.5° C, rendering evidence for freezing of almost pure water and thus confirming the conclusions drawn from the water-potential measurements. The leaves proved to be frost resistant down to temperatures between -5° C and -15° C, as depending on the species. They tolerated short supercooling periods which were necessary in order to start ice nucleation. Extracellular ice caps and ice crystals in the intercellular space were observed when cross sections of frozen leaves were investigated microscopically at subfreezing temperatures.

Notes: Abstract Turgor (Ψp) and osmotic potential (Ψs) in epidermal and mesophyll cells, in-situ xylem water potential (Ψ-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (Ψp-epi, Ψp-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.Δg/Ψp-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.

Notes: Abstract The partitioning of nitrogen deposition among soil, litter, below- and above-ground biomass of trees and understory vegetation was investigated in a 15-year-old Picea abies (L.) Karst. plantation in the Fichtelgebirge, Germany, by labeling with 62 mg of15N tracer per square meter in March 1991. Ammonium and nitrate depositions were simulated on five plots each, by labeling with either15N-NH4 + or15N-NO3 −, and the15N pulse was followed during two successive growing seasons (1991 and 1992). Total recovery rates of the15N tracer in the entire stand ranged between 93 and 102% for both nitrogen forms in 1991, and 82% in June 1992. δ5 N ratios increased rapidly in all compartments of the ecosystem. Roots and soils (to 65 cm depth) showed significant15N enrichments for both15N-treatments compared to reference plots. Newly grown spruce tissues were more enriched than older ones, but the most enriched δ15N values were found in the understory vegetation. Although spruce trees were a much larger pool (1860 g biomass/m2) than understory vegetation (Vaccinium myrtillus 333 g/m2, Calluna vulgaris 142 g/m2, Deschampsia flexuosa 22 g/m2), the ericaceous shrubs and the perennial grass were a much greater sink for the15N label. Eight months after labeling, 9% of the ammonium and 15% of the nitrate label were found in the understory. P.abies retained only 3% of the15N-ammonium and 7% of the15N-nitrate. The main sink for both15N tracers was the soil, where 87% of the ammonium and 79% of the nitrate tracer were found. The organic soil horizon (5-0 cm depth) contained 63% of the15N-ammonium and 46% of the15N -nitrate suggesting strong immobilization by microorganisms of both N forms. Eight months after tracer application, about 16% of both15N-tracers was found below 25 cm soil depth. This 16% corresponds well to a 20% decrease in the recovery of both15N tracers after 15 months and indicates a total loss out of the ecosystem. Highly enriched δ15N values were found in fruit bodies of fungi growing in reference lots (no15N addition), although soils did not show increased δ15N ratios. No transfer of15N-tracer between fungi and spruce or understory vegetation was apparent yet.