Jan-Erik Hällgren

Paternal inheritance of chloroplast DNA in Larix.

Restriction enzyme analysis was used to determine the inheritance of chloroplast DNA in conifers. The plant material studied included five individual trees of European larch (Larix decidua Mill.) and Japanese larch (Larix leptolepis Sieb. & Zucc.) and six hybrids from controlled crosses between these species. The chloroplast DNA fragment patterns generated by Bam-HI and Bcl-I were species-specific. Paternal inheritance of chloroplast DNA patterns was found in most Larix crosses. One hybrid showed maternal chloroplast DNA patterns. In addition, two other hybrids had mixed Bam-HI patterns suggesting recombination between maternal and paternal chloroplast DNA. The mechanisms favoring paternal inheritance in conifers are not known. Paternal inheritance of chloroplast DNA is suggested it to be a general phenomenon in conifers.

The effects of water availability on transpiration, water potential and growth of Picea abies during a growing season

Abstract Transpiration, pre-dawn shoot water potential and growth of Norway spruce trees were measured for one growing season, under three water regimes, giving a seasonal water input of 696 mm (irrigated), 579 mm (control) and 270 mm (drought). Potential transpiration was calculated from the Penman-Monteith equation, using maximal canopy conductance as a function of vapour pressure deficit. Sap flow was measured by a tree-trunk heat balance method and the data were expressed per unit ground surface. This allowed calculation of canopy conductance ( g c ) for the three treatments. The median of the daytime g c was about 1.1 cm s −1 for non-limiting water conditions, and about 0.5 cm s −1 for the conditions of a moderate water deficit. Pre-dawn water potential did not decrease until daily transpiration rates were substantially reduced, with corresponding g c of 0.2 cm s −1 . The type of control which limits transpiration is discussed. The basal-area growth was related closely to the amount of water infiltrated into the soil. The estimated long-term tree water-use efficiency (the total dry matter produced per unit of water transpired) was on average 4.8 g kg −1 , with insignificant differences between the treatments. Seasonal transpiration reached 380 mm, 325 mm and 209 mm for the irrigated, control and drought treatment, respectively.

Studies of 13C in the foliage reveal interactions between nutrients and water in forest fertilization experiments

Addition of N to an initially N-limited forest increases foliage biomass, demand for water and the probability of water stress. Effects of water and N on tree growth are thus compounded. The 13C abundance of plant tissues is directly correlated with water use efficiency (WUE), and could be used to disentangle the effect of water alone on carbon fixation. However, the 13C abundance may also be directly influenced by changes in rates of photosynthesis related to variations in N status, and by variations in N metabolism via non-RuBisCo carboxylations, and indirectly by effects of N source on WUE. We studied the 13C abundance of current needles from top whorls in two long-term fertilization experiments, one in Norway spruce (Picea abies Karst.) and one in Scots pine (Pinus sylvestris L.). As predicted, N fertilization increased foliage biomass and δ (‰). In the experiment with spruce this effect on 13C abundance was correlated with volume production and foliage biomass in a dry year, but was not seen in a wet year after 19 years of continuous annual N fertilization, which rules out the possible influences of N metabolism and changes in rates of photosynthesis. In the experiment with pine, which was at a drier site, needles from N-fertilized plots had a higher 13C abundance in three dry years, but not significantly so in a wet year. We suggest that effects of N source (NH4+ or NO3−) on 13C abundance are unlikely to be important under these experimental conditions. The balance between demand and supply of water should thus be the major determinant of the 13C abundance of current needles on top whorls. This opens possibilities to conduct retrospective studies of the role of water supply in fertilization experiments.

Measurements of abundances of 15N and 13C as tools in retrospective studies of N balances and water stress in forests: A discussion of preliminary results

Preliminary attempts to make retrospective studies of N balances and water stress in forest fertilization experiments by analyzing changes in the abundances of 15N and 13C, respectively, are discussed. Most evidence is from the Swedish Forest Optimum Nutrition Experiments, which have been running for two decades. Annual additions of N have been given either alone or in combination with other elements, notably P and K, every third year. Processes leading to loss of N, e.g. volatilization of ammonia, nitrification followed by leaching or denitrification, and denitrification alone, discriminate against the heavy isotope 15N. A correlation was found between fractional losses of added N and the change in δ15N (‰) during 19 years in current needles in a Scots pine forest, irrespective of source of N. Isotope effects were larger on urea than on ammonium nitrate plots (2 as compared to 9 δ15N (‰)) because of ammonia volatilization and higher rates of nitrification. They developed gradually over time, which opens possibilities to analyse the development of N saturation. However, the analysis may be confounded by shifts in 15N abundance of fertilizer N. In another trial, N isotope effects could be seen in both plants and soils 10 years after the last fertilization; they were smaller in soils because of a large pretreatment memory effect, but we expect them to persist there for decades.The enzyme RuBisCo discriminates strongly against the heavy isotope 13C during photosynthesis, but this effect becomes less expressed as stomata close because of water stress. The supply of N may also affect the δ13C (‰) via effects on rates of photosynthesis, and the source of N may have an influence directly via non-RubisCo carboxylations, and indirectly via effects on water use efficiency. In a trial with Norway spruce, the effect of N fertilization on the δ13C (‰) of current needles was strongly correlated with production and weakly so with foliar biomass a dry year, but not a wet year. This suggested that these variations are primarily related to induced differences in the balance between supply and demand for water. Hence, studies of {au13}C abundance can disentangle the role of water as such from its effects on mineralization of N and flow of N.

Assessment of transpiration estimates for Picea abies trees during a growing season

SummaryThe tree-trunk heat balance method with internal heating and sensing of temperature was used to estimate sap-flow rate of spruce trees in a stand in southern Sweden. Sap-flow rate values were scaled up to stand transpiration and utilised for calculation of canopy conductance. The calculated values provided the basis for a function relating canopy conductance to vapour pressure deficit, which was implemented in the Penman-Monteith equation. The stand was mostly growing in non-limiting soil water conditions (irrigation regime applied during dry periods). The whole-season transpiration was assessed by two different approaches and then compared: the sap-flow rate measurements were scaled to stand transpiration and the adapted Penman-Monteith estimate. They gave similar results: the transpiration totals differed by 3% and the coefficient of determination of the linear regression was r2 = 0.89. Similarly good was the assessment for a set of rainy days. The Penman-Monteith estimate adapted in this way proved to be reasonably precise and reliable in this forest stand and usable even in wet conditions. The seasonal transpiration of the spruce stand was 392 mm according to the adapted Penman-Monteith equation. Mean daily transpiration was 1.8 mm and daily maximum transpiration was 4.8–4.9 mm as estimated by sap-flow rate measurements.

Simulated and measured water uptake by Picea abies under non-limiting soil water conditions

Abstract A one-dimensional non-steady-state soil-plant-atmosphere continuum (SPAC) model was applied to a stand of Picea abies trees in southern Sweden. The simulated root water uptake was compared with measured sap flow under non-limiting soil water conditions. Sap flow was measured during the growing season using the tree-trunk heat balance method. The model included four resistances against water (soil-root, plant, stomatal and aerodynamic), one pool of easily available plant water and one compartment of intercepted water on the needle surface. The bulk stomatal resistance was estimated as the product of the combined effect of two independent variables—vapour pressure deficit and solar radiation. Good agreement between simulated and ‘measured’ water uptake was obtained both on short- and long-term scales, and the model explained 92–93% of the variation of measured uptake for both hourly and daily values. The pool of easily available water was found to be small (0.5 mm), i.e. of the same order as for agricultural crops.

The chloroplast genomes of conifers lack one of the rRNA-encoding inverted repeats

SummaryChloroplast DNA from species of five different conifer genera was extracted and studied by Southern blot analysis. For all these species, hybridization with heterologous probes specific for 16 S and 23 S rDNA detected only one chloroplast DNA fragment per enzyme digest. This observation suggests that the 16 S and 23 S rRNA genes are not duplicated in these genomes. The unique 16 S rDNA-containing BamHI fragment from Pinus contorta Dougl. was cloned and restriction mapped. Apart from the 16 S rRNA gene, this fragment also contained the psbC and psbD genes. It is concluded that the chloroplast genomes of a wide taxonomic range of conifers lack one of the inverted repeat elements and that a dislocation of the psbDC gene cluster has occurred in P. contorta.

Xylem sap composition: A tool for investigating mineral uptake and cycling in adult spruce

Xylem sap composition of spruce is influenced by several factors, such as the sampled organ, the sampling period, the availability of soil nutrients, and the soil water potential. Based on literature data and ongoing investigations carried out with adult trees, we present an overview on the main factors influencing xylem sap concentrations of Norway spruce. Direct measurements of nutrient fluxes in the xylem sap are then used to suggest a general scheme of mineral element cycling within adult trees. In Norway spruce (Picea abies Karst.), nutrient concentration in the xylem sap was higher in twigs and fine roots compared to the bottom of the trunk, the highest concentrations beeing observed in spring during the shoot elongation. Xylem sap concentrations were higher in spruce growing at nutrient rich sites than at poor sites. The combination of twig and trunk xylem sap analysis, together with xylem flow measurements in the trunk during the course of a vegetation period allowed the quantification of mineral fluxes via xylem sap flow in the trunk and twigs. These results were compared to gross mineral uptake measurements at the same site. Ca flux in the trunk xylem sap was lower than the gross uptake of Ca. Mg flux in trunk sap was approximately equivalent to Mg gross uptake whereas P and K fluxes in trunk sap were much higher than the gross uptake. Fluxes of Ca, Mg, K and P in the twig sap were much higher than that in trunk sap. Data suggest that internal cycling is responsible for a large part of the nutrient fluxes in the xylem sap of the crown. Xylem sap composition thus appears to be a tool which can complement other sources of information on mineral uptake and cycling in adult spruce.

Effects of summer frost oh the gas exchange of field‐grown Pinus sylvestris L. seedlings

The effect of night temperatures on the gas exchange of naturally established Scots pine (Pinus sylvestris L.) seedlings was investigated at a site with frequent summer frosts in northern Sweden. During the growing season, a reversible inhibition of light‐saturated rates of CO2‐assimilation was observed after nightfrosts. Stomatal conductance decreased or increased in parallel with changes in the CO2‐assimilation rate. The inhibition of photosynthesis was not primarily the result of stomatal closure, since intercellular CO2‐concentration increased after night frosts. Naturally regenerated Scots pine seedlings survived repeated frosts during the growing season and showed no visible damage although air temperatures as low as ‐8.2°C were recorded during the period of shoot elongation

The relationship between photosynthesis measured by 14C incorporation and by uptake of inorganic carbon in unicellular algae

The traditional 14C method of estimating photosynthetic rates was compared with net photo-synthesis in two species of unicellular algae. Net photosynthesis was defined as the disappearance of inorganic carbon from the algal medium and was determined using a described infra-red gas analyser (IRGA) technique. For Amphidinium carterae Hulbert, the 14C method always led to lower calculated rates of photosynthesis than the IRGA technique. This difference was, on average, ≈100%. For Scenedesmusobliquus (Turpin) Kutzing, the 14C method could lead to over-estimates, good agreement with, or underestimates of net photosynthesis. Although the under-estimates were only of the order of 15%, the over-estimates were in some cases > 100%. There is some indication that respiration rate is an important factor in the relationship between photosynthetic rates calculated by the two methods for this organism.