Carbon uptake and respiration in above-ground parts of a Larix decidua × leptolepis tree

Summary Shade needles of hybrid larch (Larix decidua × leptolepis) had the same rates of photosynthesis as sun needles per dry weight and nitrogen, and a similar leaf conductance under conditions of light saturation at ambient CO₂ (A_max). However, on an area basis, A_max and specific leaf weight were lower in shade than in sun needles. Stomata of sun needles limited CO₂ uptake at light saturation by about 20%, but under natural conditions of light in the shade crown, shade needles operated in a range of saturating internal CO₂ without stomatal limitation of CO₂ uptake. In both needle types, stomata responded similarly to changes in light, but shade needles were more sensitive to changes in vapor pressure deficit than sun needles. Despite a high photosynthetic capacity, the ambient light conditions reduced the mean daily (in summer) and annual carbon gain of shade needles to less than 50% of that in sun needles. In sun needles, the transpiration per carbon gain was about 220 mol mol^–1 on an annual basis. The carbon budget of branches was determined from the photosynthetic rate, the needle biomass and respiration, the latter of which was (per growth and on a carbon basis) 1.6 mol mol^–1 year^–1 in branch and stem wood. In shade branches carbon gains exceeded carbon costs (growth + respiration) by only a factor of 1.6 compared with 3.5 in sun branches. The carbon balance of sun branches was 5 times higher per needle biomass of a branch or 9 times higher on a branch length basis than shade branches. The shade foliage (including the shaded near-stem sun foliage) only contributed approximately 23% to the total annual carbon gain of the tree.     

Photosynthetic response to varying light intensity in ecotypes of Solanum dulcamara L. from shaded and exposed habitats

Summary Within the widespread species Solanum dulcamara, contrasting ecotypes were found which are physiologically adapted to the light intensities prevailing in their natural habitats. When grown under a high light intensity, an ecotype from a shaded habitat exhibits signs of damage. Another one from an exposed habitat has higher rates of photosynthetic CO₂ uptake when grown under strong as compared to weak light and does not show damage. This differential response becomes even more evident when leaves of both ecotypes are grown to maturity under weak light and are subsequently subjected to strong light for some time. The quantum requirement for photosynthesis increases in the shade-, but not in the sun-ecotype. The sun type increases its rate of photosynthesis under saturating light intensities after a few days in strong light.No significant difference in physical resistances to gas diffusion could be found to explain the highly differing rates of photosynthesis. With the increase in photosynthetic capacity in leaves of the sun type, protein content, activity of RuDP carboxylase, and concentration of Fraction I protein increased likewise. It is suggested that de novo synthesis of photosynthetic enzymes in fully expanded leaves of the sun ecotype following treatment with strong light is the cause of its increased capacity for CO₂ fixation.     

Spacial distribution of photosynthetic capacity and performance in a mountain spruce forest of Northern Germany

Summary Biomass distribution and diurnal CO₂ uptake under natural conditions were investigated on Picea abies in a mountainous climate (Solling, Northwest Germany). Spruce has a remarkable variability in leaf characteristics. Even on a single branch in the lower sun crown, needle dry weight and surface area change considerably from the branch base to the tip and accoring to exposure. Only about 18% of the total biomass of the tree was current year's growth, about 40% of the needles were 4 years and older reaching a maximal age of 12 years. The main growing zone was at the border of upper shade and lower sun crown and the main accumulation of dry weight was at a greater tree height than was observed for maximal growth of needle numbers or surface area. The annual, new growth shifted toward the upper sun crown. Maximal daily CO₂ uptake was highest in the lower sun crown on days with variable cloud cover when temperatures were moderate and water vapor pressure deficits were low. Also the annual CO₂ uptake was highest in the lower sun crown, where 4-year-old and older needles contributed about 35% to the annual CO₂ uptake of the tree. Current year growth contributed about 15% of the total CO₂ gain. The upper and lower sun crowns produce about 70% of the total carbon gain. The carbon balance of spruce and the distribution of the production process in relation to needle age and crown level are discussed.     

Rapid field determination of photosynthetic capacity of cut spruce twigs (Picea abies) at saturating ambient CO2

Summary Routine field determination of the parameters characterizing the activity of the photosynthetic apparatus is often difficult when attached branches of tall trees have to be used for gas exchange measurement. If severed twigs could be used, determining these parameters would be greatly facilitated. Because stomatal conductance changes when twigs or leaves are detached, CO₂ assimilation is usually altered. Thus, measurements made at ambient CO₂ concentration fail to accurately assess the activity of the photosynthetic apparatus because photosynthetic rates greatly depend on the supply of carbon dioxide. However, when photosynthetic carboxylation reactions are saturated by increased CO₂ partial pressure in the mesophyll, CO₂ assimilation rates no longer depend on instantaneous stomatal conductance, as shown by gas exchange measurements of spruce (Picea abies) twigs prior to and following detachment. Because net photosynthesis following detachment at saturating CO₂ remains constant for a minimum of 15 min, photosynthetic measurements of severed twigs may be reliable. This length of time is sufficient for detaching and recutting the twig, assembling a portable minicuvette system, re-establishing steady-state conditions with the gas analyser system, and reading the data over a reasonable period of time. The method described measures the maximal photosynthetic CO₂ assimilation of spruce needles of a single age-class from detached spruce twigs under the following conditions: saturating light, saturating external CO₂-partial pressure, standardized temperature and air humidity in the field. The method is applicable as a routine procedure to characterize the status of the photosynthetic apparatus of spruce trees that may be damaged in the process of forest decline.     

Photosynthesis and nitrogen allocation in needles in the sun and shade crowns of hybrid larch saplings: effect of nitrogen application

We studied the effects of applying 50 kg(N) ha^?1 year^?1 of nitrogen (N) on needle photosynthesis, N allocation and nutrient content in the sun- and shade crowns of the hybrid larch F₁ (Larix gmelinii var. japonica × L. kaempferi). The light-saturated net photosynthetic rate (P _Nmax) was not significantly affected by N application or crown position, although the contents of N, P, K, and chlorophyll (Chl), and the maximum rates of carboxylation and electron transport were lower in needles of the shade crown than of the sun crown. This difference was mainly due to an increase in the intercellular CO₂ concentration (C _i) in the needles of the shade crown. Analysis of N allocation in photosynthetic systems revealed that more N was allocated to functions related to electron transport and ribulose-1,5-bisphosphate (RuBP) regeneration in needles of the shade crown. N allocation in needles of the hybrid larch F₁ was regulated mainly by the light conditions, rather than by N application     

Crassulacean acid metabolism in the shade. Studies on an epiphytic fern,Pyrrosia longifolia,and other rainforest species from Australia

Summary Crassulacean acid metabolism (CAM) was studied in a tropical epiphytic fern, Pyrrosia longifolia, from a fully sun-exposed and from a very shaded site in Northern Queensland, Australia. Measurements of instantaneous net CO₂ exchange showed carbon gain via CO₂ dark fixation with some net CO₂ uptake also occuring during late afternoon, in both sun and shade fronds. Maximum rates of net CO₂ uptake and the nocturnal increase in titratable acidity were lower in shade than in sun fronds. ¹³C values of sun and shade fronds were not significantly different, and ranged between-14 and-15 suggesting that, in the long term, carbon gain was mainly via CO₂ dark fixation. Sun fronds had a higher light compensation point of photosynthesis than shade fronds but the same quantum yield. Yet there was no acclimation of photosynthetic O₂ evolution, (measured at 5% CO₂) in sun and shade fronds and photosynthesis saturated at between 200 and 400 mol quanta m^-2 s^-1. Use of higher light intensities for photosynthesis of sun fronds was probably precluded by low nutrient availability. Total nitrogen was less than 1% of dry weight in fully expanded sun and shade fronds. Exposure of shade fronds to full sunlight for 6 h led to a 60% decline in the quantum yield of photosynthesis and to a decline in variable fluorescence measured at room temperature. Photoinhibition by high light was also observed in Hoya nicholsoniae, a rainforest climber growing in deep shade. This species also exhibited CAM as demonstrated by nocturnal net CO₂ uptake, nocturnal acidification and a ¹³C value of-14. Photosynthetic O₂ evolution in this species was saturated at 2.5% of full sunlight. Two species of Dendrobium (Orchidaceae) from sun-exposed sites, one species exhibiting CAM and the other one exhibiting net CO₂ uptake exclusively during daytime via conventional C₃ photosynthesis, showed similar light response curves and the same quantum yield for photosynthetic O₂ evolution.     

Photosynthetic capacity of loblolly pine (Pinus taeda L.) trees during the first year of carbon dioxide enrichment in a forest ecosystem

Our objective was to assess the photosynthetic responses of loblolly pine trees (Pinus taeda L.) during the first full growth season (1997) at the Brookhaven National Lab/Duke University Free Air CO₂ Enrichment (FACE) experiment. Gas exchange, fluorescence characteristics, and leaf biochemistry of ambient CO₂ (control) needles and ambient + 20 Pa CO₂ (elevated) needles were examined five times during the year. The enhancement of photosynthesis by elevated CO₂ in mature loblolly pine trees varied across the season and was influenced by abiotic and biotic factors. Photosynthetic enhancement by elevated CO₂ was strongly correlated with leaf temperature. The magnitude of photosynthetic enhancement was zero in March but was as great as 52% later in the season. In March, reduced sink demand and lower temperatures resulted in lower net photosynthesis, lower carboxylation rates and higher excess energy dissipation from the elevated CO₂ needles than from control needles. The greatest photosynthetic enhancement by CO₂ enrichment was observed in July during a period of high temperature and low precipitation, and in September during recovery from this period of low precipitation. In July, loblolly pine trees in the control rings exhibited lower net photosynthetic rates, lower maximum rates of photosynthesis at saturating CO₂ and light, lower values of carboxylation and electron transport rates (modelled from A–C_i curves), lower total Rubisco activity, and lower photochemical quenching of fluorescence in comparison to other measurement periods. During this period of low precipitation trees in the elevated CO₂ rings exhibited reduced net photosynthesis and photochemical quenching of fluorescence, but there was little effect on light- and CO₂-saturated rates of photosynthesis, modelled rates of carboxylation or electron transport, or Rubisco activity. These first-year data will be used to compare with similar measurements from subsequent years of the FACE experiment in order to determine whether photosynthetic acclimation to CO₂ occurs in these canopy loblolly pine trees growing in a forest ecosystem.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary Photosynthetic rates and nutrient contents of spruce needles were measured in a region with high levels of air pollution in NE Bavaria, Germany (FRG), and compared to spruce grown under clean air conditions at Craigieburn, in the South Island of New Zealand (NZ). The absolute rates of CO₂ uptake, the slope of the CO₂ response curve at 240 l l^–1 internal CO₂ concentration, and the change of photosynthetic rates with needle age at ambient and saturated CO₂ concentrations were virtually identical at both measuring sites. These results confirm an earlier conclusion, that there is no long-term effect of atmospheric pollutants directly on photosynthetic CO₂ uptake rates with persistent exposure at the FRG site to high levels of anthropogenic air pollution. Photosynthetic capacity at saturating CO₂ concentration was three times higher in the NZ spruce. Needles with high photosynthetic capacity in NZ had lower nitrogen and higher calcium concentrations per unit dry weight but higher concentrations of nitrogen, phosphorus, potassium, magnesium and calcium per unit leaf area, and twice the specific leaf weight.     

Seasonal development of the photosynthetic performance of Norway spruce (Picea abies [L.] Karst.) under magnesium deficiency

In order to investigate the influence of different magnesium nutrition on photosynthesis, one hundred 6-year-old spruce trees derived from one clone were planted in October 1990 into a special out-door experimental construction, where they were cultivated in sand culture with an optimal supply of nutrients, except magnesium, via circulating nutrient solutions. Magnesium was added to the nutrient solutions in three different concentrations, varying from optimal to severe deficient supplies. During the first vegetative period in 1991, photosynthetic performance and carboxylation efficiency were measured under saturating light, controlled CO₂ conditions, optimal temperature and humidity, using a minicuvette system.During summer, the trees under moderate magnesium deficiency developed tip yellowing symptoms on older needles, while the youngest needles remained green with unchanged chlorophyll contents. Trees under severe magnesium deficiency showed yellowing symptoms on all needle age classes combined with decreased chlorophyll contents in the youngest needles as well. In comparison with the controls, the photosynthetic performance of the 1-year-old needles was significantly lower in both deficiency treatments. The same was observed in the youngest needles of the trees under severe deficiency. Trees under moderate deficiency treatment decreased in photosynthetic performance during the summer without reduction of chlorophyll contents. The reduction of photosynthetic rates corresponded to a decrease in carboxylation efficiency, which is taken as a measure of the activity of the enzyme ribulose-1,5-bisphosphate carboxylase. This reduction, together with the observed increase of carbohydrate contents in needles of trees growing under magnesium deficiency, led to the assumption that the photosynthetic carbonfixation is reduced as a consequence of the accumulation of carbohydrates.     

Magnesium deficiency treatment causes reductions in photosynthesis of well-nourished Norway spruce

In order to investigate effects of magnesium deficiency on Norway spruce [Picea abies (L.) Karst.] photosynthesis, 100 well-nourished 5-year-old spruce trees were grown in sand culture, individually supplied with circulating nutrient solutions. Mineral nutrients were added to the nutrient solutions in optimal quantities and optimal relations to nitrogen. Magnesium was supplied at 0.203, 0.041 and 0.005 mM in order to simulate optimal nutrition, moderate deficiency and severe deficiency. Parameters of photosynthetic gas exchange, chlorophyll, magnesium and starch concentrations were determined in current-year and 1-year-old needles during one growing season. By mid May — 6 months after onset of the Mg deficiency treatments in late autumn — CO₂-assimilation rates of 1-year-old needles were significantly decreased independent of the severity of the deficiency treatment, whereas the chlorophyll concentrations did not differ from the controls. The occurrence of yellowing symptoms during July did not further influence the Mg deficiency effect on photosynthesis. In contrast to 1-year-old needles, significant reductions of photosynthesis and chlorophyll in current-year needles were only caused by severely deficient Mg supply. Mg deficiency affected carboxylation efficiency but not light use efficiency. From the accumulation of starch in the needles, up to 30-fold of the controls, the conclusion has been drawn that reactions of CO₂-fixation were affected by reduced carbohydrate export. The light-dependent pigment reduction, leading to the typical tipyellowing of needles, clearly reflects a secondary effect of Mg deficiency.     

Inhibition of photosynthesis in current-year needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during autumn and early winter

Inhibition of photosynthesis was followed during autumn and early winter in current-year sun and shade needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] by simultaneous measurements of photosynthetic O₂ evolution and chlorophyll a fluorescence at 20 °C. The CO₂-saturated rate of O₂ evolution was generally higher in sun needles of fertilized trees than in those of unfertilized trees over a wide range of incident photon flux densities (PFDs). Furthermore, the maximum photo-chemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F_V/F_M) was generally higher for sun needles of fertilized trees. The depression of f_v/f_m during frost periods was more pronounced in sun needles than in shade needles, indicating that winter inhibition in Norway spruce is strongly light-dependent. However, the inhibition of the rate of O₂ evolution at high PFDs in needles of fertilized trees during early winter was partly independent of the light regime experienced by those needles in the field, which appeared to result in a pronounced decrease in the proportion of oxidized PS II reaction centres in shade needles. A nearly identical linear relationship between the quantum yield of PS II electron transport determined by chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PFD) was obtained for the investigated types of needles during autumn and early winter. Except for shade needles of fertilized trees, this appeared to be largely achieved by adjustments in thermal energy dissipation within PS II.     

Photosynthesis and transpiration of healthy and diseased spruce trees in the course of three vegetation periods

Summary CO₂- and H₂O-gas exchange of 20- to 25-year-old spruce trees from a plantation in the Hunsrück mountains were investigated over a period of 3 years. All measurements were made as pair comparisons, i.e., in each case the gas exchange of a damaged tree and of a relatively healthy tree in its immediate vicinity was measured simultaneously. A second plantation in the Westerwald mountains consisted of 18-year-old apparently healthy spruce trees. Pair comparison at this location meant comparison of two healthylooking trees. The investigations at both locations included diurnal course measurements of photosynthesis and transpiration, and light saturation curves and CO₂-saturation curves of photosynthesis. The reduced photosynthesis parameters of the phenotypically damaged trees at the Hunsrück location indicates massive damage to the photosynthetic apparatus. Measurements of H₂O-gas exchange showed that there are disturbances in stomatal regulation of the needles of damaged trees. As a result, the water use efficiency of these needles proved to be significantly lower. In addition, apparent photorespiration of the damaged trees was decreased, whereas their light- and CO₂-compensation points and their dark respiration were increased. In contrast to the Hunsrück plantation, no such effects were detectable when the healthy-looking Westerwald trees were subjected to pair comparison of gas exchange. Reduced photosynthetic capacity and disturbances of the stomatal regulation of the phenotypically damaged Hunsrück trees may be due to damage in the cellular membranes. Furthermore, a comparison of three growing seasons led to the conclusion that the gas exchange of spruce trees in their natural habitat is markedly influenced by climatic conditions.     

An in vivo analysis of the effect of SO2 fumigation on photosynthesis in Zea mays.

Fumigation of leaves with SO₂ can reduce the capacity for photosynthetic CO₂ uptake even in the absence of visible symptoms of damage. In vitro studies suggest that this invisible injury to intact leaves could be affected by damage to each of the main stages in the photosynthetic process. Reduced stomatal apertures may also reduce photosynthesis following SO₂ fumigation. The responses of CO₂ uptake by leaves to intercellular CO₂ concentration and to absorbed light provide information for quantitative separation of the in vivo contribution of the different stages of photosynthesis to reduction in overall rate. This study uses these techniques to examine the basis of reduction in CO₂ uptake in Zea mays cv. LG11 leaves following short-term fumigation with SO₂. Fumigation with 33 μmol m^–3 SO₂ for 30 min reduced light saturated CO₂ uptake by about one-third. An even greater reduction in light limited CO₂ uptake was observed and with no significant change in light absorptance this was attributed to a reduced quantum yield of photosynthesis. The light saturated CO₂ uptake rate and the stomatal conductance decreased in parallel. However, the relationship of CO₂ uptake to the intercellular CO₂ concentration suggested that the reduced stomatal conductance did not account for the reduced rate of CO₂ uptake following fumigation. Both the initial slope and plateau of this relationship were significantly reduced, suggesting that both carboxylation efficiency and capacity for regeneration of CO₂ acceptor were diminished by SO₂ fumigation. The operating intercellular CO₂ concentration indicated that both processes were co-limiting, before and after fumigation. The time required for induction of photosynthetic CO₂ uptake on illumination was approximately doubled following SO₂ fumigation, showing that fumigation impairs the ability of the photosynthetic apparatus to adapt to fluctuations in light level.     

Needle age dependence of photosynthesis along a light gradient within an <Emphasis Type="Italic">Abies alba</Emphasis> crown

Age and shade effects on needle structure and photosynthesis were determined within a lower part of Abies alba crown along a horizontal increasing gradient of branches self-shading. It was hypothesized that a decrease in net CO₂ assimilation rate with increasing needles’ age would be related to: (1) structural age-related changes in needles, (2) reduction of stomatal conductance, (3) nitrogen translocation from old to young needles, and (4) decrease in efficiency of photochemical processes. Leaf mass-to-area ratio increased non-linearly with needle age. In a needle cross section, distance between the vascular bundles decreased, and height of palisade parenchyma cells increased with age. The structural changes observed in our study might lead to an increase in internal resistance to CO₂ with greater needle age. Total needle nitrogen concentration linearly decreased with age due to dilution and/or translocation to younger needles. When expressed per needle area, nitrogen content was reduced in 6-year-old needles compared with younger ones. Net CO₂ assimilation rate per needle area decayed and was accompanied by a decrease in transpiration and water and photosynthetic nitrogen use efficiency. Old needles maintained high photochemical efficiency which compensated to some extent for light deficit in their micro-light environments. Our results have suggested that there is a mechanism controlling the relation between efficiency of light and dark photosynthetic processes along the needle age and shade gradient in A. alba crown.     

Net photosynthesis and transpiration of Pinus montana on east and north facing slopes at alpine timberline

Summary Potted Pinus montana seedlings, age 4 years, transplanted on adjacent east and north facing slopes 25 m apart at alpine timberline (2,020 m a.s.l.) were measured for net photosynthesis and transpiration under ambient conditions using climatised Koch-Siemens cuvettes. Concurrent recordings were made of air temperature, atmospheric water vapour pressure deficit and illuminance at each site.On a typical summers day the northern aspect averaged 9% less light, 1.8°C cooler air temperatures and 25% lower v.p.d. levels than the eastern aspect. The order of these differences was found to increase in the autumn. Net photosynthetic rates of seedlings on the northern aspect were on average 28% lower than the rates of seedlings on the warmer eastern aspect. Differences in transpiration rates were even greater with north slope seedlings averaging rates 42% lower than east slope seedlings.Maximum CO₂ uptake rate per hour of east slope seedlings was 3.2 mg CO₂ g^-1 d.w.h^-1 but average rates when light was not limiting were around 2.0 mg CO₂ g^-1 d.w.h^-1. Corresponding values for the north slope seedlings were 3.0 mg CO₂ and 1.8 mg CO₂ g^-1 d.w.h.^-1 respectively.Light intensities below 10 klx, when photosynthesis was strongly limited by light, totalled 48% of available daylight hours on the east slope and more than 50% on the north slope.Net photosynthesis was largely unaffected by air temperature between 10°C and the recorded maximum at either site (24°C east, 20°C north) and there was no apparent response to v.p.d. at levels up to 10 mbar. However the consistently higher net photosynthesis of east slope seedlings under all combinations of weather conditions indicated a possible acclimatisation of seedlings at each site.     

Protective and injuring action of visible light on photosynthetic apparatus in wheat plants during hyperthermia treatment

Illumination of wheat (Triticum aestivum L.) leaves during heat treatment produced either additional injury or protection of photosynthetic apparatus depending on irradiance and the heating dose. Furthermore, illumination of leaves during hyperthermia exerted differential impacts on thermal tolerances of photosynthesis and photosystem II-driven electron transport assessed from the reduction of 2,6-dichlorophenolindophenol (DCPIP). Measurements with infrared gas analyzer showed that mild heating of leaves in darkness (10 min at 38–40°C) had stronger inhibitory effect on CO₂ uptake than heating of leaves exposed to low and moderate complex irradiances (3–30 klx), as well as excessive irradiance (75–100 klx). When the leaves were heated at higher temperatures (42–44°C), the low and moderate irradiances had a protective action, while high-intensity light aggravated the inhibition of photosynthesis. Illumination of leaves with weak light during heat treatment mitigated the impairment of chloroplast ultrastructure, whereas irradiation with high-intensity light (100 klx) destroyed the sensitive population of chloroplasts. The heat-stimulated photoinhibition was stronger for leaf photosynthesis than for DCPIP reduction in chloroplasts isolated from heat-treated leaves. No correlation was observed between the extent of violaxanthin deepoxidation, zeaxanthin accumulation, and the protective effect of light on photosynthetic apparatus during heat treatments.     

Further Studies on Differentiation of Photosynthetic Properties in Sun and Shade Ecotypes of Solidago virgaurea

Measurements of the fraction of the incident light absorbed by diverse Solidago leaves revealed that differences in light harvesting capacity cannot explain the differences in efficiency of utilization of weak light in photosynthesis that have previously been shown to exist between sun and shade ecotypes when these have been grown in strong light and between identical clones of shade ecotypes when grown at different light intensities. Photosynthesis measurements at low and normal oxygen concentrations, provided no evidence that a different degree of inhibition of photo-synthetic CO₂ uptake by atmospheric oxygen is responsible for the observed differences in photosynthetic efficiency, at low or high light intensities. These results support the conclusion that the markedly less efficient use of weak light by shaded habitat clones grown in strong as compared with weak light is caused primarily by damage to the photosystems, or to a site close to them. Measurements of Emerson enhancement and of light-induced absorbance changes provide some evidence that photoreaction II is more affected than I. Enzyme extracts prepared from clones native to an exposed habitat were found to contain considerably higher activities of carboxydismutase (ribulosc-l,5-diphos-phate carboxylase) than from clones native to a shaded habitat when the plants were previously grown at a moderately high light intensity. Exposed habitat clones apparently have a genetically determined, higher capacity to produce the carboxyla-tion enzyme than shaded habitat clones. The high degree of correlation found when the light-saturated rate of CO₂ uptake in vivo of a number of individual Solidago leaves is plotted against the carboxydismutase activities found in the extracts of these same leaves suggests that low carboxydismutase activity is one of the intrinsic properties responsible for the low capacity for light-saturated photosynthesis of clones from shaded habitats. It is concluded from this and other investigations that differentiation between plants from habitats with contrasting light intensities, whether unrelated species or ecotypos of the same species, probably involves the capacity of several component steps of the photosynthetic process.     

Influence of shoot structure on light interception and photosynthesis in conifers

The influence of shoot structure on net photosynthesis was evaluated under field conditions for the central Rocky Mountain (United States) conifers Picea engelmannii (Parry ex Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.). In all species, the greater number of needles per unit stem length on sun shoots correlated with a smaller silhouette leaf area to total leaf area ratio (STAR). Decreased STAR was due primarily to greater needle inclination toward the vertical, plus some needle mutual shading. However, photosynthesis expressed on a total leaf area basis did not decrease in sun shoots (lower STAR) but remained nearly constant at approximately 3 micromoles per square meter per second over a wide range of STAR (0.1 to 0.3). Relatively low light saturation levels of 200 to 1400 microeinsteins per square meter per second and diffuse light to 350 microeinsteins per meter per second maintained photosynthetic flux densities in inclined and/or shaded needles at levels comparable to those in unshaded needles oriented perpendicular to the solar beam. As a result, net CO₂ uptake per unit stem length increased as much as 2-fold in sun shoots (low STAR) in direct proportion to increasing needle density.     

Photosynthetic performance,chloroplast pigments,and mineral content of various needle age classes of spruce (Picea abies) with and without the new flush: an experimental approach for analysing forest decline phenomena

Summary Damage in the older needles of Norway spruce [Picea abies (L.) Karst.] in the Fichtelgebirge (NE Bavaria, FRG) appears to result primarily from nutrient imbalances rather than from direct effects of air pollutants on the mesophyll of the needles. Support for this conclusion was obtained by altering the nutrition of older needles through the removal of terminal buds on several branches from a damaged and an undamaged spruce tree in spring. Various photosynthetic parameters, as well as the chloroplast pigment and nutrient concentrations, of 1- to 3-year-old needles on manipulated branches were compared with those of branches on which the new flush was allowed to develop during the course of the growing period. Removal of terminal buds affected only the 1-year-old needles. Elimination of the new flush resulted in a higher Ca and Mn content of the needles of the undamaged tree. This treatment also resulted in an increase of the photosynthetic capacity (under saturating light and CO₂ conditions), carboxylation and light use efficiency, as well as net photosynthesis under natural conditions of the 1-year-old needles on the yellow chlorotic tree. This was accompanied by higher chlorophyll concentrations and an increase in Mg, Ca, Mn, and Zn content, and no visible signs of chlorosis developed in the experiment. By contrast, the needles of twigs in which the new flush was allowed to develop exhibited reductions in mineral content in the middle of the year. This was especially true for the elements Mg and Ca, and was accompanied by needle chlorosis and a depression of the capacity of photosynthesis. Thus it appears that there is a close relationship between the development of needle damage and nutrient imbalances in spruce. The retranslocation of elements from the 1-year-old needles to the new flush seems to play a major role in the development of needle bleaching. This approach thus supports the hypothesis described above and confirms a preliminary test with a similar experimental design, which had been conducted earlier.     

Photosynthetic action spectra of trees: I. Comparative photosynthetic action spectra of one deciduous and four coniferous tree species as related to photorespiration and pigment complements

Comparative isoenergetic action spectra of net photosynthesis for intact, current year foliage of five tree species were determined from 400 to 710 nm by CO₂ exchange analysis. The blue (400 to 500 nm) peak of net photosynthetic activity for the green broadleaves of red alder (Alnus rubra Bong.) was reduced to a plateau for the green needle-leaves of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and Sitka spruce (Picea sitchensis [Bong.] Carr.), a shoulder for the blue-green needles of Colorado spruce (Picea pungens Engelm.), and a reduced shoulder for the blue-white needles of Blue spruce (Picea pungens var. hoospii). These differences were attributable neither to a differential blue light stimulation of photorespiration nor to a differential presence of a nonplastid screening pigment. The conifers all had similar carotenoid-chlorophyll ratios, with approximately 50% more carotenoid relative to chlorophyll as compared to red alder. Blue light absorption and low efficiency of energy transfer by the carotenoids probably accounts for the low net photosynthetic activity of the green conifers in blue light as compared to red alder. Leaf form per se (broad versus needle) had no distinguishable influence on these results.