Maximum Rates Of Net Photosynthesis Research Articles

Photosynthesis and respiration in air-grown and CO 2-grown photoautotrophic cell suspension cultures of carnation

Photoautotrophic carnation ( Dianthus caryophyllus L.) cell-suspension cultures were grown either in the presence of air enriched with CO 2 (1%, v/v) or in atmospheric air. Photosynthesis, respiration and carbohydrate content of the two types of cells were compared. Chlorophyll content and maximal rates of net photosynthesis were very similar in both cultures. High rates of photosynthesis were correlated with the exponential phase of growth and low cellular sucrose and glucose contents. The apparent affinity for CO 2 for photosynthesis was low compared to leaves of C 3 plants and only slightly higher in air-grown cells than in CO 2-grown cells. Air-grown cells had a lower growth rate, a lower carbohydrate content, and a lower respiration rate than CO 2-grown cells. Measurements of fumarase activity (a mitochondrial marker enzyme) suggest that the number of mitochondria per cell might be lower in air-grown cells than in CO 2-grown cells. Compared with intact leaves of carnation. isolated cells had about 4-times less chlorophyll and 5-times less net photosynthetic activity per mg of dry wt. However, measurements of gross and net photosynthesis with 18O 2 and a mass spectrometry technique indicate that 65–70% of the energy produced by the photosynthetic apparatus of isolated cells was used for CO 2 fixation at saturating levels of CO 2, a result roughly comparable to the 80–85% recorded with intact carnation leaves.

Rhythms during extended dark periods determine rates of net photosynthesis and accumulation of starch and soluble sugars in subsequent light periods in leaves of Sorghum

Photosynthesis and photosynthate partitioning in leaves of Sorghum bicolor (L.) Moench exhibited a cyclic dependence on the duration (10-62 h) of dark periods inserted prior to bright light test periods (550 μmol·s(-1)·m(-2), photosynthetic photon flux). Maximum rates of net photosynthesis and of accumulation of starch and soluble sugars were, in the order given, two-, three- and fourfold greater than minimum values. Between 14 and 53% of photosynthate was retained in leaves depending on the length of the dark period. These changes were sufficient to account for the previously described stimulatory effect of short daylengths (i.e., long nights) on carbohydrate accumulation in leaves (N.J. Chatterton and J.E. Silvius, 1980, Physiol. Plant. 49, 141-144). The freerunning periods for the rhythmic dependence on darkness, determined either directly or by curve fitting, were about 24 h for net photosynthesis, 23 h for starch accumulation, and 26 h for solublesugar cccumulation. The deviation from period lengths of 24 h for carbohydrate accumulation indicates that these rhythms are probably endogenous and circadian. Initial maxima were observed after 14 h of darkness for photosynthesis, after 18-22 h for starch, and after 26 h for soluble sugars. The differences in period length and phase indicate that at least three separate rhythms underlie the dependence of photosynthate partitioning in Sorghum on darkness. Periods of low leaf dry-matter accumulation coincided approximately with periods of high net photosynthesis. As a result, maximum photoassimination and maximum export were synchronized and, furthermore, occurred at about the same time as expected light periods.

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius, a European mediterranean semi-deciduous shrub.

During five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO2 uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO2 partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO2- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 μmol m-2 s-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 μmol m-2 s-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.

Photosynthese und Wasserhaushalt der immergrünen mediterranen Hartlaubpflanze Arbutus unedo L. im Jahreslauf am Freilandstandort in Portugal: II. Charakteristische Gaswechselparameter von CO2-Aufnahme und Transpiration

The annual course of certain parameters that characterize the photosynthetic behavior of leaves was derived from diel courses of CO 2 gas exchange and transpiration of intact Arbutus unedo leaves during the period August 1982 to September 1983. The diel courses were shown in a preceeding paper. These data were analyzed along with microclimatic conditions and plant water status and led to the following: — The daily rate of CO 2 uptake (NP ges ) increased steadily from very low rates during the summer dry period throughout the autumn and winter. In February the rates were about eight fold greater than at the end of the dry period. The new leaves, which flushed in March, exhibited these high rates by April and remained at this level of activity until the end of May. At this time midday stomatal closure was first apparent and NP ges decreased throughout the summer to similar rates as were found at the end of the dry period in the previous year. — The total daily water loss (Tr ges ) was very low during the two dry periods. It increased slightly in autumn due to the greater stomatal conductance and remained at a comparatively low level throughout the winter because of the small water saturation deficit of the air at this time of year. The increase in air temperature in spring along with plentiful soil moisture resulted in a steep increase in Tr ges . From the end of May, Tr ges decreased steadily until the end of the dry period. — Due to the very effective stomatal regulation of water loss, the transpiration ratio (TQ = Tr ges /NP ges ), exhibited comparatively low values (similar to those, which were found in early spring) during the two dry periods. The lowest values occurred during the winter. Rather high values were found in the late spring as well as on some days following rain during the dry period, in which case the plants did not exhibit the coordination between stomatal aperture and photosynthetic activity in the mesophyll as was usually exhibited. — The daily maximum rates of net photosynthesis (NP max ) and transpiration (Tr max ) were linearly correlated with NP ges and Tr ges , respectively. Therefore, they can be considered as representative parameters for the annual course of CO 2 gain and transpiration. During the winter, NP max and the daily maximum of photosynthetically active radiation (PAR max ) occurred frequently at the same time of day because photosynthesis was usually light limited at that time of year. From late spring and through the summer, the NP max occurred progressively earlier in the day, since the stomata began to close during the morning hours as water stress became more pronounced. Throughout the year, NP max occurred always within the same temperature range of about 25°C and at almost the same water vapour pressure deficit between leaf and air (ALVPD), about 14 mbar bar − 1 . Thus, stomatal control of transpiration seems to occur only at values greater than 14 mbar bar − 1 . From these data it can be concluded, that the photosynthetic apparatus seems to behave rather “conservatively” in relation to the annual fluctuation of ambient conditions. An important aspect of acclimation to these annual fluctuations is accomplished by stomatal control, which shifts the main period of photosynthetic activity to different times of day when the most favourable ambient conditions occur. — NP max and the respective stomatal conductance (G NPmax ) were linearly correlated. Therefore, following normal diffusion law assumptions, NP max always occurred at similar internal CO 2 partial pressure (p i ≈ 208 μ bar). — During the dry period, NP max as well as G NPmax were approximately linear correlated with the predawn water potential ( ψ pd ). During the rest of the year there was no apparent correlation of these two parameters with the water status of the plants. — There was no marked correlation between the annual course of TQ and that of Because of the effective stomatal control of water loss during the dry period, TQ was similar in the summer ( ψ pd < —45 bar) and in the winter ( ψ pd = —10 bar). — A short term improvement of plant water conditions following rain during the dry period resulted in only a temporary increase of stomatal conductance but no marked increase of photosynthetic activity.

The Seasonal Pattern of Growth and Production of a Temperate C4Species,Cyperus longus

Collins, R. P. and Jones, M. B. 1986. The seasonal pattern of growth and production of a temperate C4 species, Cyperus longus.—i. exp. Bot. 37: 1823-1835. Growth and production of the temperate C4 species Cyperus longus L. was measured throughout a growing season in an established plot in Eastern Ireland. The maximum standing live biomass reached was 2-5 kg m~2. Estimates of unit leaf rate (ULR) and leaf area index (LAI) were made. The product of these quantities gave the crop growth rate (CGR) each week. C. longus was found to maintain high values of LAI throughout the summer, with a maximum value of about 13 in early August. CGR reached a peak in early July. The optimum LAI was 11-6. Temperatures at five levels in the plant canopy, and the amount of solar radiation intercepted by the canopy were measured continuously during the summer. The mean daily rate of leaf extension was positively correlated with the mean daily air temperature above the canopy but the temperature coefficient of the process was low compared with other temperate species. The percentage of solar radiation intercepted by the canopy increased rapidly in early summer, and canopy closure had occurred by mid-June. Rates of net photosynthesis were measured on young and old leaf material in situ at the time of peak LAI. In young leaves the maximum rates of net photosynthesis were higher than those published for a range of temperate C3 species, but similar to those found in another temperate C4 species, Spartina townsendii.

Low-temperature acclimation of net photosynthesis in the crustaceous lichen Caloplaca trachyphylla

The seasonal net photosynthetic and respiratory response matrix is presented for Caloplaca trachyphylla (Tuck.) A. Zahlbr. at 7, 14, 21, 28 and 35 °C, under 0, 300, 600, 900, and 1200 μE m−2 s−1 illumination, over a full range of thallus hydration, in January, May, and July. The maximum rates of net photosynthesis recorded during the summer period, at 14–21 °C, under 1200 μE m−2 s−1 illumination are ca. 3.0 mg CO2 h−1 g−1. They are interpreted in terms of the thallus temperatures documented during early-morning dew events and during sporadic thunderstorm activity. During winter, net photosynthetic rates at 7 °C increase significantly from the summer rates of ca. 2.0 mg CO2 h−1 g−1 to ca. 3.2 mg CO2 h−1 g−1. This low-temperature photosynthetic-capacity change is shown to be temperature acclimation and it is suggested that the mechanism involves a change in the affinity constant (Km) of ribulose bisphosphate carboxylase. This acclimatory response can be readily induced, or reversed in winter, by air-dry storage at an appropriately high or low ambient temperature. In midsummer, however, comparable storage conditions fail to elicit an acclimatory response. The significance of low-temperature photosynthetic acclimation in C. trachyphylla is discussed in relation to winter snowmelt sequences during chinook conditions in Alberta.

Epiphytische Flechten im Bereich einer chilenischen „Nebeloase“ (Fray Jorge) II. Ökophysiologische Charakterisierung von CO2-Gaswechsel und Wasserhaushalt

In a previous communication (Part I) the epiphytic lichen vegetation in the region of the fog oasis Fray Jorge was reported. For 2 lichen communities, the habitat conditions have been characterized through microclimatological measurements. The present paper considers CO 2 gas exchange and water relations of these lichens. Under controlled conditions, photosynthesis and respiration are reported as influenced by water content of the thalli, light intensity, and temperature, for selected species ( Everniopsis trulla and Usnea lacerata from the Oropogonetum loxensis and Heterodermia spinulosa and Ramalina cactacearum from the Ramalinetum cactacearum). The maximal rates of net photosynthesis and dark respiration of the 4 species were very similar. They exhibited medium range of CO 2 exchange capacity when compared with maximum rates of species from other climatic regions. Moistened with liquid water, photosynthetic activity began at a thallus water content of 26 to 29 % (on a dry weight basis). Subsequently, net photosynthesis increased almost linearly with increasing water content. Very high water content resulted in a substantial depression of CO 2 uptake for the species of the Ramalinetum. This apparently is due to increased diffusion resistances within the fully-moistened thalli. Characteristically, such a depression is much smaller or nonexistent for the species of the Oropogonetum. Water vapor uptake in the absence of condensation by the dry lichen thalli can also enable photosynthetic activity. For example, Ramalina cactacearum already exhibited CO 2 uptake in equilibrium with a relative air humidity of 82 %, which corresponds to a water potential of —251 bar (at 10°C). In equilibrium with nearly saturated air, the lichens reached rates of photosynthesis which are similar to those which are attained at optimal water contents after moistening with liquid water. However with water vapor uptake, the same rates of net photosynthesis are possible at substantially lower thalli water contents than would be the case if the thalli were moistened with liquid water. Differences in activation of mitochondrial respiration and in the diffusive conductance of the thalli for CO 2 may be the reasons for thie phenomenon. This has important implications for production ecology. Based upon the CO 2 exchange measurements in the laboratory, the dependency of net photosynthesis of the experimental lichens on water status, light and temperature is presented in interpolation diagrams. These allow estimation of rates of CO 2 uptake for a given combination of environmental factors. With the weather data obtained from the field measurements it is thus possible at least approximately to simulate gas exchange performance of the lichens under their natural conditions. Fog condensation results in water contents of the lichen thalli which — according to the simulations — allow maximal rates of net photosynthesis. With typical fog weather conditions, a positive net CO 2 uptake is possible for the species of the Oropogonetum from briefly after sunrise until about 13 h when the moisture compensation point is finally reached. The daily photosynthetic CO 2 gain under such circumstances amounts to about 5.47 mg CO 2 per g dry weight. Conditions for production are much less favourable for the species of the Ramalinetum. However, the simulations show that in spite of the lack of moistening by fog in these habitats, water vapor uptake from the ambient air alone enables carbon gain in the morning. The duration and rate of net photosynthesis is very dependent on the humidity conditions during the night and on the rate of drying by solar radiation in the morning hours. Under conditions of high humidity at night, Heterodermia spinulosa attains a daily gain of 2.39 mg CO 2 per g. The capability of the thalli to use air humidity as their main water source and their ability to conduct positive photosynthesis at extremely low thallus water potentials, are necessary characteristics for the existence of these species in the Ramalinetum.

The influence of irradiance and external CO 2-concentration on photosynthesis of different tomato genotypes

With 4 genotypes of tomato, irradiance and CO 2-response curves of net photosynthesis were analysed by means of curve fitting. Estimated values of the light compensation point I c showed small but significant differences between the genotypes, the overall value being in the order of 8 W m −2. The photochemical efficiency (α n) and the maximum net photosynthesis per unit leaf area basis ( P nm) reached the highest values for ‘F6 IVT’ (13.3 μg CO 2 J −1 resp. 0.549 mg CO 2 m −2 s −1), the lowest value of α n with ‘Bonabel’ (9.9 μg CO 2 J −1), and the lowest value of P nm with ‘PI 114969’ (0.424 mg CO 2 m −2 s −1). The CO 2-compensation point ( C c) was relatively high (177–245 mg m −3). ‘F6 IVT’ demonstrated the highest value of C c, the lowest carboxylation efficiency and the highest maximum rate of net photosynthesis. The results clearly demonstrate that the latter genotype requires a much higher external CO 2-concentration than the other genotypes in order to exhibit the highest rate of net photosynthesis.

The influence of a severe drought on net photosynthesis of white oak (Quercus alba)

A prolonged and intense drought during the summer of 1976 caused soil water potential in the upper 45 cm of the soil profile to decrease below −2.5 MPa. Predawn xylem pressure potential (P) frequently was less than −1.8 MPa. Under conditions of high soil moisture, net photosynthesis averaged more than 8.0 mg CO2∙dm−2∙h−1 between 0500 and 1800 hours, and maximum rates of net photosynthesis were above 14.0 mg CO2∙dm−2∙h−1. Average and daily maximum net photosynthesis decreased to less than 1.0 and 8.0 mg CO2∙dm−2∙h−1, respectively, during the peak of the drought. Maximum rates of net photosynthesis observed under controlled light and temperature conditions remained above or near 14.0 mg CO2∙dm−2∙h−1 even when predawn P equalled −1.83 MPa. Net photosynthesis seldom decreased below zero if quantum flux densities were greater than the compensation point during this period of record low soil moisture. Environmental conditions, such as fog and clouds, which reduced leaf temperatures and atmospheric evaporative demand, increased net photosynthesis although predawn P was less than −1.8 MPa. During cloudy days, rates of net photosynthesis were within 80% of those observed under ideal soil moisture conditions. The photosynthetic characteristics of white oak noted during this drought are coupled with other physiological traits of white oak to explain its relative success in the oak–hickory forest type.

Moss functioning in different taiga ecosystems in interior Alaska : I. Seasonal, phenotypic, and drought effects on photosynthesis and response patterns.

Carbon dioxide exchange rates in excised 2-year-old shoot sections of five common moss species were measured by infrared gas analysis in mosses collected from different stands of mature vegetation near Fairbanks, Alaska. The maximum rates of net photosynthesis ranged from 2.65 mg CO2 g-1h-1 in Polytrichum commune Hedw. to 0.25 in Spagnum nemoreum Scop. Intermediate values were found in Sphagnum subsecundum Nees., Hylocomium splendens (Hedw.) B.S.G., and Pleurozium schreberi (Brid.) Mitt. Dark respiration rates at 15°C ranged from 0.24 mg CO2 g-1h-1 in S. subsecundum to 0.57 mg CO2 g-1h-1 in H. splendens. The dark respiration rates were found to increase in periods of growth or restoration of tissue (i.e., after desiccation). There was a strong decrease in the rates of net photosynthesis during the winter and after long periods of desiccation.Due to increasing amounts of young, photosynthetically active tissue there was a gradual increase in the rates of net photosynthesis during the season to maximum values in late August. As an apparent result of constant respiration rates and increasing gross photosynthetic rates, the optimum temperature for photosynthesis at light saturation and field capacity increased during the season in all species except Polytrichum, with a corresponding drop in the compensation light intensities. Sphagnum subsecundum seemed to be the most light-dependent species.Leaf water content was found to be an important limiting factor for photosynthesis in the field. A comparison between sites showed that the maximum rates of net photosynthesis increased with increasing nutrient content in the soil but at the permafrostfree sites photosynthesis was inhibited by frequent moisture stress.

Studies on lichen-dominated systems. XXIII. The control of seasonal rates of net photosynthesis by moisture, light, and temperature in Cladonia rangiferina

The net photosynthetic response matrix to four light levels combined with four temperatures and for a full range of thallus hydration has been defined for a Cladonia–woodland population of Cladonia rangiferina in midsummer and early winter. Maximum rates of net photosynthesis are generated between 150 and 200% thallus water content by weight between 15 and 25 °C with low rates being generated at 5 and 35 °C. Light saturation occurs above 600 μE∙m−2∙s−1. There is no evidence of acclimation of photosynthetic optima to temperature but with the onset of early winter there is a marked decrease of respiration rate. An examination of the sensitivity of Cladonia rangiferina in an air-dry state to heat stress shows a rapid decline of net photosynthesis under 45 °C daytime stress. Although 35 °C initially reduces the level of net photosynthesis to some extent, over a 3-week experimental period no further decline is observed. It is suggested that this population of Cladonia rangiferina, which is markedly restriced to a position under the shade of Ledum shrubs, is protected there from the otherwise excessive heat load which occurs on the open Cladonia stellaris surface and which typically reaches 40 °C under full radiation conditions in summer.

A Physiological Basis for Niche Separation Between Agropyron Smithii (C3) and Bouteloua Gracilis (C4)

Agropyron smithii (a C3 species) and Bouteloua gracilis (a C4 species) occur in the same habitat of the shortgrass prairie ecosystem in northeastern Colorado. The hypothesis tested in this study was that the two species differ in their fundamental niches partly as a function of their different physiological responses to temperature and moisture gradients. Growth and gas exchange patterns were measured on plants grown in controlled—environment chambers at cool (20°/15°C) and warm (35°/15°C) temperatures. Total dry mass growth of A. smithii was approximately two times greater when grown at cool compared to warm temperatures, whereas total dry mass of B. gracilis was 2.7 times greater when grown at warm compared to cool temperatures. The temperature for maximal growth was correlated with the temperature response patterns for gas exchange of leaves and roots. A smithii had maximal rates of net photosynthesis when grown at cool temperatures, while B. gracilis had maximal rates when grown at warm temperatures. In plants not water stressed, net photosynthesis was limited primarily by stomatal aperture in B. gracilis and internal leaf factors in A. smithii. Rates of net photosynthesis of both species from both growth temperatures were very sensitive to leaf water potential and decreased exponentially with decreasing leaf water potential. Root respiration, measured simultaneously with leaf gas exchange in hydroponically grown plants, was greater in A. smithii grown at 20°/15°C and in B. gracilis grown at 35°/15°C, and decreased with decreasing solution water potential. Root respiration of warm—grown A. smithii and cool—grown B. gracilis was lower than that of plants grown at more nearly optimal temperatures and did not decrease with decreasing solution water potential. These finding suggest that the temporal displacement of growth between A. smithii and B. gracilis in their native habitat is partly a function of the differential temperature responses of photosynthesis and related metabolic processes. The differences in the temperature responses of photosynthesis appear to be a consequence of the species having different photosynthetic pathways. The similarities between the species in their physiological responses to water stress suggest that seasonal moisture gradients are not as important a parameter along which niche separation has occurred as are seasonal temperature gradients.

Long-term effects of drought on wild and cultivated plants in the Negev desert : I. Maximal Rates of Net Photosynthesis.

The relation between daily maximal rates of net photosynthesis and plant water status was studied during a dry season on irrigated and non-irrigated, naturally growing, perennial wild plants.Species were examined which differ in phenology, leaf anatomy and morphology: Hammada scoparia, Artemisia herba-alba, Zygophyllum dumosum, and Reaumuria negevensis. Prumus armeniaca which was growing in the run-off farm at Avdat and which has mosomorphic leaves was included in the comparison. All plants differed in their seasonal change in plant water status, and in their seasonal change in daily maximal net photosynthesis. Rates of CO2 uptake were not uniquely related to simultanously measured leaf water potentials. Daily maximal rates of net photosynthesis of non-irrigated plants, and the difference between maximal CO2 uptake of irrigated and non-irrigated plants were examined in relation to pre-dawn water potential. Maximal net photosynthesis rates decreased very rapidly with decrease in pre-dawn water potential or, for Hammada scoparia, they decreased even with a constant level of pre-dawn water potential. Consequently, it was considered necessary to include both time and water potential in a parameter "bar day" describing the accumulated drought stress of the plants. All species showed the same relation between relative maximal net photosynthesis and drought experience as determined by cumulative daily addition of pre-dawn water potentials for the non-irrigated plants since the last rain.

A Model for C3Leaves Describing the Dependence of Net Photosynthesis on Irradiance

Measurements of the photosynthesis-light response of flag leaves in a winter wheat crop were made during the period from maximum elongation until complete senescence. Immediately following leaf elongation, the maximum rates of photosynthesis and the values of efficiency at low light were in the range 2-8-3-6 g C02 m~2 h-1 and 8-11 //g C02 J-1 respectively. The shape of the photosynthesis light response remained constant throughout and was close to a 'Blackman' type response rather than a rectangular hyperbola. The results were analysed, therefore, using a more recent model which is a non-rectangular hyperbola. Stomatal and internal resistances were equally important in limiting the maximum rate of photosynthesis. INTRODUCTION The model, derived by Marshall and Biscoe (1980), describes the photosynthesis light response (PLR) as a non-rectangular hyperbola which has four parameters: Pn ma* (maximum rate of net photosynthesis), Rd (rate of dark respiration), a (photochemical efficiency of photosynthesis at low light intensity), and d (ratio of physical to total diffusion resistance to C02). In this paper the effect of age on flag leaf photosynthesis is assessed in terms of these individual parameters. Com parisons and investigations need not be limited to effects of age alone but may also include effects of treatments, for example, differing levels of nitrogen fertilization or water stress. However, such comparisons are valid only if the calculated values of the parameters are reliable and representative of the actual measurements. This was shown to be the case in Marshall and Biscoe (1980) and further proof is given in the Results section of this paper. Having applied the model as an analytical tool to summarize information about the PLRs, the causes of changes with age in calculated parameter values are then investigated with reference to the physical and chemical processes they represent. 1 Present address: Broom's Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk, IP28 6NP. This content downloaded from 207.46.13.15 on Fri, 26 Aug 2016 05:36:03 UTC All use subject to http://about.jstor.org/terms 42 Marshall and Biscoe—Flag Leaf Photosynthesis MATERIALS AND METHODS Using an infrared gas analysis system (Marshall and Biscoe, 1977), detailed measurements of PLRs were made on the flag leaf in a winter wheat crop in 1977, from maximum elongation (5 June 1977) until complete senescence, a period of 65 d. The crop underwent normal farm management and the season was both cooler and wetter than average. The model was fitted to all sets of measurements with the aid of a computer program from the NAG library (fitting routine E04JAF) as described in Marshall and Biscoe (1980). RESULTS AND DISCUSSION Photosynthesis-light response curves Figure 1 shows three typical sets of flag leaf PLR measurements made 17, 31, and 51 d after maximum elongation (DAME). These curves show that as the leaf ages the maximum rate of net photosynthesis decreases but there appears to be relatively little change in the initial slope at low light intensity, consistent with other results where detailed PLR measurements have been presented (Ludlow and Wilson, 1971; McPherson and Slatyer, 1973). The solid lines in Fig. 1 are non-rectangular hyperbolae and it can be seen that they are an acceptable fit. The calculated values of the parameters are shown in Table 1.

A Model for C3Leaves Describing the Dependence of Net Photosynthesis on Irradiance

The rectangular hyperbola is shown to be a poor description of the response of photosynthesis to irradiance for leaves of a winter wheat crop. A model is derived which combines a simplified description of the biochemical reactions occurring within the chloroplast with the physical diffusion of CO2 from the atmosphere. The model is a non-rectangular hyperbola and uses four parameters: Pnmax (maximum rate of net photosynthesis), Rd (rate of dark respiration), α (photochemical efficiency of photosynthesis at low light), and θ (ratio of physical to total resistance to diffusion of CO2). The model is shown to be a significant improvement on the rectangular hyperbola. It provides good fits both to our own measurements and to some data from the literature, and gives consistent and reliable estimates of the four parameters.

Corrigendum - Effects of a water stress on the photosynthesis and respiration of wheat ears

An experiment was conducted on Huntsman winter wheat to investigate the effects of a water stress applied at anthesis on the carbon dioxide exchange of the ears during grain filling. The water stress was created by excluding rain from the soil, not the foliage, of plants growing in the field. Control plants were well watered throughout the period when the treatment was imposed. At intervals for 32 days after anthesis, detailed measurements were made of the photosynthetic rate of ears at different irradiances and rates of ear dark respiration. The measurements were analysed by using the photosynthesis-light response model developed by Marshall and Biscoe (1980) for wheat leaves with a modification for the pathway of respiration from the grains to the glumes. The model is a non-rectangular hyperbola and uses four parameters: Pn,max (maximum rate of net photosynthesis), Rd (rate of dark respiration), � (photochemical efficiency at low light), and F (ratio of physical to total resistance to diffusion of carbon dioxide). Analysis showed that in wheat ears during grain filling, photosynthesis can be treated as occurring predominantly in the glumes and respiration in the grains. The shape of the photosynthesis-light response curves for ears from both treatments were similar, but differed from those for wheat leaves because the maximum rates of photosynthesis were reached more gradually with increasing irradiance. However, the measured response curves were still better fitted by the model than a rectangular hyperbola which has often been used in the past. The water stress at anthesis decreased the maximum rate of ear photosynthesis by 0.8 g carbon dioxide m-2 h-1 throughout the grain-filling period. Initially, the rates of ear respiration were the same, but 32 days after anthesis the treatment had decreased ear respiration rate from 0.04 to 0.01 g carbon dioxide h-1/grain.

Primary productivity of Zostera muelleri Irmisch ex Aschers. In Westernport Bay (Victoria, Australia)

From simultaneous measurements of photosynthetically active radiation (PAR) and gas exchange under both exposed and submerged conditions in situ in Westernport Bay it is shown that the response of photosynthesis by Zostera muelleri ex Aschers. to light can be approximated by a Blackman limiting response curve. The maximum rate of net photosynthesis under submerged conditions is ca. 230 μg CO 2 m −2 leaf area s −1, this rate being reduced by about one-third on exposure. Under submerged conditions the light compensation point corresponds to a photon flux density of 25 μmol m −2s −1, and photosynthesis becomes light-saturated at a photon flux density of about 120 μmol m −2s −1. Estimates of net primary production for each month of the year were calculated from the light response curve and solar radiation records. Photosynthesis is shown to be light saturated for the greater part of each day, and the seasonal variation in net primary production is therefore influenced strongly by day length. Annual net primary production by Z. muelleri amounts to 4.3 g dry matter g −1 dry wt. of plant (17.2 g dry matter g −1 dry wt. of leaf), indicating a turnover of about four times per year. For the whole of Westernport Bay, annual net primary production by Z. muelleri, which covers an area of 38 km 2, is estimated to be 16.3 × 10 9 g dry wt. Taking into account primary production by other species of seagrass and benthic algae, which together cover an area of 181 km 2, it is evident that Westernport Bay is indeed a highly productive ecosystem.

Effects of a water stress on the photosynthesis and respiration of wheat ears

An experiment was conducted on Huntsman winter wheat to investigate the effects of a water stress applied at anthesis on the carbon dioxide exchange of the ears during grain filling. The water stress was created by excluding rain from the soil, not the foliage, of plants growing in the field. Control plants were well watered throughout the period when the treatment was imposed. At intervals for 32 days after anthesis, detailed measurements were made of the photosynthetic rate of ears at different irradiances and rates of ear dark respiration. The measurements were analysed by using the photosynthesis-light response model developed by Marshall and Biscoe (1980) for wheat leaves with a modification for the pathway of respiration from the grains to the glumes. The model is a non-rectangular hyperbola and uses four parameters: Pn,max (maximum rate of net photosynthesis), Rd (rate of dark respiration), � (photochemical efficiency at low light), and F (ratio of physical to total resistance to diffusion of carbon dioxide). Analysis showed that in wheat ears during grain filling, photosynthesis can be treated as occurring predominantly in the glumes and respiration in the grains. The shape of the photosynthesis-light response curves for ears from both treatments were similar, but differed from those for wheat leaves because the maximum rates of photosynthesis were reached more gradually with increasing irradiance. However, the measured response curves were still better fitted by the model than a rectangular hyperbola which has often been used in the past. The water stress at anthesis decreased the maximum rate of ear photosynthesis by 0.8 g carbon dioxide m-2 h-1 throughout the grain-filling period. Initially, the rates of ear respiration were the same, but 32 days after anthesis the treatment had decreased ear respiration rate from 0.04 to 0.01 g carbon dioxide h-1/grain.

Moss production in a black spruce Picea mariana forest with permafrost near Fairbanks, Alaska, as compared with two permafrost‐free stands

During the 1975 and 1976 seasons the net primary production of five common bryophytes in different stands of mature vegetation near Fairbanks, Alaska was investigated.Overall annual moss production at the intensive black spruce site was about 120 g m−1 yr−1 or about twice as high as the corresponding annual spruce production. Maximum rates of net photosynthesis varied from 2.7 mg CO2 g−1 h−1 in Polytrichum commune Hedw. to 0.6 mg CO2 g−1 h−1 in Sphagnum nemoreum Scop. The photosynthesis of overwintered leaves early in the season was low and as a result of new growth a steady increase in net photosynthesis occurred throughout the season. Leaf water content was found to be the most important limiting factor for growth under natural conditions.There was a strong increase in growth and photosynthesis of Sphagnum nemoreum after application of N and P, indicating nutrient deficiency.

Studies on lichen-dominated systems. XXI. The control of seasonal rates of net photosynthesis by moisture, light, and temperature in Stereocaulon paschale

Changes in rates of respiration and net photosynthesis in Stereocaulon paschale (L.) Hoffm. in relation to temperature, moisture, and light have been documented for a woodland population collected in the Abitau–Dunvegan Lake area of the Northwest Territories. The temperature optimum for net photosynthesis is about 20–30 °C and not 0 °C as has been previously reported in the literature. Maximal rates of net photosynthesis are only developed at low levels (100–150% water content by weight) of thallus saturation, suggesting a good degree of adaptation to the xeric conditions normally found in spruce–lichen woodland. Similarly the relatively high light requirement of 1000 μE ∙ m−2 ∙ s−1 correlates well with the open nature of lichen woodland and especially with the disappearance of Stereocaulon paschale in old woodland with the closure of the canopy.The sensitivity of Stereocaulon paschale, when dry, to thermal stress is examined in detail and even the moderate thallus temperature of 35 °C is shown to be highly stressful to this species. Surface temperatures in the low arctic are often above 35 °C and over recently burnt surfaces, temperatures above 45 °C occur on a regular basis. Thus the delayed entry of Stereocaulon paschale into the successional sequence reflects its extreme sensitivity to thermal stress. With the gradual accumulation of an organic layer and an enhanced retention of soil moisture over the initial 60 years of succession there is a significant reduction in both the sensible heat flux and the surface temperatures. At the same time, tree growth leads to a sharp decrease in direct radiation, and the combination of these two developments results in a much cooler surface and one which can be colonized by Stereocaulon.It is suggested that thermal stress tolerance of lichens in an air-dry state, to either high or low temperatures may be centrally important to their overall ecology. Furthermore the net photosynthetic response pattern to temperature, moisture, light, and time of year may, in fact, play a subordinate role in the ecology of a lichen.