J. D. Tenhunen

Changes in photosynthetic capacity, carboxylation efficiency, and CO2 compensation point associated with midday stomatal closure and midday depression of net CO2 exchange of leaves of Quercus suber.

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a “standard” time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO2 uptake. Midday depression of CO2 uptake is the result of i) a decrease in CO2-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance

SummaryThalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 μE m-2 s-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO2 concentrations from 200 to 1,000 ppm, CO2 uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO2 concentration, and it was fully absent at 1,600 ppm ambient CO2. Under these conditions of CO2 saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO2 uptake of the water saturated lichen at lower (e.g. natural) ambient CO2 must be due exclusively to increased resistance to CO2 diffusion from the external air to the sites of carboxylation.

Use of an analytical model to study limitations on net photosynthesis in Arbutus unedo under field conditions

SummaryFrom field gas-exchange measurements on Arbutus unedo growing in Portugal, parameter values necessary to apply an analytical, physiologicallybased model of C3 photosynthesis were obtained. The model successfully simulated measured diurnal photosynthetic responses in Arbutus during periods without water stress, under both natural and CO2-saturating conditions. The model was used to analyze those factors limiting primary productivity during each of the experimental days. Due to a large investment in ribulose bisphosphate (RuBP) regeneration capacity, irradiance was rarely limiting, even during cloudy periods, but the limitation imposed by stomatal conductance was quite large, averaging over 30%. The fact that experimental leaves were maintained in a horizontal position is at least partially responsible for these results. Possible other reasons for this apparent excess of RuBP regeneration capacity visa-vis RuBP carboxylase-oxygenase concentration are discussed.

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

SummaryGas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO2 pressure remained near 220 μbar during diurnal courses in the spring. On midsummer days, leaf internal CO2 decreased from a late morning value of 200 μbar to a late afternoon value of approximately 150 μbar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO2 supply due to stomatal closure reduced net CO2 uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO2 compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO2 saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar-1.

Irradiance and temperature effects on photosynthesis of tussock tundra Sphagnum mosses from the foothills of the Philip Smith Mountains, Alaska

SummaryPhotosynthetic characteristics of three species of Sphagnum common in the foothills of the Brooks Range on the North Slope of Alaska were investigated. Generally, light-saturated rates of net photosynthesis decreased in the order S. squarrosum, S. angustifolium, and S. warnstorfii when plants were grown under common growth chamber conditions. For field-grown S. angustifolium, average light compensation point at 10°C was 37 μmol m-2s-1 photosynthetic photon flux density (PPFD), and light saturation occurred between 250 and 500 μmol m-2 s-1. At 20°C, compensation point increased to 127 μmol m-2s-1 and the PPFD required for light saturation increased to approximately 500 μmol m-2s-1, while maximum rates of CO2 uptake increased only slightly. Light response curves of chamber-grown plants exhibited substantially lower compensation points and higher light-saturated rates of CO2 assimilation than field-grown material, due perhaps to a higher percentage of green, photosynthetically competent tissue. All three species exhibited broad responses to temperature, with optima near 20°C, and maintained at least 75% of maximum assimilation between approx. 13° and 30°C. Rates at 5°C were approx. 50% of maximum. Studies of the microclimate of Sphagnum at the field research site suggest that CO2 uptake should occur at near light-saturated rates during the day in open tussock tundra but that PPFD may often be limiting under Salix and Betula canopies in a water track drainage. Simulations using a simple model provided a seasonal estimate of 0.78 g dry weight (DW) of S. angustifolium produced from each initial g of photosynthetic tissue under willow canopies, assuming no water limitations. Although the simulation model suggests that production would be 66% higher in open tussock tundra, S. angustifolium is rarely found in this potentially more stressful habitat. To explain the relative abundance of Sphagnum in shaded water track areas as compared to open tussock tundra, we postulate that the vascular plant canopies provide protection from adverse effects of high temperatures, excess irradiance and reduced water availability. Under conditions of normal water availability, removal of the vascular plant cover did not affect the tissue water content of S. squarrosum, but resulted in a strong decrease in photosynthetic capacity, accompanied by chlorophyll bleaching. These results suggest that photoinhibition may limit production under certain conditions.

Midday stomatal closure in Mediterranean type sclerophylls under simulated habitat conditions in an environmental chamber

SummaryShrubs of the Mediterranean sclerophyllous species Arbutus unedo and Quercus ilex were studied under simulated habitat conditions in an environmental chamber. Temperature, humidity, and light intensity were altered stepwise to simulate diurnal changes in conditions similar to those measured in an evergreen macchia in Sobreda, Portugal. Leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of gas exchange. Increasing atmospheric stress in the form of higher temperature and lower humidity on successive days gradually results in midday depression of transpiration rate and net photosynthesis rate of leaves due to midday stomatal closure.

Midday Stomatal Closure in Mediterranean Type Sclerophylls under Simulated Habitat Conditions in an Environmental Chamber: I. Comparison of the Behaviour of Various European Mediterranean Species

Summary Well watered shrubs of ten Mediterranean sclerophyllous species were studied. under simulated habitat conditions in an environmental chamber. Temperature, air humidity and light intensity were altered stepwise to simulate a diurnal course of weather conditions similar to that measured in an evergreen macchia in Portugal on a typical cloudless summer day. Twigs with leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of CO 2 exchange and transpiration. The species investigated differed considerably in stomatal responses at midday to high air temperature and steep leaf to air vapor pressure gradient. Arbutus unedo, Laurus nobilis, Phillyrea angustifolia, Quercus coccifera, Q. ilex, and Q. suber exhibited pronounced midday closure of stomata, which resulted in a strong depression of net photosynthesis and transpiration rates, followed by reopening of stomata and higher rates of CO 2 uptake and water loss. Arbutus andrachne, Nerium oleander, Pistacia lentiscus , and Smilax aspera , on the other hand, showed only a single maximum in stomatal conductance. We conclude, that the differences may reflect species-specific sensitivities of the stomatal apparatus to atmospheric stress in the form of high temperature and low air humidity. All of the species investigated exhibited a lower leaf conductance and a lower net photosynthesis in the afternoon when compared under the same environmental conditions as in the morning. This time dependent response was independent of the absolute degree of maximal conductance and independent of whether or not the diurnal course of stomatal conductance was two-peaked. These observations indicate that it is necessary in the future to consider more carefully the effects of time dependent processes on regulation of stomatal conductance.

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

SummaryDuring 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.

Changes during summer drought in leaf CO2 uptake rates of macchia shrubs growing in Portugal: Limitations due to photosynthetic capacity, carboxylation efficiency, and stomatal conductance

The classically described stresses to which plants are subjected in mediterranean type climates are water stress and heat stress during the summer and cold stress during the winter. The general behavioral tendencies in gas exchange of leaves of shrubs subjected to these climate conditions were summarized nicely by Larcher (1961).

Water content effects on photosynthetic response of Sphagnum mosses from the foothills of the Philip Smith Mountains, Alaska

SummaryIn tussock tundra areas of the foothills north of the Brooks Range, Alaska, up to two-thirds of annual precipitation may occur during intermittent summer thunderstorms. The seasonal pattern in capitulum water content of Sphagnum spp. depends on the frequency and duration of these precipitation events, on the microtopography of the habitat including depth of thaw, and on morphological characteristics of the individual species. The response of net photosynthesis to varying water content in Sphagnum squarrosum and S. angustifolium growing under willow canopies in a tussock tundra area near the Dalton Highway on the North Slope of Alaska was examined in the field. After a period in June required to develop photosynthetic capability, capitula water content was essentially optimal for photosynthesis in the range from 6 to 10 g H2O/g DW. Above this range, the rate of CO2 uptake was reduced, presumably due to limitations on CO2 diffusion to the photosynthetically active sites. At water contents below the optimum, net photosynthesis fell rapidly until reaching compensation at approximately 1 g H2O/g DW. Dependent on changes in weather conditions, average water content of Sphagnum samples collected in the field occasionally fell below 5 g H2O/g DW. During a particularly dry period, water content of individual Sphagnum hummocks fell below 1 g H2O/g DW, indicating that water stress does limit Sphagnum photosynthetic production in this habitat.