Higher Rates Of Net Photosynthesis Research Articles

Carbohydrate metabolism in one‐ and two‐year‐old spruce needles, and stem carbohydrates from three months before until three months after bud break

Starch and sucrose metabolism of one‐ and two‐year‐old needles of Norway spruce (Picea abies [L.] Karst., about 30 years old) was investigated from three months before until three months after bud break at a natural site. We distinguish different metabolic states according to the extractable activities of enzymes (α‐amylase [EC 3.2.1.1], ADP‐glucose pyrophosphorylase [AGP, EC 2.7.7.27], D‐enzyme [EC 2.4.1.25], starch phosphorylase [STP. EC 2.4.1.1]), sucrose phosphate synthase [SPS, EC 2.4.1.14], sucrose syntbase [SS, EC 2.4.1.13]. acid invertase [AI, EC 3.2.1.261) and pool sizes of related metabolites (starch, glucose, fructose, sucrose, raffinose, stachyose, fructose 6‐phosphate [F6P], glucose 6‐phosphate [G6P], fructose 2,6‐bisphosphate [F26BP], and inorganic phosphate [P1]). The period ending with bud break was characterized by high rates of net photosynthesis, a pronounced decrease in the amount of soluble sugars, and a steep rise in starch (from the detection limit to approximately 600 nmol glycosyl units [mg dry weight]‐1). In parallel, the extractable activity of AGP increased, while D‐enzyme was on a relative high level when compared with the period after bud break. With respect to sucrose metabolism, F26BP, an inhibitor of sucrose synthesis, decreased from 1 to 0.4 pmol (mg dry weight)‐1. This was complemented by SPS activity, which was due to both increased protein levels shown by immunoblotting and activation under metabolite control (high levels of G6P and a low Pi/G6P ratio). This indicates a high capacity of synthesis of starch and sucrose in the period before bud break. These observations are in accordance with estimates of photosynthetic carbon gain, which indicate that in early spring large amounts of carbon from current photosynthesis are exported out of the needles. In addition, the content of nonstructural carbohydrates (expressed as hexoses) increased in the bark of the stem. This could also be a consequence of an enhanced carbon export from the needles.After the onset of bud break, starch concentration decreased in all tissues under investigation. In contrast, the level of total nonstructural carbohydrates in the outermost sapwood nearly doubled from bud break until the end of sampling. In the needles, net photosynthesis was reduced by about 75% and a decrease in SPS activity and protein level were found together with lower G6P concentration, and an increased Pi/G6P ratio. These results suggest that during that period sucrose synthesis was reduced in the older needles. In addition, under conditions of reduced photosynthesis, carbon demand of current year needles was in part ensured by the mobilization of starch in the older needles.Taken together our data show that before bud break carbon metabolism of mature leaves is related with the sink demands of storage organs. After bud break the accumulated assimilate pools in needles and stem, mainly the bark, are mobilized and support carbon supply to new tissues.

Dendroecological and Ecophysiological Analysis of Gap Environments in Mixed-Oak Understoreys of Northern Virginia

Ecophysiological, morphological and dendroecological attributes were examined in species growing in understorey and newly formed gap environments in mixed-Quercus forests of northern Virginia. Gap environments were characterized by higher photosynthetic photon flux density (PPFD) but were not different from non-gap control plots in their soil moisture, predawn leaf water potential (Ψ), leaf temperature or leaf to air vapour pressure deficit (VPD). In addition, few significant differences existed between regeneration responses at gap and control sites. Plants in gaps experienced higher rates of net photosynthesis (A) and leaf conductance to water vapour diffusion (g wv ) than control plants, although parameters did not differ between early vs late successional species. Gap plants also produced leaves with smaller leaf area and greater leaf thickness, leaf mass per area (LMA) and stomatal density than control plants. All gap species experienced accelerated height growth compared to non-gap plants, with early successional Sassafras albidum (Sassafras) and Liquidambar styraciflua (Sweetgum) exhibiting the highest rates. In contrast, average radial growth in small, understorey trees was not significantly different between habitat type, although Quercus alba (White Oak) did respond significantly to the most recent gaps. Many small trees (6-12 cm d.b.h.) were surprisingly old - up to 100 years in age. Despite severe suppression periods in these trees, they experienced release events during their lifetime, although not necessarily to the most recent gaps. This may partially explain the lack of correlation between ecophysiological and growth parameters.

The Influence of Temperature Upon Physiological Processes in Early-Flowering and Late-Flowering Strains of Thlaspi arvense L.

This study was initiated to determine net photosynthesis, respiration, and stomatal conductance for early-flowering (EF) and late-flowering (LF) strains of Thiaspi arvense L., and to determine the relative impact of temperature on these processes in the two strains. The strains have been shown to differ by a single gene. Vernalized EF and LF plants were grown under controlled environments at day temperatures of 1 50C, 200C, and 250C (1 6-h photoperiod) each, with 1 50C nights. During the period of floral initiation for the EF strain, net photosynthesis, respiration, and stomatal conductance were determined in both strains. Plants of the LF strain had higher net photosynthesis rates and lower respiration rates. Net photosynthesis was most efficient at 200C days, while respiration rates increased with increasing temperature. Stomatal conductance was greater in the EF strain, which also had larger stomata than plants of the LF strain. Generally, EF plants grew more quickly in terms of shoot dry weight than LF plants. However, at 250C, the LF plants had significantly more shoot dry weight. We suggest that differential net photosynthesis and respiration rates of the EF and LF strains of T. arvense may influence the relative amounts of carbohydrates and nitrogen available to the shoot apices and therefore affect the time to floral transition.

Influence of climate and fertilization on net photosynthesis of mature slash pine.

Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only a small decrease in rates of net photosynthesis from late summer through winter demonstrating that appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially increased leaf area and aboveground biomass, it only slightly increased the rate of net photosynthesis. Simultaneous measurements of gas exchange in fertilized and unfertilized (control) plots allowed the detection of a small, but statistically significant difference in average net photosynthesis of 0.14 micro mol m(-2) s(-1). Irradiance, and to a lesser extent air temperature, were the environmental factors that exerted the most control on net photosynthesis. The highest rates of net photosynthesis occurred between air temperatures of 25 and 35 degrees C. Because air temperatures were within this range for 46% of all daylight hours during the year, air temperature was not often a significant limitation. Soil and atmospheric water deficits had less effect on photosynthesis than irradiance or air temperature. Although the depth to the water table changed during the year from 10 to 160 cm, predawn and midday xylem pressure potentials only changed slightly throughout the year. Predawn values ranged from -0.63 to -0.88 MPa in the control plot and from -0.51 to -0.87 MPa in the fertilized plot and were not correlated with water table depth. There was no correlation between xylem pressure potentials and net photosynthesis, presumably because water uptake was adequate. Although vapor pressure deficits reached 3.5 kPa during the summer, they had little effect on net photosynthesis. Over a vapor pressure deficit range from 1.0 to 3.0 kPa, net photosynthesis only decreased 21%. No differences in responses to these environmental factors could be attributed to fertilization.

The influences of temperature and soil water on growth, photosynthesis, and nitrogen fixation of red alder (Alnusrubra) seedlings

A 3 × 2 factorial experiment was conducted to investigate the interaction of temperature and soil water status on the growth, photosynthetic, transpiration, and nitrogen fixation rates of 2-month-old red alder (Alnusrubra Bong.) seedlings. Three day: night temperature treatments, 15:10 °C, 20:10 °C, and 25:10 °C were used. Two soil-water treatments kept pots between 85 and 100% of field capacity (wet) and 70–85% of field capacity (dry). Treatment effects on growth, net photosynthetic, transpiration and nitrogen fixation rates, plant water potential, and foliar nutrient concentration were measured over a 9-week period. The greatest seedling growth occurred at 25 °C day temperatures, while 20 and 25 °C days produced the greatest nodule growth. The allocation of biomass to roots increased with decreasing temperature. The highest rates of net photosynthesis occurred at 15 and 20 °C whereas transpiration was greatest at 25 °C. Plant water stress was greatest at 25 °C. The difference in plant water potential between the wet and dry treatments was only 0.04 MPa, which was not great enough to produce significant effects on growth or photosynthesis. Nitrogen fixation rates were highest in the wet treatment seedlings at 20 and 25 °C.

Photosynthetic CO2-use efficiency in lichens and their isolated photobionts: the possible role of a CO2-concentrating mechanism

The CO2 dependence of net CO2 assimilation was examined in a number of green algal and cyanobacterial lichens with the aim of screening for the algal/cyanobacterial CO2-concentrating mechanism (CCM) in these symbiotic organisms. For the lichens Peltigera aphthosa (L.) Willd., P. canina (L.) Willd. and P. neopolydactyla (Gyeln.) Gyeln., the photosynthetic performance was also compared between intact thalli and their respective photobionts, the green alga Coccomyxa PA, isolated from Peltigera aphthosa and the cyanobacterium Nostoc PC, isolated from Peltigera canina. More direct evidence for the operation of a CCM was obtained by monitoring the effects of the carbonic-anhydrase inhibitors acetazolamide and ethoxyzolamide on the photosynthetic CO2use efficiency of the photobionts. The results strongly indicate the operation of a CCM in all cyanobacterial lichens investigated and in cultured cells of Nostoc PC, similar to that described for free-living species of cyanobacteria. The green algal lichens were divided into two groups, one with a low and the other with a higher CO2-use efficiency, indicative of the absence of a CCM in the former. The absence of a CCM in the low-affinity lichens was related to the photobiont, because free-living cells of Coccomyxa PA also apparently lacked a CCM. As a result of the postulated CCM, cyanobacterial Peltigera lichens have higher rates of net photosynthesis at normal CO2 compared with Peltigera aphthosa. It is proposed that this increased photosynthetic capacity may result in a higher production potential, provided that photosynthesis is limited by CO2 under natural conditions.

Leaf temperature effects on net photosynthesis, dark respiration, and photorespiration of seedlings of black locust families with contrasting growth rates

Rates of net photosynthesis, dark respiration, and photorespiration of six half-sib families of black locust (Robiniapseudoacacia L.) were measured at leaf temperatures ranging from 10 to 40 °C. Rates of dark respiration increased with increasing leaf temperature in all families and reached as high as 67% of gross photosynthesis at 40 °C in one family. Dark respiration of foliage accounted for 12.5 to 59% of the reduction in net photosynthesis at temperatures higher than those optimum for net photosynthesis. Rates of photorespiration peaked at 10 to 20 °C, exhibiting the same pattern as net photosynthesis, and did not contribute to the decline in net photosynthesis at high temperatures. The families with high rates of net photosynthesis also had high rates of photorespiration. Rates of dark respiration were significantly different among the families, and the slow-growing families had the highest rates of dark respiration. A significant interaction between half-sib families and leaf temperatures was noted for dark respiration. The data indicated the possibility of improving the growth of black locust by selection and breeding for large leaf area, high rates of net photosynthesis and low rates of dark respiration.

Gas exchange during a soil drying cycle in seedlings of four black walnut (Juglans nigra L.) families.

Photosynthetic and stomatal responses to a soil drying cycle were examined in half-sib seedlings of four walnut (Juglans nigra L.) families. Well-watered seedlings of an Iowa seed source had significantly higher rates of net photosynthesis than seedlings from New York or Michigan sources. This superior photosynthetic potential was associated with both greater stomatal conductance and mesophyll capacity for CO(2) fixation. In a drying soil, net photosynthesis and leaf conductance to water vapor of all families declined substantially, even under mild water stress. These responses were more strongly related to soil water status, as estimated by predawn leaf water potential, than to leaf water potential at the time of gas exchange measurement. There were no differences among families in the pattern of gas exchange response to developing water stress; however, families differed in capacity for recovery of gas exchange from water stress following rehydration. Sensitivity of photosynthesis of black walnut seedlings to water stress may be associated with poor growth and survival of this species in xeric habitats.

Resource‐Use Efficiency and Drought Tolerance In Adjacent Great Basin and Sierran Plants

Using several adjacent Great Basin desert and Sierran montane trees and shrubs, we tested the prediction that water—use efficiency (WUE) is lower in more drought—tolerant species. We also measured maximum net photosynthetic rates and photosynthetic nitrogen—use efficiency (PNUE) to determine if these attributes decrease at high WUE. Differences in the physiological responses to resource availability were more clearly evident between growth forms, trees vs. shrubs, than between vegetation types, montane vs. desert. The shrubs (Anelanchier alnifolia, Arctostaphylos patula, and Artemisia tridentata) maintained higher rates of net photosynthesis at lower predawn water potential but had lower WUE than the trees (Juniperus osteosperma, Pinus monophylla, P. jeffreyi, and P. ponderosa). PNUE was highest in the shrubs that also had the highest foliage N content, and PNUE was inversely related to WUE. However, trees had higher nutrient retranslocation efficiency and growth—based NUE than shrubs. Low WUE and high drought tolerance of shrubs suggests that maintaining high WUE under competitive water—limited conditions may not be advantageous. In contrast, conservative use of water (high WUE) and high growth—based NUE by trees lengthens the season of net carbon assimilation and more closely matches growth with low rates of nutrient availability on nutrient—poor sites.

Mikroklima, Wassergehalt und Photosynthese von Flechten in der küstennahen Nebelzone der Namib-Wüste: Messungen während der herbstlichen Witterungsperiode

The coastal fog zone of the Namib Desert (Namibia, Southwest Africa) where there are hardly any higher plants, is known for its rich lichen vegetation. The functional mechanisms that allow existence of these lichens under the special conditions of the “fog desert” are investigated. The study is part of our general efforts to analyse the ecological basis of the photosynthetic production of lichen-dominated habitats in order to explain physiological factors underlying the success of the lichens. During the fall, in April 1988, microclimate, water relations, and photosynthesis of 10 characteristic species were studied in their natural environment in one of the lichen fields north of Swakopmund. Time courses of CO2 exchange and water content were used to estimate the daily primary production and provide information about light dependence of CO2-assimilation of well-hydrated thalli, and about their light (LKP) and moisture compensation point (FKP). Fog is the most important source of water for these lichens. However, dew condensation without fog also results in high degrees of hydration. Maximal water contents of more than 150% of dry weight of the thalli are reached. Soon after sunrise, the hydrated lichens show a steep increase in the rate of net photosynthesis which initially is determined by the increase of photosynthetically active radiation. Subsequently, the lichens lose water, and metabolic activity is then limited by their hydration. Typically, photosynthesis of the drying lichens ceases 2 to 4 hours after sunrise and the water content of the thalli drops below 10%. In addition to hydration through liquid water, water vapor uptake in air of high humidity alone can reactivate the photosynthetic apparatus of the lichens. This may result in a second, smaller peak in CO2 uptake at late afternoon. This general diurnal pattern of photosynthetic activity is modified according to the actual weather conditions. There occurred one day during the measuring period where no CO2 was gained. The characteristics of a species, especially its growth form, determine maximal rate of photosynthesis and the ability of the lichen to make use of the available moisture. The multibranched fruticose species (e.g., Teloschistes capensis, Alectoria spec., Ramalina lacera) reach higher rates of net photosynthesis than the more compact foliose lichens (e.g., Xanthomaculina convoluta, X. hottentotta, Xanthoparmelia walteri), both their maximal apparent quantum use efficiency (initial slope of the light response curve) and their maximal photosynthetic rates at light saturation being greater. Light saturation of the whole thalli in their natural position takes place between 583 (Xanthomaculina hottentotta) and 1856 μE m−2 S−1 (Ramalina lacera) photosynthetically active radiation. LKP varies between 16.3 (Caloplaca elegantissima) and 31.5 μE m−2 S−1 (Xanthomaculina hottentotta), FKP after desiccation in the morning lies between 15.0 (Santessonia hereroensis) and 25.6% (Ramalina lacera) water content relative to dry weight of the thallus. Caloplaca elegantissima, the only crustose species studied which grows on stone pebbles, is characterized by medium rates of photosynthesis when related to dry weight or carbon content of the thallus and by high rates when related to chlorophyll content. Its LKP is very low. In the most productive species Teloschistes capensis, the maximal daily carbon gain during the daylight period amounts to about 0.25% of the thallus carbon content. This is ten times the relative carbon gain of the Xanthomaculina species under the same microc1imatic conditions. The results show that primary production of the cushion forming, fruticose growth form of the lichens is superior in the habitat with heavy fog formation near the coast. This accords with the fact that Teloschistes capensis is the dominant species in these lichen fields. Continuation of the work will be extended over other seasons of the year and will concentrate further on the analysis of the different growth forms as adaptations to the extreme environment. Field investigations will be complemented by measurements of the factor dependency of CO2 exchange under controlled conditions in the laboratory.

PATTERNS IN LEAF MORPHOLOGY AND PHOTOSYNTHESIS IN SHOOTS OF SASSAFRAS ALBIDUM (LAURACEAE)

Trees of Sassafras albidum (Nuttall) Nees. display leaves that fall into discrete categories of form. Unlobed leaves, with undissected margins, predominate at proximal and distal nodes of shoots, while lobed leaves are most common at intermediate nodes. In this study we investigated the hypothesis that shoots of sassafras respond to environmental changes over the growing season by generating predictable nodal patterns for leaf morphology and physiology. We recorded leaf shape category and measured leaf surface area, node number, chlorophyll content, nitrogen content, and photosynthesis in four trees. Over 1,000 measurements of photosynthesis were made in situ, using the LI‐Cor LI 6200 portable photosynthesis system. The two trees growing under heavy shade had few lobed leaves (2.0% and 18.8%) and often had negative carbon balance, with positive net photosynthesis occurring during sun flecks. The two trees growing in more open conditions had many more lobed leaves (56.3% and 61.0%) and higher, less variable net photosynthetic rates. As indicated by chlorophyll and nitrogen contents, the highest leaf photosynthetic rates occurred at intermediate nodes (nodes six to nine), and this was shifted distally along the shoot during the growing season. Leaves at intermediate nodes also had the largest surface areas and the greatest frequency of lobing. In comparative experiments with models, lobing was shown to enhance free‐convectional heat loss relative to unlobed leaves of the same surface area. Due to their large surface area, these leaves also have the highest rates of whole leaf photosynthesis. Under conditions of equivalent photosynthetically active radiation, lobed leaves had higher rates of net photosynthesis than did unlobed leaves. We conclude that shoots of sassafras produce groups of leaves with predictably different morphological and physiological specializations.

Bryophyte phytomass in tropical ecosystems

Knowledge about bryophyte production and phytomass in the tropics is poor and has been derived principally from studies of epiphytic bryophytes. Such work has been undertaken in Transvaal, Tanzania, Venezuela, Peru and Borneo. Except for the studies in Peru and Borneo, the vegetation types studied and the methods used have been too variable to allow many comparisons or generalizations to be made. However, in general it can be stated that phytomass of epiphytic bryophytes in equatorial latitudes increases from the tropical lowlands to the foreslline. This striking phenomenon has been hypothetically explained by gradients of factors such as precipitation, humidity, temperature and desiccation, or combinations of these. Recent experiments on the gas exchange of tropical-montane bryophytes have revealed that the rate of net assimilation decreases dramatically above 25oC and that high temperature combined with low light intensities, as realized in the tropical lowland forest, results in high rates of respiration and insufficient net photosynthesis. Experiments with temperate bryophytes show the same results. If can be assumed that tropical lowland species have special physiological adaptations to survive these unfavourable conditions, such as a specialized phytochrome system or effective storage of photosynthetic products. Additional factors could include the relatively high nutrient supply provided by abundant precipitation in tropical montane forests, and damage to cells by strong desiccation in the tropical lowlands.

Photosynthetic Responses of C 3 and C 4 Wetland Species in a Tropical Swamp

(1) A tropical swamp near Nairobi, Kenya containing monotypic stands of the C4 sedges Cyperuspapyrus, Cyperus latifolius and the C3 species Typha domingensis was investigated. (2) Aerial biomass of C. papyrus was 2050 g m-2, C. latifolius 2170 g m-2 and T. domingensis 1350 g m-2. The LAI of C. latifolius (15 3) was three times that of T. domingensis. The Cyperus canopies intercepted more than 95% and the Typha canopy more than 85% of incident photosynthetically active radiation. (3) Photosynthetic light response curves were constructed for the photosynthetic bracteoles of C. papyrus and leaves of C. latifolius and T. domingensis. Maximum rates of net photosynthesis were 26 8 ,umol m-2 s-' for C. papyrus, 21 8 ,umol m-2 s-' for C. latifolius and 17 8 ,umol m-2 s-' for T. domingensis. The apparent quantum yield of net photosynthesis from incident light was higher in C. papyrus than the other two species. (4) The relationship between net photosynthesis and stomatal conductance was almost linear for all species but the C4 plants achieved their highest rates of net photosynthesis at lower conductances than the C3 T. domingensis. (5) Both photosynthetic water-use efficiency and nitrogen-use efficiency in the C4 sedges were more than double the values for T. domingensis. (6) The adaptive advantage of C4 photosynthesis in wetlands is discussed and it is suggested that the higher nitrogen-use efficiency of the C4 C. papyrus may explain why it is more tolerant of oligotrophic conditions than other (mainly) C3 species.

COMPARATIVE PATTERNS OF NET PHOTOSYNTHESIS IN AN ASSEMBLAGE OF MOSSES WITH CONTRASTING MICRODISTRIBUTIONS

Effects of plant water content, previous desiccation, photon flux density, and temperature on rate of net photosynthesis were measured in the common species in the assemblage of desiccationtolerant mosses that dominates the flora on granitic boulders at a semiarid site in the inland chaparral of San Diego County, California. Species differed significantly in their responses to water content, desiccation, and light. Species previously found to occur in microsites of lower water availability attained maximum net photosynthesis at lower water contents and showed greater ability to recover from prolonged desiccation. Species found in microsites of lower light availability had higher rates of net photosynthesis at low photon flux density. Results suggest that differences in potential rates of net photosynthesis partly determine relative ability of these species to colonize xeric or dark microsites, but that other factors also determine microdistribution.

Diurnal and seasonal patterns of net photosynthesis by irrigated Chrysothamnus nauseosus under field conditions

Chrysothamnus nauseosus ssp. albicaulis (white rubber rabbitbrush) is a common deciduous C3 desert shrub which is a potential source of natural rubber and which exhibits high rates of net photosynthesis (Pn) for a woody species. When maintained under irrigated conditions in the field, rates of Pn on cloudless days were generally found to be highest during the cool morning hours (peak rates observed between about 0900 and 1000 hrs). Pn declined during mid-day hours and was approximately 75% of that observed in the early morning for the remainder of the light period. This decline in Pn was accompanied by about 26 μmol CO2.m(-2).s(-1) in mid-May (approximately 30 days after bud break) to about 48 μmol CO2.m(-2).s(-1) in early August. Thereafter, Pn declined gradually until leaf fall in November. These data indicate that rabbitbrush is capable of relatively high rates of Pn throughout a substantial portion of the growing season when maintained under irrigated conditions. This may, at least in part, account for the relatively high rates of growth observed under conditions of ample soil moisture.

Variation in Amounts of Pyruvate, Orthophosphate Dikinase, and Some Other Enzymes of the C4 Pathway in Some Wheat Species

First leaves and flag leaves of the wheat species Triticum aestivum cv Anza (6x), T. boeoticum Boiss (2x) L. were examined for content of pyruvate, orthophosphate dikinase (PPDK), phosphoenolpyruvate carboxylase (PEPC), and ribulose 1,5-bisphosphate carboxylase (RuBPC) by protein blot analyses using antibodies to maize leaf enzymes and by activity assays. In agreement with previous reports, the amount of RuBPC per mesophyll cell was about 3 times more in the hexaploid species, T. aestivum, than in the diploid species, T. boeoticum, both in first leaves and in flag leaves. In contrast, the level of PPDK polypeptide was nearly 3-fold higher per unit leaf area in the first leaf and 63% higher in the flag leaf of this diploid species compared to this hexaploid species. There was no significant difference in the levels of polypeptide and enzyme activity of PEPC between diploid and hexaploid wheat. Despite this significantly greater level of PPDK in the diploid species, the actual amount of PPDK could still supply only a limited amount of the enzyme activity necessary to provide phosphoenolpyruvate (PEP) for any putative intracellular C(4) carbon shuttle providing carbon to RuBPC. Thus, this difference in enzyme amount could not by itself account for the reported high rates of net photosynthesis at high light intensity in T. boeoticum. Together with reported anatomical differences between the diploid and hexaploid species, however, this biochemical difference may be of physiological importance.

Whole Plant Photosynthesis and Dry-matter Partitioning in Fruiting and Deblossomed Day-neutral Strawberry Plants

Abstract On a whole-plant basis, deblossomed ‘Tribute’ strawberry plants had higher rates of net photosynthesis (Pn) and greater leaf area than fruiting plants during the last 3 weeks of a 6-week fruiting cycle. Leaves of both fruiting and deblossomed plants had a greater dry weight and total nonstructural carbohydrate (TNSC) content than the roots, crown, or fruit. At the end of the fruiting cycle, dry weight and TNSC content of leaves were higher for deblossomed plants than fruiting plants. There was no difference in crown or root dry weights between treatments. Pn was highly correlated with plant dry weight and TNSC during the 3rd, 5th, and 6th weeks of the fruiting cycle.

Responses of C4 Grasses to Atmospheric CO2 Enrichment. II. Effect of Water Stress1

Changes in leaf water potential, photosynthesis, and stomatal conductance in response to atmospheric COs enrichment at two watering regimes were studied in :Four species of C4 grasses: Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350 and 675 μmol mol‐1 CO2 with 1000 μmol m‐2 s‐1 photosynthetic photon tux density (PPFD). One group plants of each of the COs concentrations was subjected to water stress by withholding irrigation for 10 days starting 21 days after planting. Water potentials of the leaves of plants grown at 350 #tool mol‐1 CO2 declined more rapidly and reached lower values at the end of the stress period than the water potential of those grown at 675 μmol mol‐1 CO2. High COs‐grown plants maintained higher turgor pressures than low CO2‐grown plants. All plants had lower rates of photosynthesis during water stress compared to non‐stressed conditions. However, the net photosynthetic rates of the plants grown at high CO2 concentration did not decline as rapidly as the rates of plants grown at low CO2 when subjected to water stress. The maintenance of turgor along with CO2 enrichment during water stress development in plant leaves may be responsible for the higher rates of net photosynthesis observed in high CO2‐grown plants compared to those in low CO2‐grown plants.

Reaktivierung der Photosynthese trockener Flechten durch Wasserdampfaufnahme aus dem Luftraum: Artspezifisch unterschiedliches Verhalten

Summary It is known in principle, that dry thalli of lichens are able to take up water vapor from air of high relative humidity and that this can stimulate CO2 exchange so that high rates of net photosynthesis are reached without exposing the lichens to liquid water. The present investigation compars different types of lichens from different climatic regions with respect to their ability to be activated by water vapor uptake. Thalli of dry lichens (kept in a desiccator with silicagel) were enclosed in gas exchange chambers and treated for 50 to 100 h with air of a relative humidity of 96.5% at 15°C (corresponding to a water potential of ca. —45 bar); subsequently they were sprayed with liquid water. During the course of the experiment, their rate of CO2 exchange in the light (150 μE m−2 s−1 photosynthetically active radiation) and dark respiration was monitored. With all of the heteromerous lichen species with green algae, water vapor uptake leads to reactivation of metabolism. Photosynthetic CO2-assimilation as well as mitochondrial respiration, increases during the course of treatment with moist air and reaches an equilibrium value when water potential of the thallus has equilibrated with that of the surrounding air. However, there are essential differences in the length of time required for reactivation of different species. Several fruticose and folious lichen species (e.g., Parmelia hypoleucina, Thamnolia vermicularis) reach 50% of maximum net photosynthesis already after less than 5 h treatment of the initially dry thalli with moist air. Alpine and Antarctic Umbilicaria-species need 15 to 16 h, and the Antarctic Caloplaca regalis requires approximately 34 h. Habitat adaptations are clearly recognizable. This is apparent for instance for species from a Chilean fog oasis, which depend almost entirely on air humidity as a source for water. There are also distinct differences between lichen types in the response to moistening with liquid water. With some species, spraying with water after water vapor uptake results in an increase of net photosynthesis. With other species there is often a decrease. In extreme cases, CO2 exchange decreases below the compensation point in the light, which appears to be due to a strong and reversible stimulation of mitochondrial respiration by liquid water. The gelatinous lichens are of exceptional interest. None of the investigated Collemataceae (2 Collema, 1 Leptogium species) is able to make use of water vapor for attain positive net photosynthesis rates. With these lichens, photosynthetic primary production depends solely on hydration with liquid water. This must strongly limit the distribution of these species. The results indicate that the gelatinous species must represent an independent life form among lichens. Differences in anatomical and morphological structure of their thallus appear also to be associated with differences in function which profoundly alters the way in which photosynthetic metabolism responds to available water in the environment.

Responses of cowpea (Vigna unguiculata (L.) Walp.) and mung bean (Vigna radiata (L.) Wilczek) to irrigation

Cowpea and mung bean were subjected to three irrigation schedules during summer dry months (May–June). In general, cowpea had higher rates of net photosynthesis (Pn,Figs. 1, 2), dark respiration (Rd,Table 1), absorption of photosynthetically active radiation (PAR, Table 2), and growth (Fig. 7) than mung bean. Mung bean reflected more PAR and maintained a slightly cooler canopy than cowpea (Table 2). Moisture stress decreased Pn, Rd,absorption coefficient of PAR, evaporative cooling and growth in both the species. However, the reductions in Pnrates of stressed leaves were more than the decreases in Rd.Restoration of water supply to stressed cowpea resulted in a more rapid recovery of growth as compared to mung bean.