Photosynthetic Decline Research Articles

Low stomatal and internal conductance to CO2 versus Rubisco deactivation as determinants of the photosynthetic decline of ageing evergreen leaves

A novel A-Ci curve (net CO2 assimilation rate of a leaf -An- as a function of its intercellular CO2 concentration -Ci) analysis method (Plant, Cell & Environment 27, 137-153, 2004) was used to estimate the CO2 transfer conductance (gi) and the maximal carboxylation (Vcmax) and electron transport (Jmax) potentials of ageing, non-senescing Pseudotsuga menziesii leaves in relation to their nitrogen (N) content and protein and pigment composition. Both gi and the stomatal conductance (gsc) of leaves were closely coupled to Vcmax, Jmax and An with all variables decreasing with increasing leaf age. Consequently, both Ci and Cc (chloroplastic CO2 concentration) remained largely conserved through successive growing seasons. The N content of leaves, as well as the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other sodium dodecyl sulfate-soluble proteins, increased during the first three growing seasons, then stabilized or decreased only slightly afterwards. Thus, the age-related photosynthetic nitrogen use efficiency (PNUE) decline of leaves was not a consequence of decreased allocation of N towards Rubisco and other proteins involved in bioenergetics and light harvesting. Rather, loss of photosynthetic capacity was the result of the decreased activation state of Rubisco and proportional down-regulation of electron transport towards the photosynthetic carbon reduction (PCR) and photorespiratory (PCO) cycles in response to a reduction of CO2 supply to the chloroplasts' stroma. This study emphasizes the regulatory potential and homeostaticity of Cc- rather than photosynthetic metabolites or Ci- in relation to the commonly observed correlation between photosynthesis and gsc.

Photosynthetic decline in flag leaves of two field-grown spring wheat cultivars with different senescence properties

Photosynthetic characteristics were compared between two field-grown spring wheat ( Triticum aestivum L.) cultivars, Ningmai 8 (NM8) and its half-father sibling Ningmai 9 (NM9), to investigate the differences in photosynthetic decline of flag leaves between these two cultivars with different senescent appearance after emergence through senescence. Maximum photosynthetic activities for these two cultivars were observed at around 10 days after emergence (DAE). Photosynthetic functionally, therefore, 10 DAE was taken to define the point at which leaf senescence was initiated. During the photosynthetic rapid decline occurring after 27 DAE, as compared with NM8, NM9 showed a significantly higher Chl content, a higher Chl a/ b ratio, light-saturated photosynthetic rate ( P Ca = 360 ) and light- and CO 2-saturated photosynthetic rate ( P max), carboxylation efficiency (CE), but a significantly lower dark respiration rate ( R D) and CO 2 compensation point ( C comp). Thylakoid membranes in NM9 also showed a slightly higher electron transport activity of PS II, but similar absorption spectra properties measured at room temperature as compared to NM8. The slower photosynthetic decline may contribute some to its higher grain yield in NM9. Due to the slower decrease of Chl, CE and P Ca = 360 , and the lower R D in NM9, flag leaf senescence was delayed in comparison to the earlier senescent NM8. Compared with its half-father sibling NM8, NM9 had a later onset and a slower rate of flag leaf senescence, which may be partly responsible for its higher grain yield.

Native Mediterranean Pyrus Rootstock, P. amygdaliformis and P. elaeagrifolia, Present Higher Tolerance to Salinity Stress Compared with Asian Natives

The salt tolerance of native Asian pear species, Pyrus betulaefolia Bunge., P. pyrifolia Nakai, and P. xerophila Yu, and native Mediterranean ones, P. amygdaliformis Vill. and P. elaeagrifolia Pall., were examined by irrigating them with 75 mM and 150 mM NaCl solutions for 30 days. Native Mediterranean species did not develop leaf injury during the 30-day NaCl treatments, but native Asian species developed leaf injury. Concerning the shank, fine roots, and whole plant, bioregional differences in Na and Cl contents were small. However, the Na and Cl contents of the leaves of native Mediterranean species were markedly less than those of native Asian species in both NaCl concentrations. Therefore, the native Mediterranean P. amygdaliformis and P. elaeagrifolia may have some salt exclusion mechanisms in the shank which restrict Na and Cl transport to the leaves. The stem water potential and Ci/Ca ratios were decreased by NaCl treatments in native Mediterranean and native Asian species. Thus, the photosynthetic decline by NaCl treatment observed in these species might have been caused by stomatal closure, which is likely to be induced by decreased water potential in the plant body. However, the photosynthetic rate of native Mediterranean species under NaCl stress was higher than native Asian species. Therefore, low concentrations of Na and Cl in the leaves of native Mediterranean species might prevent crucial decreases of photosynthetic rates under NaCl stress. The present results imply that native Mediterranean P. amygdaliformis and P. elaeagrifolia have a higher salt tolerance than native Asian P. betulaefolia, P. pyrifolia and P. xerophila. These native Mediterranean species would be useful in providing genetic resources as salt-tolerant rootstock for the Japanese pear.

Effects of Hydroperiod and Iron on Typha latifolia Grown in a Phosphorus-Enhanced Medium

This study was designed to mimic a phosphorus-(P)-enhanced environment and focused on the effects of hydroperiod and dissolved iron (Fe) concentrations on Typha latifolia. The physiological responses and nutrient uptake capabilities of plants were quantified. The research was conducted in a greenhouse utilizing a factorial design with three soil-moisture treatments (permanently flooded, periodically flooded, drained) and three Fe levels (0 mg Fe L − 1, control; 1 mg Fe L − 1; 10 mg Fe L − 1). All treatments also received P at 0.2 mg l − 1. Plant gas exchange, growth, biomass, and tissue Fe and P concentrations were measured. Permanent flooding enhanced height growth, shoot weight, and root weight, and led to high Fe concentrations in the roots. Plants receiving the 10 mg Fe L −1 treatment had taller shoot heights compared with those receiving the control and 1 mg Fe L −1 treatments. It appeared that a concentration of 10 mg Fe L − 1 did not significantly affect T. latifolia under moderately reduced soil conditions. However, low soil-redox potential levels below +70 mV and iron levels of 10 mg L − 1 may eventually lead to a photosynthetic decline in T. latifolia. Results highlighted two important findings: (1) Fe treatment and moisture regimes, as tested, did not affect tissue P concentrations and thus the ability to uptake and sequester P, and (2) a treatment of 10 mg Fe L − 1 may have led to some initial beneficial effects in T. latifolia, such as producing the greatest shoot growth, but a photosynthetic decline was noted in the last week as redox potential levels fell below +100 mV. This effect potentially hinders the use of Fe to aid P retention in wetlands, due to the possibility of inducing physiological dysfunction in some species, such as noted in T. latifolia.

Acidic mist reduces foliar membrane-associated calcium and impairs stomatal responsiveness in red spruce

Acidic deposition can leach essential pools of calcium (Ca) directly from plant foliage. Because of the central role of Ca in environmental signal transduction, disruptions of labile foliar Ca pools could impair physiological responses to a variety of environmental stimuli and stressors. We investigated the possibility that acidic mist-induced depletion of membrane-associated Ca (mCa), which is one form of labile Ca, may alter stomatal responsiveness to water stress, a process known to include Ca in signal transduction cascades. Red spruce (Picea rubens Sarg.) seedlings were exposed to either pH 3.0 or pH 5.0 mist treatments for one growing season. Foliar nutrition was assessed following treatments, and declines in stomatal conductance and net photosynthesis were measured on current-year shoots following stem excision. Seedlings exposed to pH 3.0 acidic mist treatments had reduced mCa relative to the pH 5.0 treated seedlings. Seedlings subjected to the pH 3.0 acidic mist treatment exhibited impaired stomatal functions, including a smaller maximum aperture, slower closure and an increased lag time between stomatal closure and photosynthetic decline following experimental water stress. Delayed stomatal closure could undermine desiccation avoidance mechanisms. Previous work has demonstrated that acidic mist treatments deplete mCa in red spruce and impair cold tolerance, with similar effects in other species. The results we present provide further evidence that acidic mist-induced mCa depletion may cause disruption of a broad range of plant stress responses.

Photosynthesis and Nutritive Value in Leaves of Three Warm‐Season Grasses before and after Defoliation

Forage yields are influenced by plant response to defoliation. We examined the photosynthesis and nutritive value of first (first leaves) and third (third leaves) fully expanded leaves (numbered from the apex) in three warm‐season (C4) grasses. Net photosynthetic rates at uniform temperature and light both before and after a 2‐wk exposure to full sunlight and the effect of leaf position on nutritive value were determined on vegetative tillers in well‐established swards of bermudagrass [Cynodon dactylon L. (Per.) cv. Tifton 44], caucasian bluestem [Bothriochloa caucasica (Trin.) C.E. Hubb.], and Atlantic coastal panicgrass [Panicum amarum var. amarulum (Hitchcock and Chase) P.G. Palmer] growing on a Cecil clay loam (clayey, kaolinitic, thermic Typic Hapludults). Bermudagrass had the greatest level of crude protein (CP), followed by panicgrass and bluestem. Fiber was greater in the first leaves than in the third leaves for bermudagrass and panicgrass but not for bluestem. Photosynthetic rates of panicgrass and bluestem third leaves estimated 2 wk after defoliation of the surrounding canopy were less than estimates made before defoliation in the first leaves but were similar to the third leaves before canopy defoliation. The third leaves of bermudagrass 2 wk after defoliation had lesser photosynthetic rates per unit chlorophyll than the first or third leaves before defoliation. Photosynthetic rates were correlated with hemicellulose across leaf classes and species (r2 = 0.93). The photosynthetic decline observed in third leaves of bermudagrass compared with panicgrass and bluestem is evidence of variation in leaf response after defoliation among warm‐season grasses.

Physiological responses of black willow (Salix nigra) cuttings to a range of soil moisture regimes

We quantified the physiological responses of black willow to four soil moisture regimes: no flooding (control, C), continuous flooding (CF), periodic flooding (PF), and periodic drought (PD). Stomatal limitation was one of the factors that led to the reduced photosynthetic capacity in CF cuttings. Under PD, stomatal closure, decreased leaf chlorophyll content, and increased dark fluorescence yield contributed to photosynthetic decline. CF cuttings accumulated the lowest shoot biomass while the final height and root growth were most adversely affected by PD. PF cuttings tended to allocate more photoassimilates to root growth than to shoots.

Antioxidant Levels Decrease in Primary Leaves of Barley during Growth at Ambient and Elevated Carbon Dioxide Levels

Previous studies revealed that the primary leaves of barley (Hordeum vulgare L. cv. Brant) that were grown in high‐light growth chambers in elevated CO2 (100 Pa) developed symptoms of senescence earlier than primary leaves of barley plants concurrently grown in ambient CO2 (36 Pa). The onset of senescence in these leaves was hypothesized to be associated with a decline in antioxidant capacity that resulted in higher levels of active oxygen species, which caused oxidative stress. In this new study, barley was grown from planting in high‐light growth chambers supplied with either ambient CO2 (36 Pa) or elevated CO2 (100 Pa). From 9 to 17 d after planting (DAP), primary leaf chlorophyll level as well as photosynthetic oxygen evolution per unit fresh mass decreased in plants in elevated CO2 to a greater extent than in plants in ambient CO2. Exposure of plants to elevated CO2 resulted in higher levels of primary leaf–free glucose, sucrose, and starch than in plants concurrently exposed to ambient CO2; this was most evident during 9–13 DAP. However, between 13 and 17 DAP, levels of foliar carbohydrates decreased to a greater extent in elevated CO2 than in ambient CO2, indicating that photosynthetic carbon metabolism was more repressed in this period in elevated CO2 treatments. In ambient CO2 over the period of 9–17 DAP, primary leaf total ascorbic acid (ASCtot) and total glutathione (GSHtot) levels decreased 38% and 51%, respectively. In elevated CO2 over this same time period, ASCtot and GSHtot decreased 45% and 63%, respectively. Since ASCtot and GSHtot are antioxidants involved in protecting the chloroplast from damage by active oxygen species such as hydrogen peroxide, we conclude that the decrease in ASCtot and GSHtot were factors in elevated CO2‐induced photosynthetic decline in barley primary leaves. Further, we interpret the greater decrease in ASCtot in elevated CO2 to be caused by a greater decrease in photosynthate partitioning and photosynthate availability, which supports ASC synthesis. We also suggest that decrease in glutathione in elevated CO2 may be related to an inhibition by elevated CO2 of photorespiratory carbon metabolism.

Gas Exchange and Co-regulation of Photochemical and Nonphotochemical Quenching in Bean during Chilling at Ambient and Elevated Carbon Dioxide

The effects of elevated (700 micromol mol(-1)) and ambient (350 micromol mol(-1)) CO(2) on gas exchange parameters and chlorophyll fluorescence were measured on bean (Phaseolus vulgaris) during 24 h chilling treatments at 6.5 degrees C. Consistent with previous research on this cultivar, photosynthetic decline during chilling was not significantly affected by CO(2) while post-chilling recovery was more rapid at elevated compared to ambient CO(2). Our primary focus was whether there were also CO(2)-mediated differences in demand on nonphotochemical quenching (NPQ) processes during the chilling treatments. We found that photosystem II quantum yield and total NPQ were similar between the CO(2) treatments during chilling. In both CO(2) treatments, chilling caused a shift from total NPQ largely composed of q(E), the protective, rapidly responding component of NPQ, to total NPQ dominated by the more slowly relaxing q(I), related to both protective and damage processes. The switch from q(E) to q(I) during chilling was more pronounced in the elevated CO(2) plants. Using complementary plots of the quantum yields of photochemistry and NPQ we demonstrate that, despite CO(2) effects on the partitioning of NPQ into q(E) and q(I) during chilling, total NPQ was regulated at both CO(2) levels to maximize photochemical utilization of absorbed light energy and dissipate only that fraction of light energy that was in excess of the capacity of photosynthesis. Photodamage did occur during chilling but was repaired within 3 h recovery from chilling in both CO(2) treatments.

Does foliage on the same branch compete for the same water? Experiments on Douglas-fir trees

Do branchlets within a branch have autonomous water supplies, or do they share a common water supply system? We hypothesized that if branchlets shared a common water supply, then stomatal conductance (g s) on sunlit foliage would increase with reduced transpiration of competing foliage on the branch. We reduced transpiration of other foliage on the branch through bagging and shading, and we monitored the gas-exchange responses of the remaining sunlit foliage on the branch relative to control branches for several age classes of Douglas-fir trees (aged ~10 years, 20 years, and 450 years old). Contrary to our hypothesis, we found no increases in g s in either young or old trees following transient reductions in the amount of transpiring leaf area. The diurnal change in water potential, mid-day stomatal closure and associated photosynthetic decline occurred at the same time and were of the same magnitude on both treated and untreated branches, with the exception of photosynthesis in one 450-year-old tree. Hydraulic conductance measurements of branch junctions indicate that xylem within branches is only partially interconnected which would reduce the effectiveness of shading as a means of increasing water supply to the remaining sunlit foliage. The lack of a response implies that when a branch is in partial shade, the remaining sunlit foliage has no advantage with respect to water status over foliage on a branch completely in the sun.

Leaf senescence-like characteristics contribute to cotton's premature photosynthetic decline.

Leaf and canopy photosynthesis of cotton (Gossypium hirsutum L.) declines as the crop approaches cutout, just as the assimilate needs for reproductive growth are peaking. Our objective with this study was to determine whether this decline is due to remobilization of leaf components to support the reproductive growth or due to some cue from the changing environmental conditions during the growing season. Field studies were conducted in 1995-1996 at Stoneville, Mississippi, using six cotton genotypes and two planting dates (early and late), which produced two distinctly different cotton populations reaching cutout at different times. Among the six genotypes were a photoperiod sensitive line (non-flowering) and its counter part which had photoperiod insensitive genes backcrossed four times to the photoperiod sensitive line (flowering). This pair was used to assess the degree that the photosynthetic decline could be attributed to reproductive sink development. Leaf CO(2)-exchange rate (CER) and chlorophyll (Chl) fluorescence measurements were taken in mid-August, a period corresponding to cutout for the early planted plots, and those leaves were collected. Leaf Chl level, soluble protein level, various soluble carbohydrate levels and Rubisco activities were assayed on those leaves. Averaged across years, leaf CER and soluble protein levels were reduced approximately 14% and 18%, respectively, for the early planted compared to the late planted cotton. Neither leaf Chl levels or Chl fluorescence Fv/Fm values for Photosystem II yield were altered by the planting date. In 1996, leaves from the non-flowering line had 12% greater Chl and 20% greater soluble protein levels than the flowering line. However, in 1996, the CER of the early planted non-flowering line was reduced 10% compared to the late planted. Although remobilization of leaf N to reproductive growth appears to be the principle component causing the cutout photosynthetic decline, the data also indicate that environmental factors can play a small role in causing the decline.

Evidence that Premature Senescence Affects Photosynthetic Decline of Wheat Flag Leaves during Growth in Elevated Carbon Dioxide

Net CO2 exchange (NCE) rates and various flag leaf constituents were measured in wheat plants (Triticum aestivum L.) grown in field chambers at either 350 or 700 p.L L-' CO2. Rates of NCE decreased with leaf age in both CO2 treatments during 1997. A stimulation of NCE initially occurred in response to CO2 enrichment, but this was not observed on the final eight of 33 experimental days. Net photosynthetic rates in response to growth at elevated CO2 were decreased ca. 22% on average for all measurement dates in 1997 and when all NCE rates measured at elevated CO2 for 1995-1997 were averaged (P ' 0.0001). Soluble protein, ct-amino nitrogen, and Chl a + b concentrations were significantly lower (P - 0.0001) in elevated compared with ambient C02-grown wheat flag leaves in 1997. The treatment by date interactions for these flag leaf constituents were nonsignificant (P 2 0.05). Flag leaf storage carbohydrates were measured on 10 dates in 1997, but only starch and sucrose were affected by the elevated CO2 treatment. An increase of acid proteinase activity was observed on the last two measurement dates of this study. However, changes of acid proteinase activity were unaffected by CO2 enrichment (P ? 0.05) and only occurred during late stages of senescence. These findings supported the suggestion that premature senescence contributed to the photosynthetic decline observed in wheat flag leaves during growth at elevated CO2. Changes of ox-amino nitrogen were correlated with photosynthetic decline, but acid proteinase activity probably was under endogenous control.

Yellowing and photosynthetic decline of barley primary leaves in response to atmospheric CO2 enrichment

The photosynthetic response of barley (Hordeum vulgare L. cv. Brant) primary leaves was studied as a function of chlorosis induced by CO2 enrichment. Leaf yellowing, measured as changes of chlorophyll a and b, was more extensive in controlled environments at elevated (680 ± 17 µl l−1) than at ambient (380 ± 21 µl l−1) CO2. Stomatal conductance of primary leaves was decreased by growth in elevated CO2 between 11 and 18 days after sowing (DAS) when measured at both 380 and 680 µl l−1 CO2. Internal leaf CO2 concentration (Ci) was also lower for elevated‐ compared to ambient‐CO2‐grown primary leaves between 11 and 14 DAS. Results suggest that non‐stomatal factors were responsible for the decreased photosynthetic rates of elevated‐ compared to ambient‐CO2‐grown primary leaves 18 DAS. Various photochemical measurements, including quantum absorptance (α), minimal (F0), maximal (Fm), and variable (Fv) chlorophyll fluorescence, as well as the Fv/Fm ratio, were significantly decreased 18 DAS in the elevated‐ compared to ambient‐CO2 treatment. Photochemical (qP) and nonphotochemical (qN) chlorophyll fluorescence quenching coefficients of 18‐day‐old primary leaves did not differ between CO2 treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated‐ compared to ambient‐CO2‐grown primary leaves 18 DAS. Concentrations of α‐amino N (i.e. free amino acids) in barley primary leaves were increased by CO2 enrichment 10 DAS, but subsequently, α‐amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated‐ than in ambient‐CO2‐grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO2‐dependent yellowing of barley primary leaves.

Photosynthetic decline and pigment loss during autumn foliar senescence in western larch (Larix occidentalis).

We measured needle pigment content and photosynthetic rates of 1-year-old western larch (Larix occidentalis Nutt.) during autumn foliar senescence. Chlorophyll (Chl) and carotenoid (xanthophyll + b-carotene) contents of needles declined 11 and 17%, respectively, before CO(2) assimilation rate began to decline. Chlorophyll a/b ratio, Chl/carotenoid ratio, photochemical efficiency (F(v)/F(m)), and photochemical quenching did not begin to decline until late in senescence. Internal CO(2)/ambient CO(2) did not change during needle yellowing. In seedlings in warmed soil (average 3 degrees C above natural conditions), the decline in needle chlorophyll content was delayed by 10 days and the decline in CO(2) assimilation rate was delayed by 5 days, compared with seedlings in soil at ambient temperature. In seedlings exposed to an extended 16-h photoperiod, the decline in needle chlorophyll content was delayed by 32 days, and the decline in CO(2) assimilation rate was delayed by 21 days, compared with seedlings exposed to natural day lengths. In addition to delaying the onset of needle senescence, the treatments affected the sequence of events during senescence. Differences among treatment groups provide evidence that the onset of pigment loss and photosynthetic decline and the sequence of events during needle senescence are affected by soil temperature and day length.

The influence of the parasitic angiospermStriga gesnerioideson the growth and photosynthesis of its host,Vigna unguiculata

Cowpea plants (Vigna unguiculata) infected with the root hemiparasitic angiosperm Striga gesnerioides accumulated less biomass than uninfected plants over a growth period of 60 d. The allometric relationship between shoot and root dry weight was similar in parasitized plants relative to control plants, as was the proportion of dry matter partitioned into leaf, stem and root tissue. However, infected plants failed to make any significant investment of dry matter in pods. The rate of photosynthesis of the youngest fully expanded leaf of parasitized plants was significantly lower than for control plants. The lower rates of photosynthesis were not attributable to stomatal limitation, a loss of chlorophyll or to an accumulation of carbohydrate. The depression of photosynthesis in the young leaves was transient. As control leaves aged, photosynthesis declined. This also occurred in Striga infected plants, but to a lesser extent resulting in higher rates of photosynthesis in mature leaves when compared to those of uninfected plants. The foliar nitrogen content of parasitized plants was higher than control plants consistent with the slower rate of photosynthetic decline of older leaves. The data are discussed with respect to the influence of parasitic weeds on host growth and photosynthesis.

Evidence of Reduced Photosynthetic Rates in Old Trees

Abstract Wood production generally declines as forests age. The decrease in production has been attributed to higher respiration associated with increasing biomass, but direct measurements fail to support this assumption. An alternate hypothesis is that the growth decline is caused by reduced net photosynthesis. To test this hypothesis, we compared the net photosynthetic rates of foliage from trees that had ceased height growth with foliage from actively growing, mature trees. Results showed that net photosynthesis per unit area of 1-yr-old foliage from old Pinus contorta and P. ponderosa averaged 14-30% lower than the same-aged foliage from younger trees. Computer simulations from a previous study indicated that differences of this magnitude are sufficient to explain the lower wood production in old trees. The cause of the photosynthetic decline is not clear, but it did not appear to be due to a reduction in the maximum capacity for photosynthesis. Ratios of stable carbon isotopes from leaves and wood indicated a greater stomatal limitation of photosynthesis in older trees. A possible relationship between lower hydraulic conductance in the vascular systems of older trees leading to greater stomatal closure is discussed. For. Sci. 40(3):513-527.

Atmospheric CO 2 enrichment: Kinetics of chlorophyll a fluorescence and photosynthetic CO 2 uptake in individual, attached cotton leaves

Chl fluorescence and gas exchange of attached cotton leaves ( Gossypium hirsutum L.) were measured in ambient air and in a highly CO 2-enriched atmosphere (4000 μl 1 −1 CO 2; photosynthetic saturation). In the short term (hours to one day), net CO 2 uptake approximately doubled in all leaves examined. Photochemical (q P) and nonphotochemical (q NP) quenching of chlorophyll fluorescence, and calculated linear photosynthetic electron flow, did not change significantly when CO 2 rose from 250 to 4000 μl 1 −1 CO 2. These results show that high CO 2 concentration did not inhibit photosynthesis in any leaf. In contrast, the long-term response of leaves to atmospheric CO 2-enrichment was variable. Some leaves sustained the initial high level of photosynthetic stimulation for more than a week while in others photosynthetic CO 2-uptake declined more or less. These leaves turned yellowish-green although chlorophyll content declined little. Variance in the degree of leaf yellowing was also encountered in experiments with clover when sets of plants were CO 2-enriched. Gas exchange and chl fluorescence results suggest that yellowing of cotton leaves in high CO 2 was not equivalent to ‘natural’ senescence although some chlorophyll fluorescence parameters changed similarly. During extended high CO 2 treatment the level of q NP increased notably in the yellowing leaves. The high levels of q NP and relaxation kinetics of chl fluorescence quenching recorded upon darkening demonstrate that thylakoid energization increased during the decline of photosynthetic CO 2 uptake in high CO 2. This shows that the photosynthetic decline was not caused by decreasing thylakoid energization because of physical damage by oversized starch grains. Calculated photosynthetic electron flow declined little suggesting that CO 2 at ribulosebisphosphate carboxylase-oxygenase fell and thus photorespiration rose. With regard to growth limitation in high CO 2 concentration, these results support the concept that high CO 2 concentration tends to induce low inorganic phosphate concentrations (Morin et al. Plant Physiol. 99, 89–95, 1992; Duchein et al. J. Exp. Bot. 44, 17–22, 1993) which can limit chloroplast ATP synthase and thus increase thylakoid energization. It is proposed that the different responses of individual leaves to atmospheric CO 2 enrichment reflects variety among leaves in the phosphate status or in the capacity for Pi-recycling (assimilate utilization).

EFFECTS OF TRELLIS FORM ON RADIATION INTERCEPTION WATER USE AND PHOTOSYNTHESIS IN GRAPEVINES

Manipulating radiation interception by using horizontal and vertical trellis forms was tested for effects on water loss and photosynthesis on sunny days. Water loss from 15 potted Pinot Noir vines of each form was estimated by weighing the vines at two hour intervals during the day. Radiation interception for vines and gas exchange of individual exposed leaves was measured over the day to evaluate the leaf activity patterns in relation to exposure patterns. Similarly, diurnal whole vine photosynthesis measurements were taken by using clear plastic “balloon” whole vine chambers. The horizontal form had higher radiation interception than the vertical form during the day, resulting in higher water loss from the horizontal form. Under N-S orientation, the horizontal form showed a gradual decrease in exposed leaf and whole vine photosynthesis starting in late-morning while the vertical form had a more uniform pattern over the day. The long periods of leaf exposure of horizontal canopies appears to induce a photosynthetic decline that also reduces afternoon transpiration. Vertical forms have two populations of leaves that are exposed for shorter periods, thus less photosynthetic decline occurs. Vertical trellis forms may have potential for improving water use efficiencies in arid regions with high radiation.

Effects of organic pollution on transplanted aquatic bryophytes

Abstract Some ecophysiological variables of aquatic bryophytes transplanted from their original collection sites in the River Iregua (La Rioja, N. Spain) to two stations of different water quality were examined as indicators of organic pollution. The variables were chlorophyll and phaeopigment levels, some pigment indexes (OD665/0D665 - acidified, OD430/OD410, OD430/0D665 and OD480/0D665), net photosynthesis, dark respiration and tissue Nand P levels.Net loss or cessation of uptake of Nand P, together with photosynthetic decline, were firstly detected as a consequence of organic pollution. Later, pigment composition was also affected, phaeopigment ratios being the most sensitive to organic pollution. Respiration did not show consistent changes. Tissue Nand P contents are particularly recommended as organic pollution bioindicators because of their applicability in a range of pollution intensities, species sensitivities and exposure periods and their direct physiological relationship to anoxia in waters.Amo...

High SO2 resistance of Clethra barbinervis established in a smoke‐polluted area of Ashio, Tochigi Prefecture, Japan

AbstractThe effect of SO2 on the photosynthesis of Clethra barbinervis collected from a smoke‐polluted area near the Ashio copper smelter in Tochigi Prefecture was compared with C. barbinervis collected from a nonpolluted district in Tsukuba, Ibaraki Prefecture and Quercus mongolica var. grosseserrata grown in a nonpolluted field in Nagano Prefecture. The plants were exposed to 0.5–1.5 p.p.m. SO2 for 90 min (short‐term) and to 0.3 p.p.m. SO2 for 31–39 days (long‐term). The Clethra plants from both sites had a lower intrinsic stomatal conductance and photosynthetic rate than Quercus plants. Short‐and long‐term fumigation caused stomatal closure in Quercus plants, but had little effect on the stomatal conductance of Clethra plants. Under short‐term fumigation, nonstomatal photosynthetic inhibition per unit of absorbed SO2 was smallest in Clethra plants from Ashio. Long‐term fumigation caused photosynthetic decline and visible foliar injury to Quercus plants, but had no effect on Clethra plants from Ashio. Consequently, Clethra plants from Ashio had a higher photosynthetic rate than Quercus plants after long‐term fumigation. These results suggest that C. barbinervis populations in the smoke‐polluted area of Ashio had evolved high SO2 resistance connected with SO2 detoxification ability in mesophyll cells.