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1.
Experiments were conducted to examine the development of photosynthetic carbon metabolism in Peperomia scandens, a tropical epiphyte. Leaves were sampled during a 10-day period when they were between 30 to 165 days old. P. scandens exhibits a C 3 to CAM-cycling to CAM shift during maturation with the magnitude of CAM increasing with age. Initially, during both day and night, no significant CO 2 uptake or diurnal acid flux was evident. C 3 gas exchange was detected at 41 days of age with a gradual shift towards CAM gas exchange maximized thereafter. An acidity flux of 130 to 150 microequivalents per gram fresh weight was evident by 41 days. Between 40 and 90 days, the leaves shifted their CO 2 uptake pattern from a daytime to a nighttime peak. After 90 days, the leaves remained in CAM. The δ 13C values became progressively less negative as the leaves matured. In the 30-day-old leaves, the δ 13C value was −21.1% while in the 165-day-old leaves the δ 13C value was −18.3%. The time-dependent shift from C 3 to CAM-cycling to CAM in P. scandens does not appear to result from changes in water, light, or temperature regimes since these variables were constant for all leaves sampled. 相似文献
2.
BackgroundIn obligate Crassulacean acid metabolism (CAM), up to 99 % of CO 2 assimilation occurs during the night, therefore supporting the hypothesis that CAM is adaptive because it allows CO 2 fixation during the part of the day with lower evaporative demand, making life in water-limited environments possible. By comparison, in facultative CAM (inducible CAM, C 3-CAM) and CAM-cycling plants drought-induced dark CO 2 fixation may only be, with few exceptions, a small proportion of C 3 CO 2 assimilation in watered plants and occur during a few days. From the viewpoint of survival the adaptive advantages, i.e. increased fitness, of facultative CAM and CAM-cycling are not obvious. Therefore, it is hypothesized that, if it is to increase fitness, CAM must aid in reproduction. ScopeAn examination of published reports of 23 facultative CAM and CAM-cycling species finds that, in 19 species, drought-induced dark CO 2 fixation represents on average 11 % of C 3 CO 2 assimilation of watered plants. Evidence is discussed on the impact of the operation of CAM in facultative and CAM-cycling plants on their survival – carbon balance, water conservation, water absorption, photo-protection of the photosynthetic apparatus – and reproductive effort. It is concluded that in some species, but not all, facultative and cycling CAM contribute, rather than to increase carbon balance, to increase water-use efficiency, water absorption, prevention of photoinhibition and reproductive output.Key words: Facultative CAM, CAM-cycling, water, crassulacean acid metabolism, deficit 相似文献
3.
Photosynthetic gas exchange and malic acid fluctuations were monitored in 69 well-watered plants from five morphologically similar species of Talinum in an investigation of the ecophysiological significance of the Crassulacean acid metabolism (CAM)-cycling mode of photosynthesis. Unlike CAM, atmospheric CO 2 uptake in CAM-cycling occurs exclusively during the day; at night, the stomata are closed and respiratory CO 2 is recaptured to form malic acid. All species showed similar patterns of day-night gas exchange and overnight malic acid accumulation, confirming the presence of CAM-cycling. Species averages for gas exchange parameters and malic acid fluctuation were significantly different such that the species with the highest daytime gas exchange had the lowest malic acid accumulation and vice versa. Also, daytime CO 2 exchange and transpiration were negatively correlated with overnight malic acid fluctuation for all individuals examined together, as well as within one species. This suggests that malic acid may effect reductions in both atmospheric CO 2 uptake and transpiration during the day. No significant correlation between malic acid fluctuation and water-use efficiency was found, although a nonsignificant trend of increasing water-use efficiency with increasing malic acid fluctuation was observed among species averages. This study provides evidence that CO 2 recycling via malic acid is negatively correlated with daytime transpirational water losses in well-watered plants. Thus, CAM-cycling could be important for survival in the thin, frequently desiccated soils of rock outcrops on which these plants occur. 相似文献
4.
High levels of variability in gas exchange characteristics and degree of CAM-cycling were found in the same and different individuals of Talinum calycinum Engelm. collected from rock outcrops in Missouri. Differences in CO 2 assimilation were mostly correlated with differences in shoot conductance to CO 2 not shoot internal CO 2 concentration. As found previously, CAM acid fluctuations were evident in well-watered plants exhibiting C 3 gas exchange patterns (CAM-cycling) and also in drought-stressed plants with stomata closed, or nearly so, day and night (CAM-idling). Drought stress also resulted in rapid stomatal closure, conserving water during droughts. Maximal CO 2 uptake rates occurred below 35°C; higher temperatures induced decreases in CO 2 assimilation and conductance while shoot internal CO 2 concentrations remained similar. Plant water-use-efficiency was severely curtailed at temperatures above 30°C. Tissue acid fluctuations were the result of changes in malic acid concentrations. Calculations of the amount of water potentially conserved by CAM-cycling yielded values of approximately 5 to 44% of daytime water loss. Thus, CAM-cycling may be an important adaptation minimizing water loss by perennial succulents growing in shallow soil on rock outcrops. 相似文献
5.
Peperomia has species that may be C 3, show Crassulacean acid metabolism (CAM), or CAM-cycling. Species that show CAM progress from C 3 to CAM through CAM-cycling during leaf development. In CAM and CAM-cycling species, CAM metabolism is predominately in the upper multiple epidermis and lower spongy mesophyll, whereas C 3 metabolism is localized mostly in the palisade mesophyll. Using specific protein and cDNA probes prepared from P-enolpyruvate carboxylase (PEPc) and ribulose-1,5-bisphosphate carboxylase (Rubisco), we have now studied the differential distribution of photosynthetic metabolism in Peperomia leaves using the technique of tissue printing. The tissue printing studies detected Rubisco protein in leaves of C 3 P. orba, but not PEPc. Young C 3 leaves of P. scandens and P. camptotricha showed Rubisco protein, but not PEPc; however, the mature leaves of these two species that have CAM showed PEPc protein and RNAs in both the multiple epidermis and spongy mesophyll. In contrast, Rubisco protein and RNAs were present throughout the leaf. The tissue printing data are consistent with our previously published data showing the differential distribution of photosynthetic metabolism in leaves of Peperomia. Although the tissue printing technique is qualitative, coupled with quantitative data it has proven useful for the study of function related to structure. 相似文献
6.
Under well-watered conditions in the laboratory, Sedum pulchellum assimilated CO 2 only during the day, yet exhibited small nocturnal increases in tissue acid content followed by deacidification in the light (CAM-cycling). When drought-stressed, little CO 2 was fixed in the day and none at night, yet even greater acid fluctuations were observed (CAM-idling). Calculations indicate that water savings associated with CAM-cycling when water is available are small. Water saving is more likely to be significant during CAM-idling when water supply is limited and stomata are closed day and night. Thus, in this species, CAM-idling may be of greater benefit to the plant, relative to CAM-cycling, in surviving habitats prone to frequent drought stress.Abbreviations A
CO 2 exchange rate
- CAM
Crassulacean acid metabolism
- c i
shoot internal CO 2 concentration
- g c
shoot conductance to CO 2
- PPFD
photosynthetic photon flux density
- WUE
water-use efficiency
Supported by National Science Foundation Grant No. DMB 8506093. 相似文献
7.
The diurnal variations in volume and in specific weight were determined for green stems and leaves of Crassulacen acid metabolism (CAM) plants. Volume changes were measured by a water displacement method. Diurnal variations occurred in the volume of green CAM tissues. Their volume increased early in the light period reaching a maximum about mid-day, then the volume decreased to a minimum near midnight. The maximum volume increase each day was about 2.7% of the total volume. Control leaves of C 3 and C 4 plants exhibited reverse diurnal volume changes of 0.2 to 0.4%. The hypothesis is presented and supported that green CAM tissues should exhibit a diurnal increase in volume due to the increase of internal gas pressure from CO 2 and O 2 when their stomata are closed. Conversely, the volume should decrease when the gas pressure is decreased. The second hypothesis presented and supported was that the specific weight (milligrams of dry weight per square centimeter of green surface area) of green CAM tissues should increase at night due to the net fixation of CO2. Green CAM tissues increased their specific weight at night in contrast to control C3 and C4 leaves which decreased their specific weight at night. With Kalanchoë daigremontiana leaves, the calculated increase in specific leaf weight at night based on estimates of carbohydrate available for net CO2 fixation was near 6% and the measured increase in specific leaf weight was 6%. Diurnal measurements of CAM tissue water content were neither coincident nor reciprocal with their diurnal patterns of either volume or specific weight changes. 相似文献
8.
Peperomia camptotricha, a tropical epiphyte from Mexico, shows variable forms of Crassulacean acid metabolism (CAM). Young leaves exhibit CAM-cycling, while mature leaves show an intermediate type of metabolism, between CAM and CAM-cycling, having approximately the same amount of nighttime gas exchange as daytime. Metabolism of young leaves appears independent of daylength, but mature leaves have a tendency toward more CAM-like metabolism under short days (8 hours). Large differences in the physical appearance of plants were found between those grown under short daylengths and those grown under long daylengths (14 hours). Some anatomical differences were also detected in the leaves. Water stress caused a switch to CAM in young and mature leaves, and as water stress increased, they shifted to CAM-idling. 相似文献
9.
The freshwater macrophyte, Ottelia alismoides, is a bicarbonate user performing C4 photosynthesis in the light, and crassulacean acid metabolism (CAM) when acclimated to low CO 2. The regulation of the three mechanisms by CO 2 concentration was studied in juvenile and mature leaves. For mature leaves, the ratios of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) are in the range of that of C4 plants regardless of CO 2 concentration (1.5–2.5 at low CO 2, 1.8–3.4 at high CO 2). In contrast, results for juvenile leaves suggest that C4 is facultative and only present under low CO 2. pH-drift experiments showed that both juvenile and mature leaves can use bicarbonate irrespective of CO 2 concentration, but mature leaves have a significantly greater carbon-extracting ability than juvenile leaves at low CO 2. At high CO 2, neither juvenile nor mature leaves perform CAM as indicated by lack of diurnal acid fluctuation. However, CAM was present at low CO 2, though the fluctuation of titratable acidity in juvenile leaves (15–17 µequiv g ?1 FW) was slightly but significantly lower than in mature leaves (19–25 µequiv g ?1 FW), implying that the capacity to perform CAM increases as leaves mature. The increased CAM activity is associated with elevated PEPC activity and large diel changes in starch content. These results show that in O. alismoides, carbon-dioxide concentrating mechanisms are more effective in mature compared to juvenile leaves, and C4 is facultative in juvenile leaves but constitutive in mature leaves. 相似文献
10.
The submerged aquatic plant Isoetes howellii Engelmann possesses Crassulacean acid metabolism (CAM) comparable to that known from terrestrial CAM plants. Infrared gas analysis of submerged leaves showed Isoetes was capable of net CO 2 uptake in both light and dark. CO 2 uptake rates were a function of CO 2 levels in the medium. At 2,500 microliters CO 2 per liter (gas phase, equivalent to 1.79 milligrams per liter aqueous phase), Isoetes leaves showed continuous uptake in both the light and dark. At this CO 2 level, photosynthetic rates were light saturated at about 10% full sunlight and were about 3-fold greater than dark CO 2 uptake rates. In the dark, CO 2 uptake rates were also a function of length of time in the night period. Measurements of dark CO 2 uptake showed that, at both 2,500 and 500 microliters CO 2 per liter, rates declined during the night period. At the higher CO 2 level, dark CO 2 uptake rates at 0600 h were 75% less than at 1800 h. At 500 microliters CO 2 per liter, net CO 2 uptake in the dark at 1800 h was replaced by net CO 2 evolution in the dark at 0600 h. At both CO 2 levels, the overnight decline in net CO 2 uptake was marked by periodic bursts of accelerated CO 2 uptake. CO 2 uptake in the light was similar at 1% and 21% O 2, and this held for leaves intact as well as leaves split longitudinally. Estimating the contribution of light versus dark CO 2 uptake to the total carbon gain is complicated by the diurnal flux in CO 2 availability under field conditions. 相似文献
11.
In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO 2 fixation (C 3) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C 3-type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO 2 concentration and that these changes in CO 2 could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Well-watered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal ( ca. 330 ppm), and high (950 ppm) CO 2 during the day with normal concentrations of CO 2 during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO 2 uptake, and phosphoenolpyruvate carboxylase activity, remained unaffected by low, normal, or high CO 2 treatments. In well-watered plants, however, both low and high ambient concentrations of CO 2 tended to reduce organic acid concentrations, low concentrations of CO 2 reducing the organic acids more than high CO 2. It was concluded that exposing the plants to the CO 2 concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism. 相似文献
12.
The relationship between malic acid production and carbon assimilation was examined in the submerged aquatic Crassulacean acid metabolism (CAM) plant, Isoetes howellii Engelmann. Under natural conditions free-CO 2 level in the water was highest at 0600 hours and 14CO 2 assimilation rates in I. howellii were also highest at this time. After 0900 hours there was a similar pattern in (a) rate of free-CO 2 depletion from the water, (b) reduction of carbon assimilation rates, and (c) rate of deacidification in leaves. Rates of daytime deacidification increased under CO 2-free conditions and as irradiance intensity increased. Nighttime CO 2 uptake was estimated to contribute one-third to one-half of the total daily gross carbon assimilation. CO 2 uptake, however, accounted for only one-third to one-half of the overnight malic acid accumulation. Internal respiratory CO 2 may be a substrate for a large portion of overnight acid accumulation as leaves incubated overnight without CO 2 accumulated substantial levels of malic acid. Loss of CAM occurred in emergent leaf tips even though submerged bases continued CAM. Associated with loss of CAM in aerial leaves was an increase in total chlorophyll, a/b ratio, and carotenoids, and a decrease in leaf succulence. δ 13C values of I. howellii were not clearly distinguishable from those for associated non-CAM submerged macrophytes. 相似文献
13.
By measuring titratable acidity, gas exchange parameters, mesophyll succulence, and 13C/ 12C ratios, we have shown that Cissus quadrangularis L. has C 3-like leaves and stems with Crassulacean acid metabolism (CAM). In addition, the nonsucculent leaves show the diurnal fluctuations in organic acids termed recycling despite the fact that all CO 2 uptake and stomatal opening occurs during the day. Young succulent stems have more C 3 photosynthesis than older stems, but both have characteristics of CAM. The genus Cissus will be a fruitful group to study the physiology, ecology, and evolution of C 3 and CAM since species occur that exhibit characteristics of both photosynthetic pathways. 相似文献
14.
Diurnal patterns of CO 2 exchange and fluctuations of tissuemalic acid concentrations were investigated in three speciesof Commelinaceae: Callisia fragrans and Tripogandra multiflorafrom Jamaica, and Tradescantia brevifolia from southern Texas.Very low levels of CAM gas exchange were induced by droughtstress in C. fragrans and T. multiflora. In addition, past indicationsof CAM-cycling in the two Jamaican species were confirmed indrought-stressed plants; however, only C. fragrans exhibitedCAM-cycling under well-watered conditions. CAM-cycling underdrought stress was also found in T. brevifolia. This constitutesthe first report of CAM ( sensu lato) in the genus Tradescantia.The importance of low-level CAM in these three species is discussedas a potential adaptation to drought. Copyright 1994, 1999 AcademicPress Callisia fragrans, Tradescantia brevifolia, Tripogandra multiflora, Commelinaceae, CO 2 exchange, Crassulacean acid metabolism, CAM-cycling, CAM-idling, drought stress, malic acid fluctuations 相似文献
15.
Net CO 2 uptake over 24-hour periods was examined for the leaves and for the stems of 11 species of cacti representing all three subfamilies. For Pereskia aculeata, Pereskia grandifolia, and Maihuenia poeppigii (subfamily Pereskioideae), all the net shoot CO 2 uptake was by the leaves and during the daytime. In contrast, for the leafless species Carnegiea gigantea, Ferocactus acanthodes, Coryphantha vivipara, and Mammillaria dioica (subfamily Cactoideae), all the shoot net CO 2 uptake was by the stems and at night. Similarly, for leafless Opuntia ficus-indica (subfamily Opuntioideae), all net CO 2 uptake occurred at night. For leafy members of the Opuntioideae ( Pereskiopsis porteri, Quiabentia chacoensis, Austrocylindropuntia subulata), at least 88% of the shoot CO 2 uptake over 24 hours was by the leaves and some CO 2 uptake occurred at night. Leaves responded to the instantaneous level of photosynthetically active radiation (PAR) during the daytime, as occurs for C 3 plants, whereas nocturnal CO 2 uptake by stems of O. ficus-indica and F. acanthodes responded to the total daily PAR, as occurs for Crassulacean acid metabolism (CAM) plants. Thus, under the well-watered conditions employed, the Pereskioideae behaved as C 3 plants, the Cactoideae behaved as CAM plants, and the Opuntioideae exhibited characteristics of both pathways. 相似文献
16.
PAM ( Pulse Amplitude Modulation) fluorometer techniques directly measure the light reactions of photosynthesis that are otherwise difficult to estimate in CAM (Crassulacean Acid metabolism) plants such as pineapple ( Ananas comosus comosus cv. Phuket). PAM machines calculate photosynthesis as the Electron Transport Rate (ETR) through PSII (4 electrons per O 2 produced) as mol m ?2 s ?1. P vs. E curves fitted the waiting-in-line function (an equation of the form $ {\hbox{ETR}} = \left( {{\hbox{ET}}{{\hbox{R}}_{{ \max }}} \times {\hbox{E}}/{{\hbox{E}}_{\rm{opt}}}} \right).{{\hbox{e}}^{{1} - {\rm{E}}/{\rm{Eopt}}}} $ ) allowing half-saturating and optimal irradiances (E opt) to be estimated. Effective Quantum Yield (Y max), Electron Transport Rate (ETR max) and the Non-Photochemical Quenching parameter, NPQ max all vary on a diurnal cycle but the parameter qN max does not show a systematic variation over a diurnal period. Phuket pineapple is a “sun plant” with Optimum Irradiance (E opt) from 755 to 1,130 μmol m ?2 s ?1 (400–700 nm) PAR but photosynthetic capacity is very low in the late afternoon even though light conditions are favourable for rapid photosynthesis. Total CO 2 fixed nocturnally as C4-dicarboxylic acids by leaves of the Phuket pineapple was only ≈0.14 gC m ?2 d ?1 (0.012 mol C m ?2 d ?1). Titratable acid of leaves was depleted about 3 pm (15:00) and shows a classical CAM diurnal cycle. The Phuket pineapple variety only stored enough CO 2 as C4 acids to account for only about 2.5% of photosynthesis (P g) estimated using the PAM machine (≈5.6 gC m ?2 d ?1). Phuket pineapples are classifiable as CAM-Cycling plants but nocturnal fixation of CO 2 is so low compared to the more familiar Smooth Cayenne variety that it probably recycles only a small proportion of the respiratory CO 2 produced in leaves at night and so even CAM-cycling is only of minor importance to the carbon economy of the plant. Unlike the Smooth Cayenne pineapple variety, which fixes large amounts of CO 2 nocturnally, the Phuket pineapple is for practical purposes a C3 plant. 相似文献
17.
Crassulacean acid metabolism (CAM) was investigated in leaves and stems of the succulent C 4 dicot Portulaca oleracea L. Diurnal acid fluctuations, CO 2 gas exchange, and leaf resistance were monitored under various photoperiod and watering regimes. No CAM activity was seen in well watered plants grown under 16-hour days. Under 8-hour days, however, well watered plants showed a CAM-like pattern of acid fluctuation with amplitudes of 102 and 90 microequivalents per gram fresh weight for leaves and stems, respectively. Similar patterns were also observed in detached leaves and defoliated stems. Leaf resistance values indicated that stomata were open during part of the dark period, but night acidification most likely resulted from refixation of respiratory CO 2. In water-stressed plants maximum acid accumulations were reduced under both long and short photoperiods. At night, these plants showed short periods of net CO 2 uptake and stomatal opening which continued all night long during preliminary studies under natural environmental conditions. Greatest acid fluctuations, in P. oleracea, with amplitudes of 128 microequivalents per gram fresh weight, were observed in water-stressed plants which had been rewatered, especially when grown under short days. No net CO 2 uptake took place, but stomata remained open throughout the night under these conditions. These results indicate that under certain conditions, such as water stress or short photoperiods, P. oleracea is capable of developing an acid metabolism with many similarities to CAM. 相似文献
18.
For the leaf succulent Agave deserti and the stem succulent Ferocactus acanthodes, increasing the ambient CO 2 level from 350 microliters per liter to 650 microliters per liter immediately increased daytime net CO 2 uptake about 30% while leaving nighttime net CO 2 uptake of these Crassulacean acid metabolism (CAM) plants approximately unchanged. A similar enhancement of about 30% was found in dry weight gain over 1 year when the plants were grown at 650 microliters CO 2 per liter compared with 350 microliters per liter. Based on these results plus those at 500 microliters per liter, net CO 2 uptake over 24-hour periods and dry weight productivity of these two CAM succulents is predicted to increase an average of about 1% for each 10 microliters per liter rise in ambient CO 2 level up to 650 microliters per liter. 相似文献
19.
The occurrence of the Crassulacean acid metabolism (CAM) was studied in four epiphytic species of the Gesneriaceae: two neotropical species, Codonanthe crassifolia and Columnea linearis, and two paleotropical species, Aeschynanthus pulcher and Saintpaulia ionantha. Gas exchange parameters, enzymology, and leaf anatomy, including mesophyll succulence and relative percent of the mesophyll volume occupied by airspace, were studied for each species. Codonanthe crassifolia was the only species to show nocturnal CO 2 uptake and a diurnal organic acid fluctuation. According to these results, Codonanthe crassifolia shows CAM-cycling under well-watered conditions and when subjected to drought, it switches to CAM-idling. Other characteristics, such as leaf anatomy, mesophyll succulence, and PEP carboxylase and NADP malic enzyme activity, indicate attributes of the CAM pathway. All other species tested showed C 3 photosynthesis. The most C 3-like species is Columnea linearis, according to the criteria tested in this investigation. The other two species show mesophyll succulence and relative percent of the leaf volume occupied by airspace within the CAM range, but no other characters of the CAM pathway. The leaf structure of certain genera of the Gesneriaceae and of the genus Peperomia in the Piperaceae are similar, both having an upper succulent, multiple epidermis, a medium palisade of one or a few cell layers, and a lower, succulent spongy parenchyma not too unlike CAM photosynthetic tissue. We report ecophysiological similarities between these two distantly related families. Thus, the occurrence of CAM-cycling may be more common among epiphytic species than is currently known. 相似文献
20.
A comparison of carbon metabolism in the constitutive crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. and the C 3-CAM intermediate Clusia minor L. was undertaken under controlled environmental conditions where plants experience gradual changes in light intensity, temperature and humidity at the start and end of the photoperiod. The magnitude of CAM activity was manipulated by maintaining plants in ambient air or by enclosing leaves overnight in an atmosphere of N 2 to suppress C 4 carboxylation. Measurements of diel changes in carbonisotope discrimination and organic acid content were used to quantify the activities of C 3 and C 4 carboxylases in vivo and to indicate the extent to which the activities of phosphoenolpyruvate carboxylase (PEPCase), ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and decarboxylation processes overlap at the start and end of the photoperiod. These measurements in vivo were compared with measurements in vitro of changes in the diel sensitivity of PEPCase to malate inhibition. The results demonstrate fundamental differences in the down-regulation of PEPCase during the day in the two species. While PEPCase is inactivated within the first 30 min of the photoperiod in K. daigremontiana, the enzyme is active for 4 h at the start and 3 h at the end of the photoperiod in C. minor. Enclosing leaves in N 2 overnight resulted in a two-to threefold increase in PEPCase-mediated CO 2 uptake during Phase II of CAM in both species. However, futile cycling of CO 2 between malate synthesis and decarboxylation does not occur during Phase II in either species. In terms of overall carbon balance, C 4 carboxylation accounted for ≈ 20% of net daytime assimilation in both species under control conditions, increasing to 30–34% after a night in N 2. Although N 2-treated leaves of K. daigremontiana took up 25% more CO 2 than control leaves during the day this was insufficient to compensate for the loss of CO 2 taken up by CAM the previous night. In contrast, in N 2-treated leaves of C. minor, the twofold increase in daytime PEPCase activity and the increase in net CO 2 uptake by Rubisco during Phase III compensated for the inhibition of C 4 carboxylation at night in terms of diel carbon balance. 相似文献
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