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1.
The response of photosynthetic CO 2 assimilation to salinization in 19 year old Prunus salicina was evaluated under field conditions for a 3 year period. The observed decline in CO 2 assimilation capacity was apparently related to increasing leaf chloride (Cl −) content, and independent of changes in leaf carbohydrate status. The response of net CO 2 assimilation (A) to leaf intercellular CO 2 partial pressure (C i) indicated that the reduction in the capacity for A with Cl − was not the result of decreased stomatal conductance but a consequence of nonstomatal inhibition. The nonstomatal limitations to CO 2 assimilation capacity, as determined by the response of A to C i and biochemical assay, were related to a decline in the activity of ribulose 1,5-bisphosphate carboxylase (Rubpcase) and the pool size of triose phosphate, ribulose 1,5-bisphosphate (Rubp) and phosphoglycerate with increasing salinity. Lack of agreement between the initial slope of the A to C i response curve and Rubpcase activity suggests the occurrence of heterogeneous stomatal apertures with the high salinity treatment (28 millimolar). Prolonged exposure to chloride salts appeared to increase the Rubp or Pi regeneration limitation, decrease Rubpcase activity and reduce leaf chlorophyll content. Observed changes in the biochemical components of CO 2 fixation may, in turn, affect total leaf carbohydrates, which also declined with time and salinity. The reduction in Rubpcase activity was apparently a consequence of a reduced Rubpcase protein level rather than either a regulatory or inhibitory effect. 相似文献
2.
Differences in the photosynthetic performance between pairs of heat tolerant (HT) and heat sensitive (HS) accessions of tuber-bearing Solanum species were measured at 40 °C, after treating plants at 40/30 °C. After 1 to 9 days of heat treatment, both HT and HS accessions showed progressive inhibitory effects, primarily decreased rates of CO 2 fixation, and loss of leaf chlorophyll. These effects were most pronounced in the HS accessions. Stomatal conductivity and internal CO 2 concentrations were lower for both accessions at 40 °C especially for the HS accessions, suggesting that at ambient CO 2 concentrations, stomatal conductance was limiting CO 2 availability at the higher temperature. In the HT accessions, stomatal limitations were largely attributed to differences in vapor pressure deficit between 25° and 40 °C, while the HS accessions exhibited significant nonstomatal limitations. The young expanding leaves of the HS accession showed some HT characteristics, while the oldest leaves showed severe senescence symptoms after 9 days at 40/30 °C. The data suggest that differences in heat sensitivity between HT and HS accessions are the result of accelerated senescence, chlorophyll loss, reduced stomatal conductance, and inhibition of dark reactions at high temperature. 相似文献
3.
(±) Abscisic acid (ABA) injected into petioles of attached transpiring leaves of Pharbitis nil Chois. cv violet reduced the photosynthetic capacity of the mesophyll of these leaves as well as the stomatal conductance to CO 2 diffusion. Greater than 75% of the injected ABA was recovered as ABA, suggesting that ABA rather than some metabolite thereof was the active compound. The nonstomatal effect of ABA increased from 30% reduction in photosynthesis at 0.25 micromolar ABA in the leaf blade to 90% reduction at 18 micromolar. Despite the effect of ABA on the nonstomatal component of leaf net CO 2 uptake, it was calculated that a substantial part of the reduction in leaf net CO 2 uptake (50-80%) could be accounted for by the effect of ABA on stomatal conductance. 相似文献
4.
Some evidence indicates that photosynthetic rate ( A) and stomatal conductance ( g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a “messenger” from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO 2 concentration ( ci), which is in turn a function of g, such a correlation may be partially mediated by ci if g is to some degree an independent variable. Among individual sunlit leaves in a cotton ( Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g ( r2 = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of ci. This approach revealed that, in the absence of ci effects, mesophyll photosynthetic capacity was unrelated to g ( r2 = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO 2, however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable ci. This “residual” correlation implies the existence of a messenger in CO 2-enriched plants. Enriched CO 2 also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO 2. In a CO 2-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced. 相似文献
5.
The physiological and physical components of the feedback loop involving intercellular CO 2 concentration (c i) and stomata are identified. The loop gain (G) is a measure of the degree of homeostasis in a negative feedback loop [the expression 1/(1-G) represents the fraction to which feedback reduces a perturbance]. Estimates are given for the effects of G on responses of stomata and c i to changes in ambient CO 2 concentration, light intensity, and perturbations in the water relations of a leaf. At normal ambient CO 2 concentration, the gain of the loop involving stomatal conductance and c i was found to be −2.2 in field-grown Zea mays, −3.6 if plants of this species were grown in a growth chamber, and zero in well watered Xanthium strumarium in the vegetative state. 相似文献
6.
The effect of long-term (7 days) and shortterm (up to 2 h) treatment of barley plants with jasmonic acid (JA) on the components contributing to stomatal and nonstomatal limitation of photosynthesis was studied. Net CO 2 assimilation rate ( A) responses to intercellular CO 2 concentration ( C
i
), i.e., A/C
i
curves, were used to assess the photosynthetic ability. Long-term treatment of barley plants with JA led to a noticeable decrease in both the initial slope of the A/C
i
curves and the maximum A at saturating C
i
. The proportion of stomatal and nonstomatal factors in limitation of photosynthesis depended on the applied JA concentration. Short-term treatment with JA affected neither the stomatal conductivity for CO 2 nor the rate of photosynthetic CO 2 assimilation. We suggest that JA may affect photosynthesis indirectly, either as a stress-modulating substance, or through the alterations in gene expression. 相似文献
7.
Leaf gas exchange characteristics of a desert annual ( Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 ( Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from −0.50 to −2.9 megapascals. At an ambient [CO 2] of 330 microliters per liter, T. kotschyi had higher conductance and CO 2 assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO 2 concentration (C i) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO 2] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C i and lower water use efficiency. Under water deficits, however, C i at 330 microliters per liter ambient [CO 2] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C i response curves had higher A at C i values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis. 相似文献
8.
Light saturated photosynthesis (A) in field saplings of shade tolerant, intermediate, and intolerant tree species was analyzed
for stomatal and nonstomatal limitations to test differences between species and sun and shade phenotypes during drought.
Throughout the study, photosynthesis was highest and mesophyll limitations of A (L m) lowest in the intolerant species in both open and understory habitats. The shade tolerant species exhibited the only drought-related
decreased A and increased L m in the open, and the greatest drought-related decreased A and increased L m in the understory. Few species exhibited significant habitat or drought-related differences in stomatal conductance to CO 2 (g c), but even slight decreases in g c during drought were associated with large increases in stomatal limitations to A (L g). Combined changes in L m and L g resulted in increased relative stomatal limitation to A ( l
g) in several species during drought. Nevertheless, the overall lack of stomatal closure allowed for nonstomatal limitations
to play a major role in reduced A during drought. Higher leaf N was associated with shallower slope of the l
g versus g c relationship, an indication of greater A capacity. Photosynthetic capacity tended to be greater in the intolerant species
than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant species in the understory.
Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynthetic
light reactions. If so, reduced carbon reduction capacity of shade tolerant species or shade phenotypes may predispose them
to drought conditions, which suggests a mechanism behind the well-recognized tradeoff between drought tolerance and shade
tolerance of temperate tree species.
Received: 20 October 1995 / Accepted: 20 February 1996 相似文献
9.
The responses of steady state CO 2 assimilation rate ( A), transpiration rate ( E), and stomatal conductance ( gs) to changes in leaf-to-air vapor pressure difference (Δ W) were examined on different dates in shoots from Abies alba trees growing outside. In Ecouves, a provenance representative of wet oceanic conditions in Northern France, both A and gs decreased when Δ W was increased from 4.6 to 14.5 Pa KPa −1. In Nebias, which represented the dry end of the natural range of A. alba in southern France, A and gs decreased only after reaching peak levels at 9.0 and 7.0 Pa KPa −1, respectively. The representation of the data in assimilation rate ( A) versus intercellular CO 2 partial pressure ( Ci) graphs allowed us to determine how stomata and mesophyll photosynthesis interacted when Δ W was increased. Changes in A were primarily due to alterations in mesophyll photosynthesis. At high Δ W, and especially in Ecouves when soil water deficit prevailed, A declined, while Ci remained approximately constant, which may be interpreted as an adjustment of gs to changes in mesophyll photosynthesis. Such a stomatal control of gas exchange appeared as an alternative to the classical feedforward interpretation of E versus Δ W responses with a peak rate of E. The gas exchange response to Δ W was also characterized by considerable deviations from the optimization theory of IR Cowan and GD Farquhar (1977 Symp Soc Exp Biol 31: 471-505). 相似文献
10.
Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO 2 concentration (c i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed. 相似文献
11.
Root chilling has been shown to inhibit shoot photosynthesis yet the mechanism for such an action is not clearly understood. A study was designed to elucidate the mechanism by which root cooling may affect net photosynthesis. Roots of Artemisia tridentata seedlings were cooled from 20°C to 5°C while their shoot temperature remained at 20°C. This was conducted at two light levels (700 and 1300 μmol m ?2 s ?1). The time course of shoot net photosynthesis (A), stomatal conductance to water vapor (g s), intercellular CO 2 concentration (C i) and root respiration (R s) were determined on a whole-plant basis. Root cooling caused a 25% reduction in A at high PPFD, which was preceded by more than 50% reduction of g s and about 10% reduction in C i. A versus C i curves for single branches showed no difference between cold and warm soil temperatures, although stomatal conductance was lower for the lower soil temperature. This suggests that a stomatal limitation may have been involved in the inhibition of A. Furthermore, a concomitant decrease of as much as 23% in leaf relative water content (RWC) indicated that root cooling affected stomatal closure due to decreased water supply to the foliage. At lower PPFD, root cooling did not cause a decrease in A of the whole plant despite a moderate drop in g s, C i and RWC. Cold soil also led to a substantial and rapid reduction in root respiration rate (R s) regardless of the light level. 相似文献
12.
A unique approach was used to evaluate stomatal and nonstomatal constraints to photosynthesis in 19 naturally occurring, deciduous tree species on xeric, mesic and wetmesic sites in central Pennsylvania, USA, during relatively wet (1990) and dry (1991) growing seasons. All species exhibited significantly decreased stomatal conductance to CO 2 ( gc) in 1991 compared to 1990. The mesic species had drought related decreases in photosynthesis ( A) attributed primarily to increased absolute stomatal limitation to A ( Lg), whereas in the wet-mesic species, the absolute mesophyll limitation ( Lm) was at least as important as Lg in limiting A during drought. The xeric species maintained relatively high A during drought despite decreased gc. In the xeric and mesic species, Lm decreased and Lg increased during drought due to stomatal closure. From xeric to mesic to wet-mesic, the relative stomatal limitation ( Ig) generally decreased faster, and relative mesophyll limitations to A increased faster, with increasing gc suggesting greater photosynthetic capacity (i.e. greater potential maximum A) with increasing drought tolerance rank of species. Few species exhibited a significant drought-related decrease in photosynthetic capacity. The results of this landscape-based study indicate that the interaction of stomatal and nonstomatal limitations of A vary in a manner consistent with species' drought tolerance and site conditions, and that nonstomatal constraints to A in field plants during a moderate, season-long drought were generally not as severe as reported in controlled studies. 相似文献
13.
Two separate objectives were considered in this study. We examined (1) internal conductance to CO 2 ( gi) and photosynthetic limitations in sun and shade leaves of 60-year-old Fagus sylvatica, and (2) whether free-air ozone fumigation affects gi and photosynthetic limitations. gi and photosynthetic limitations were estimated in situ from simultaneous measurements of gas exchange and chlorophyll fluorescence on attached sun and shade leaves of F. sylvatica. Trees were exposed to ambient air (1× O 3) and air with twice the ambient ozone concentration (2× O 3) in a free-air ozone canopy fumigation system in southern Germany (Kranzberg Forest). gi varied between 0.12 and 0.24 mol m −2 s −1 and decreased CO 2 concentrations from intercellular spaces ( Ci) to chloroplastic ( Cc) by approximately 55 μmol mol −1. The maximum rate of carboxylation ( Vcmax) was 22–39% lower when calculated on a Ci basis compared with a Cc basis. gi was approximately twice as large in sun leaves compared to shade leaves. Relationships among net photosynthesis, stomatal conductance and gi were very similar in sun and shade leaves. This proportional scaling meant that neither Ci nor Cc varied between sun and shade leaves. Rates of net photosynthesis and stomatal conductance were about 25% lower in the 2× O 3 treatment compared with 1× O 3, while Vcmax was unaffected. There was no evidence that gi was affected by ozone. 相似文献
14.
Background and AimsWater and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration ( E) can be decreased by reducing leaf stomatal conductance ( gs) at the expense of lowering CO 2 uptake ( A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when gs declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration ( N) varies on time scales much longer than gs, an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO 2 concentration ( ca). MethodsOptimization theory is employed to quantify interactions between resource-use efficiencies under elevated ca and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂ A/∂ gs)/(∂ E/∂ gs) and η = ∂ A/∂ N, respectively. The relationship between the two efficiencies and related variation in intercellular CO 2 concentration ( ci) were examined using A/ ci curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO 2 Enrichment experiment (North Carolina, USA). Key ResultsOptimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated ca increased η and λ, and at given ca and needle age-class, the two quantities varied among needles in an approximately complementary manner. ConclusionsThe derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing ca influences ecosystem water use. 相似文献
15.
The relationship between single leaf photosynthesis and conductance was examined in cotton ( Gossypium hirsutum L.) across a range of environmental conditions. The purpose of this research was to separate and define the degree of stomatal and nonstomatal limitations in the photosynthetic process of field-grown cotton. Photosynthetic rates were related to leaf conductance of upper canopy leaves in a curvilinear manner. Increases in leaf conductance of CO2 in excess of 0.3 to 0.4 mole per square meter per second did not result in significant increases in gross or net photosynthetic rates. No tight coupling between environmental influences on photosynthetic rates and those affecting conductance levels was evident, since photosynthesis per unit leaf conductance did not remain constant. Slowly developing water stress caused greater reductions in photosynthesis than in leaf conductance, indicating nonstomatal limitations of photosynthesis. Increases in external CO2 concentration to levels above ambient did not produce proportional increases in photosynthesis even though substomatal or intercellular CO2 concentration increased. The lack of a linear increase in photosynthetic rate in response to increases in leaf conductance and in response to increases in external CO2 concentration demonstrated that nonstomatal factors are major photosynthetic rate determinants of cotton under field conditions. 相似文献
16.
Observations of nonuniform photosynthesis across leaves cast doubt on internal CO 2 partial pressures (p i) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring p i directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed p i to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower ( Helianthus annuus L.), soybean ( Glycine max L. Merr.), and bush bean ( Phaseolus vulgaris L.) subjected to high or low leaf water potentials (ψ w) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Δp) between ambient CO 2 and measured p i throughout the light period when water was supplied. When water was withheld, ψ w decreased and the stomata began to close, but measured p i increased until the leaf reached a ψ w of −1.76 (bush bean), −2.12 (sunflower) or −3.10 (soybean) megapascals, at which point Δp = 0. The increasing p i indicated that stomata did not inhibit CO 2 uptake and a Δp of zero indicated that CO 2 uptake became zero despite the high availability of CO 2 inside the leaf. In contrast, when sunflower leaves at high ψ w were treated with ABA, p i did not increase and instead decreased rapidly and steadily for up to 8 hours even as ψ w increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO 2 at low ψ w and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low ψ w but not at high ABA. 相似文献
17.
Young bell pepper ( Capsicum annuum L.) plants grown in nutrient solution were gradually acclimated to 50, 100, or 150 moles per cubic meter NaCl, and photosynthetic rates of individual attached leaves were measured on several occasions during the salinization period at external CO 2 concentrations ranging from approximately 70 to 1900 micromoles per mole air. Net CO 2 assimilation (A) was plotted against computed leaf internal CO 2 concentration (C i), and the initial slope of this A-C i curve was used as a measure of photosynthetic ability. During the 10 to 14 days after salinization began, leaves from plants exposed to 50 moles per cubic meter NaCl showed little change in photosynthetic ability, whereas those treated to 100 or 150 moles per cubic meter NaCl had up to 85% inhibition, with increase in CO 2 compensation point. Leaves appeared healthy, and leaf chlorophyll content showed only a 14% reduction at the highest salinity levels. Partial stomatal closure occurred with salinization, but reductions in photosynthesis were primarily nonstomatal in origin. Photosynthetic ability was inversely related to the concentration of either Na + or Cl − in the leaf laminas sampled at the end of the experimental period. However, the concentration of Cl − expressed on a tissue water basis was greater, exceeding 300 moles per cubic meter, and Cl − was more closely associated (R 2 = 0.926) with the inhibition of photosynthetic ability. Leaf turgor was not reduced by salinization and leaf osmotic potential decreased to a slightly greater extent than the osmotic potential decreases of the nutrient solutions. Concentration of accumulated Na + and Cl − (on a tissue water basis) accounted quantitatively for maintenance of leaf osmotic balance, assuming that these ions were sequestered in the vacuoles. 相似文献
18.
Individual groups of peach ( Prunus persica [L.] Batsch) seedlings stressed to −17, −26 and −36 bars recovered to control levels within 1, 3, and 4 days, respectively. Stomatal resistance was significantly correlated with both leaf water potential and net photosynthesis. In seedlings stressed to −52 bars, leaf water potential and stomatal resistance recovered sooner than net photosynthesis, despite recovery of 0 2 evolution at a rate similar to leaf water potential. Therefore, some nonstomatal factor other than reduction in photochemical activity must be responsible for the lag in recovery of CO 2 assimilation following irrigation. 相似文献
19.
Environmental factors that induce spatial heterogeneity of stomatal conductance, g
s, called stomatal patchiness, also reduce the photochemical capacity of CO 2 fixation, yet current methods cannot distinguish between the relative effect of stomatal patchiness and biochemical limitations
on photosynthetic capacity. We evaluate effects of stomatal patchiness and the biochemical capacity of CO 2 fixation on the sensitivity of net photosynthetic rate ( P
N) to stomatal conductance ( g
s), θ (θ = δ P
N/ g
s). A qualitative model shows that stomatal patchiness increases the sensitivity θ while reduced biochemical capacity of CO 2 fixation lowers θ. We used this feature to distinguish between stomatal patchiness and mesophyll impairments in the photochemistry
of CO 2 fixation. We compared gas exchange of sunflower ( Helianthus annuus L.) plants grown in a growth chamber and fed abscisic acid, ABA (10 −5 M), for 10 d with control plants (-ABA). P
N and g
s oscillated more frequently in ABA-treated than in control plants when the leaves were placed into the leaf chamber and exposed
to a dry atmosphere. When compared with the initial CO 2 response measured at the beginning of the treatment (day zero), both ABA and control leaves showed reduced P
N at particular sub-stomatal CO 2 concentration ( c
i) during the oscillations. A lower reduction of P
N at particular g
s indicated overestimation of c
i due to stomatal patchiness and/or omitted cuticular conductance, g
c. The initial period of damp oscillation was characterised by inhibition of chloroplast processes while stomatal patchiness
prevailed at the steady state of gas exchange. The sensitivity θ remained at the original pre-treatment values at high g
s in both ABA and control plants. At low g
s, θ decreased in ABA-treated plants indicating an ABA-induced impairment of chloroplast processes. In control plants, g
c neglected in the calculation of g
s was the likely reason for apparent depression of photosynthesis at low g
s.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
Inhomogeneous photosynthetic activity has been reported to occur in drought-stressed leaves. In addition, it has been suggested that these water stress-induced nonuniformities in photosynthesis are caused by “patchy” stomatal closure and that the phenomenon may have created the illusion of a nonstomatal component to the inhibition of photosynthesis. Because these earlier studies were performed with nonacclimated growth chamber-grown plants, we sought to determine whether such “patches” existed in drought-treated, field-grown plants or in chamber-grown plants that had been acclimated to low leaf water potentials (ψ leaf). Cotton ( Gossypium hirsutum L.) was grown in the field and subjected to drought by withholding irrigation and rain from 24 d after planting. The distribution of photosynthesis, which may reflect the stomatal aperture distribution in a heterobaric species such as cotton, was assayed by autoradiography after briefly exposing attached leaves of field-grown plants to 14CO 2. A homogeneous distribution of radioactive photosynthate was evident even at the lowest ψ leaf of −1.34 MPa. “Patchiness” could, however, be induced by uprooting the plant and allowing the shoot to air dry for 6 to 8 min. In parallel studies, growth chamber-grown plants were acclimated to drought by withholding irrigation for three 5-d drought cycles interspersed with irrigation. This drought acclimation lowered the ψ leaf value at which control rates of photosynthesis could be sustained by approximately 0.7 MPa and was accompanied by a similar decline in the ψ leaf at which patchiness first appeared. Photosynthetic inhomogeneities in chamber-grown plants that were visible during moderate water stress and ambient levels of CO 2 could be largely removed with elevated CO 2 levels (3000 μL L −1), suggesting that they were stomatal in nature. However, advanced dehydration (less than approximately 2.0 MPa) resulted in “patches” that could not be so removed and were probably caused by nonstomatal factors. The demonstration that patches do not exist in drought-treated, field-grown cotton and that the presence of patches in chamber-grown plants can be altered by treatments that cause an acclimation of photosynthesis leads us to conclude that spatial heterogeneities in photosynthesis probably do not occur frequently under natural drought conditions. 相似文献
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