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Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies
Abstract
The effects of abscisic acid (ABA) on photosynthesis in leaves of Helianthus annuus L. were compared with those in leaves of Vicia faba L. After the ABA treatment, the response of photosynthetic CO2 assimilation rate, A, to calculated intercellular partial pressure of CO2, Pi, (A(pi) relationship) was markedly depressed in H. annuus. A less marked depression was also observed in V.faba. However, when the abaxial epidermes were removed from these leaves, neither the maximum rate nor the CO2 response of photosynthetic oxygen evolution was affected by the application of ABA.
Starch-iodine tests revealed that photosynthesis was not uniform over the leaves of H. annuus treated with ABA. The starch content was diffferent in each bundle sheath extension compartment (the smallest subdivision of mesophyll by veins with bundle sheath extensions, having an area of ca. 0.25 mm² and ca. 50 stomata). In some compartments, no starch was detected. The distribution of open stomata, examined using the silicone rubber impression techniques, was similar to the pattern of starch accumulation. In V.faba leaves, which lack bundle sheath extensions, distribution of starch was more homogeneous.
These results indicate that the apparent non-stomatal inhibition of photosynthesis by ABA deduced from the depression of A(pⁱ) relationship is an artifact which can be attributed to the non-uniform distribution of transpiration and photosynthesis over the leaf. Intercellular gaseous environment in the ABA-treated leaves is discussed in relation to mesophyll anatomy.
... Accordingly, the method estimating C c based on the Farquhar-von Caemmerer-Berry (FvCB) model (Farquhar et al. 1980) combined with chlorophyll fluorescence (Harley et al. 1992;Flexas et al. 2006) might be valid during the progressive drought in the present study, because Rubisco activity was not affected and electron flow was considered to be consumed mainly by photosynthesis and photorespiration. Conversely, as the reduction of photosynthetic rate should be attributed to the restriction of CO 2 entry by stomatal closure throughout the progressive drought (Medrano et al. 2002;Bota et al. 2004;Flexas et al. 2006), the increases in C i observed at g s < 0.08 mol m −2 s −1 might result from overestimation due to stomatal patchiness (Terashima et al. 1988;Terashima 1992) and/or cuticular transpiration (Tominaga and Kawamitsu 2015;Boyer 2015). C i -overestimation due to stomatal patchiness is pronounced in heterobaric leaves that have bundle-sheath extensions across the upper and lower epidermis (Terashima et al. 1988;Terashima 1992). ...
... Conversely, as the reduction of photosynthetic rate should be attributed to the restriction of CO 2 entry by stomatal closure throughout the progressive drought (Medrano et al. 2002;Bota et al. 2004;Flexas et al. 2006), the increases in C i observed at g s < 0.08 mol m −2 s −1 might result from overestimation due to stomatal patchiness (Terashima et al. 1988;Terashima 1992) and/or cuticular transpiration (Tominaga and Kawamitsu 2015;Boyer 2015). C i -overestimation due to stomatal patchiness is pronounced in heterobaric leaves that have bundle-sheath extensions across the upper and lower epidermis (Terashima et al. 1988;Terashima 1992). Because Japanese white birch has heterobaric leaves, we considered that stomatal patchiness and/ or cuticular transpiration should be the main causes of the increases in C i in the leaves exhibiting water deficit stress. ...
Constant mesophyll conductance (gm), and two-resistance gm model (involved in resistances of cell wall and chloroplast), where gm reaches maximum under higher CO2 concentrations, cannot describe the phenomenon that gm decreases with increasing intercellular CO2 concentration (Ci) under relatively higher CO2 concentrations. Yin et al. (2020) proposed a gm model, according to which the ratio of chloroplastic CO2 concentration (Cc) to Ci is constant in the two-resistance gm model, which can describe the decreasing gm with increasing Ci. In the present study, we investigated the relationship between Cc and Ci in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO2 concentrations, light intensities, and during progressive drought. Across the range of ambient CO2 from 50 to 1000 μmol mol−1, and light intensities of 50 to 2000 μmol m−2 s−1, measured under well irrigation, the ratio of Cc to Ci kept constant. During the progressive drought, overestimated Ci due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08 mol H2O m−2 s−1) from the A/Ci response measured under adequate irrigation. The ratio of Cc to Ci during progressive drought (predawn leaf potential reached ≈ − 2 MPa) also remained constant irrespective of soil drying rate in various pot sizes. The present study suggests the involvement of some physiologically regulative mechanisms to keep Cc:Ci ratio constant, which might act on gm in addition to the physical interaction of diffusive resistances in the cell components.
https://link.springer.com/article/10.1007/s11120-020-00788-x
... Another concern at high VPD or under water stress is the occurrence of areas where the stomata remain closed, a phenomenon called patchiness. Patchiness effectively decreases the photosynthesising leaf area, making it different from the one used in the c i calculations (Mott & Buckley, 2000;Terashima et al., 1988). ...
Understanding the short‐term responses of mesophyll conductance ( g m ) and stomatal conductance ( g sc ) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short‐term responses, which underpin CO 2 diffusion within leaves. Recent methodological advances in measuring g m using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short‐term g m responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO 2 concentration indirectly impact g m through g sc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short‐term responses of g m are not, or at least not fully, mechanistically linked to changes in g sc , cautioning against using g sc as a reliable proxy for g m . The overarching challenge lies in unravelling the mechanistic basis of short‐term g m responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of g m behaviour under changing environmental conditions.
... At the same, there appears to be evidence for PBT behavior throughout all scales of biology (Bradford, 2018). At the leaf level, stomatal opening and closing have been described in population terms (Laisk et al., 1980), and individual guard cells exhibit wide variation in their response to ABA (Terashima et al., 1988) that has been confirmed at the individual receptor level (Zhang et al., 2020). At the transcriptional level, individual cells respond to heat stress in a quantal manner, or cell by cell, such that increases in transcription of responsive genes across tissues are due to turning on more cells, rather than all cells gradually increasing their rates of transcription (Alamos et al., 2021). ...
... Moreover, sugars could also be produced as a result of partial photosynthesis due to non-uniform stomatal closure in response to increased ABA levels under drought conditions [79]. These sugars were not completely utilized for fruit volume growth even after irrigation was resumed [80]. An increase in fruit sugar content is usually associated with improved TSS [78]. ...
Evolved in South Africa and released to market in 2009, the ‘African Rose’ plum has been introduced and grown under the Egyptian semi-arid conditions since 2010. Within that time, this cultivar has faced significant fruit quality issues, mainly poor color and low total soluble solids (TSS). Several trials using foliarly applied growth regulators have been conducted, but with little conspicuous results on fruit yield and quality. There is very limited information about the relationship between irrigation regime and fruit quality for this cultivar. Therefore, a field experiment was conducted to study the effect of deficit irrigation on the quality of the ‘African Rose’ plum during the 2019 and 2020 seasons. Five-year-old hedge growing trees were subjected to three deficit irrigation regimes: 100% (control), 80%, and 60% of the crop evapotranspiration (ETc) after the pit hardening stage until the end of the harvest season (May to June period) were evaluated. Results indicated that deficit irrigation positively enhanced the levels of abscisic acid (ABA), total phenols, and anthocyanins with improved fruit TSS and maturity index, although fruit yield, acidity, size, and firmness were decreased. Deficit irrigation could be suggested as a sustainable novel solution to improve the fruit quality of the ‘African Rose’ plum grown under the semi-arid conditions of Egypt. Although the total yield and some quality characteristics were not improved, the early harvested fruit with enhanced color and taste could be a good start for additional research to solve other quality-related issues under such conditions.
... Disagreements between A and photosynthetic biochemical parameters calculated from gas exchange measurement data have been noticed (Flanagan and Jefferies, 1989). This phenomenon has been attributed to ununiformed leaf stomatal closure in stress thus the consequent overestimated Ci and underestimated A/Ci curve fitting (Brugnoli and Björkman, 1992;Downton et al., 1985;Downton et al., 1988;Flanagan and Jefferies, 1989;Terashima et al., 1988). On the other hand, measuring CO2 response curves at nonsaturating PPFD or without a match step before each CO2 gradient changing might have also led to underestimations of calculated photosynthetic biochemical activities (Chen et al., 2006). ...
The cultivation of figs (Ficus carica L.) in Taiwan has been exclusively for fresh markets and completely relied on protected cultivation to avoid the undesirable high precipitation. Consequently, high evapotranspiration, over-fertilization and the lack of nature flushing cause salt accumulation in the soil, thus hindering growth and production. In this study, three concentrations (0%, 0.1%, and 0.7% NaCl)(w./medium weight) were applied to container-grown ‘Noire de Caromb’ figs to examine the effects of salinity on growth and photosynthetic regulations. Aboveground growth and leaf gas exchange were measured after salinity treatment. Data from net assimilation rate (A) against intercellular CO2 concentration (Ci) were fitted to FvCB biochemical model. The salinity treatment with 0.1% NaCl slightly reduced the length of shoots and the number of leaves but 0.7% NaCl treatment significantly suppressed length of shoots, total leaf area, number of leaves and new leaf size. No difference in A among treatments was detected through the 95-day salinity stress period. Significant reduction in gs and transpiration rate (E), and consequent improving intrinsic water use efficiency (WUEi) were detected at 10 days after treatment in plants subjected to 0.7% NaCl. However, the regulation of gs and the positive response of WUEi appeared temporary and diminished thereafter. FvCB model fitting outputs showed that photosynthesis efficiency was reduced at 95 days after 0.7% NaCl treatment, due to suppressed maximum rate of rubisco regeneration (Vcmax) and maximum rate of electron transport (Jmax)
... Still, differential stomatal sensitivity among wheat genotypes may be useful in selecting varieties that are best suited for varying drought scenarios [24]. Usually, g s and hence A vary across the crop canopy due to stomatal patchiness and differences in microclimate [10,[27][28][29][30][31]. In some cases, the relationship between A and g s changes as leaves age, where A declines before g s , thereby decreasing WUE i [23]. ...
Leaf-level water use efficiency (WUEi) is often used to predict whole plant water use efficiency (WUEwp), however these measures rarely correlate. A better understanding of the underlying physiological relationship between WUEi and WUEwp would enable efficient phenotyping of this important plant trait to inform future crop breeding efforts. Although WUEi varies across leaf age and position, less is understood about the regulatory mechanisms. WUEi and WUEwp were determined in Australian (cv. Krichauff) and UK (cv. Gatsby) wheat cultivars. Leaf gas exchange was measured as leaves aged and evaluated in relation to foliar abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC) concentration, chlorophyll content and Rubisco activity. Carbon dioxide (CO2) assimilation (A) declined more rapidly as leaves aged in the lower WUEwp genotype Gatsby. Both ACC concentration and Rubisco activity declined as leaves aged, but neither explained the variation in A. Further, stomatal conductance (gs) and stomatal sensitivity to ABA were unchanged as leaves aged, therefore WUEi was lowest in Gatsby. Maintenance of A as the leaves aged in the Australian cultivar Krichauff enabled greater biomass production even as water loss continued similarly in both genotypes, resulting in higher WUEwp.
Leaf dark respiratory CO 2 ‐release ( R D ) is, according to some literature, dependent on the rate of leaf transpiration. If this is true, then at a given vapor pressure deficit, the leaf stomatal conductance ( g s ) will be expected to be a controlling factor of measured R D at any given time. We artificially lowered leaf g s by applying abscisic acid (ABA). Although leaf R D generally covaried temporally with g s , artificially lowering g s by applying ABA does not affect the measured leaf R D . These results indicate that observed diel fluctuations in g s are not directly influencing the measured leaf R D , thereby simplifying both future studies and the interpretation of past studies of the underlying environmental‐ and physiological drivers of temporal variation in leaf R D .
In the southeast Asian lowland dipterocarp forest of Peninsular Malaysia (Pasoh Forest Reserve), upper canopy tree species with heterobaric leaves show severe midday depression of net assimilation rate (A) accelerated by patchy stomatal closure, although it is still unclear whether it always happens or not. We added the in situ observations at various meteorological conditions over the last decade to evaluate the frequency and environmental conditions of patchy stomatal closure in Dipterocarpus sublamellatus Foxw., an emergent tree with heterobaric leaves through leaf-gas exchange measurements combined with numerical simulations and chlorophyll fluorescence imaging. Our observation revealed that on days with moderate leaf temperature (<35 °C) and vapor pressure deficit (VPD), which consist four of nine measurement days during 2003−2015, uniform stomatal behavior could explain observed A during the day. The patterns of stomatal behavior shifted from ‘uniform’ to ‘patchy’ even within a day depending on increases in leaf temperature and VPD according to irradiation of sunlight. However, it did not return from ‘patchy’ to ‘uniform’ once patchy stomatal closure happened in a day.
West Africa suffers from climate uncertainty, high levels of variability, lack of access to real-time and future climate information, and poor predictive capacity are common barriers to adaptation though the region is identified as climate-change hotspot. The regions vulnerability is heightened by its overdependence on rain-fed agriculture, with its sensitivity to climate change and variability. Rain-fed agriculture contributes 30% of GDP and employs about 70% of the population, and it is the main safety net of the rural poor. Drought affects plantain production in West Africa resulting in high level of food insecurity among the vulnerable. Under rain-fed production, achievable yield of plantain landraces are 11.0t/ha while the potential yield is 20t/ha. Plantain breeding is limited due to the complex nature of the breeding process. Plantain plant itself is a giant herbaceous plant occupying 6m2 of land hence only 1667 could occupy a hectare of land. The production system is faced with a myriad of challenges from inadequate healthy planting materials at the time of plantain through the production system to post-harvest. Unlike bananas which are mainly produce by multinational companies under irrigation, plantain production is mainly by smallholder farmers under rain-fed agriculture. Whereas banana production under irrigation is efficient, plantain production under rainfed production is haphazard and unsustainable. Farm sizes range between 0.4 to 5ha. Major limiting factors of the rainfed system is drought resulting in low yields and economic loses. Our study seeks to understand the effects of drought on the physiological responses of plantain crop under rainfed production. Different experiments were conducted to study the responses of various cultivars to on-farm rapid production of healthy planting materials. The results revealed that drought seriously affects natural regeneration of plantain planting materials as such new approaches need to be used for sufficient production. As each plantain plant produces averagely 42 leaves before flowering and each leaf has at least one axillary bud, it presupposes that 42 suckers should be produced at harvest. However, at harvest only 12 healthy suckers are produced around each plantain plant. The approach is able to exploit the full potential of every sucker to generate healthy planting materials. This technique therefore could be used to set up commercial propagation system for plantain planting material production. The study revealed Apantu and hybrid plantain FHIA-21 to show same responses. Plantain farmers can easily produce in large quantities healthy plantain planting materials using sawdust and building their own humidity chambers near water source. The study also showed that there is the potential for farmers to use this technique for establishing commercial propagation centres to generate healthy planting materials. We also studied the physiological responses of Apantu crop to water regimes and natural mycorrhization of plantains. Also, study was conducted on the fruit maturity index and the effect of climate variability on fruit micronutrient content. The natural mycorrhization of plantain roots was the first to be reported on plantain in Ghana. During the study it was observed that plantain roots were naturally colonised by mycorrhizae. However, this phenomenon was cultivar specific; with Apantu roots more colonised compared to Apem. Sustainable intensification of plantain production could be achieved through the use of beneficial soil microbes in production. Conventional production systems, however, do not promote the survival of these microbes. The study further revealed that plantains respond to water stress by reduction in stomatal density, movement and reduction in leaf area. In addition, early stages of water stress had little effect on the final yield of the crop. The anatomical and physiological studies were challenges faced in laticiferous plant like the plantain. However, drought stress at advance stage of growth of plantain adversely affect yields. Our study also showed that fruit maturity index could vary with seasons. The various maturity indices used in plantain production could not be used under rain-fed conditions; especially angularity. The angularity index used was observed not to be appropriate in the dry season. The study also revealed high α-carotene levels with seasonality and maturity in plantain. The high levels of provitamin A in plantain fruits during the dry season coincided with high incidence of suns UV index. This finding could play a significant role in the plantain industry as a food security crop the vulnerable who could not afford the high provitamin A foods especially for children under five years. However, the retention and bioavailability of the carotenoid after cooking need to be studied. In conclusion all the chapters showed clear understanding of the behaviour of plantain under severe adverse environmental conditions and conclusions drawn to guide future production of the crop. Under climate change with its complexities, further studies on plantain is needed to improve productivity to achieve food security in West Africa.
Cuticular conductance to water (gcw) is difficult to quantify for stomatous surfaces due to the complexity of separating cuticular and stomatal transpiration, and additional complications arise for determining adaxial and abaxial gcw. This has led to the neglect of gcw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of gcw, tested in two amphistomatous plant species.
What we term the ‘Red‐Light method’ is used to estimate gcw from gas exchange measurements and a known CO2 concentration inside the leaf during photosynthetic induction under red light. We provide an easy‐to‐use web application to assist with the calculation of gcw.
While adaxial and abaxial gcw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial gcw (γn) is stable within a plant species. This has implications for use of generic values of gcw when analysing gas exchange data.
The Red‐Light method can be used to estimate total cuticular conductance (gcw‐T) accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, allowing for a wide application of this technique.
Jones (1) has discussed two main reasons for attempting to manipulate stornata by breeding. The first was to maximise productivity and hence yield, by increasing rates of assimilation of CO2. An increase in stomatal conductance would be expected to achieve this in C3 species by increasing the partial pressure of CO2 within the leaf, p., which would be useful in conditions where water supply is not limiting. The second was to improve drought tolerance. A decreased stomatal conductance can aid in this direction, but at some expense to yield potential (yield in well-watered conditions).
In cotton the rate of CO2 assimilation, at O2 partial pressures up to 200 mbar, increased to a maximum and then declined as the intercellular partial pressure of CO2 was increased. The specific intercellular partial pressure of CO2 at which rate of assimilation began to decline depended on the environmental conditions. At 19 mbar partial pressure of O2 the decline occurred at CO2 partial pressure >390 µbar. At 200 mbar partial pressure of O2 it occurred at CO2 partial pressure > 534 µbar. O2 increased the CO2 partial pressure required for inhibition but it did not appear to affect the steepness of the decline of rate of assimilation with further increase in partial pressure of CO2 once the decline became apparent. The decline was more readily observed at low temperature and low O2 partial pressure, and in plants grown at low light and NO3- levels. It was also observed in cowpea and sunflower. Changes in quantum efficiency in cotton at high and low CO2 concentrations were observed. At ambient CO2 concentration (300 µbar), the quantum yields measured at 19 and 200 mbar partial pressure of O2 were 0.072 ± 0.0003 and 0.053 ± 0.0060 mol CO2 per mol absorbed quanta, respectively. In contrast, at 900 µbar CO2 partial pressure the respective values were 0.050 ± 0.0023 and 0.070 ± 0.0006 mol CO2 per mol absorbed quanta. The nature of the inhibition of CO2 assimilation by high partial pressure of CO2 is discussed.
The diurnal movements of the stomata in the leaves of various developmental stages of the rice plant under various weather conditions in submerged paddy field were studied by using the infiltration method for measuring the aperture of the stomata. The course of the diurnal movement of the stomata in the leaves of the rice plant grown under submerged paddy field was much affected by weather conditions from day to day. In a sunshiny day the aperture reached the maximum at about 8.30-9.00 a.m. and then decreased very quickly to only 1/2 or less of the maximum in the afternoon. On the contrary, in a cloudy day or not so fine day the aperture increased slowly in the morning to reach the maximum at about noon and in the afternoon the aperture was still kept wide for sometime. From the tillering stage to the heading stage the maximum of the aperture per day was practically the same irrespective of the weather condition except under very low light intensity. However after the heading stage the maximum per day became less. Furthermore, after reaching the maximum per day the aperture decreased more quickly compared with those before the heading. When the aperture became small in the afternoon of a sunshiny day the stomata of the leaves in the rice plant which were covered with polyethylene bag for 20-30 minutes opened again up to near the maximum in the day. From these facts it is assumed that the cause why the aperture of the stomata decreases in the afternoon of asunshiny day or after the heading stage may be due to the water unbalance in the leaves of the rice plant grown under submerged paddy condition.
The present study was conducted to clarify the differences of the aperture of the stomata and their diurnal movement of a leaf due to its position on the stem. In the fully-expanded leaves, the higher the position of a leaf on the stem, the wider was its stomatal aperture. The stomatal aperture of an expanding leaf was smaller compared with those of the adjacent lower fully-expanded leaf. During elongation stage of the internode the aperture of the most upper fully-expanded leaf was not so wide as in the adjacent lower one. The diurnal movements of the stomata of the leaves on a stem were different from each other depending on their position on the stem. Especially in the sunshiny day the stomatal aperture of the upper ful1y-expanded leaf did not decrease so quickly as those of the lower one in the afternoon, after reaching the maximurn per day, and this phenomenon could be more distinctly recognized after the heading stage. The results that the aperture of the stomata and their diurnal movement in the leaves were different from each other owing to their position on the stem were discussed in connection with the vascular, especially xylem, in the leaf sheath or the internode, the leaf age and the root age.
we have been making clear that the diffusion resistance of leaf-boundary layer changes with the attack angle of leaf to flow φ, through model experiments with flat plates and measurements of transpiration rate on sweet potato leaves. In this paper, effects of φ on the dry matter production DMP, the net photosynthetic rate Po, and the transpiration rate Tr of cucumber leaves are investigated.DMP under each φ condition is measured as the dry matter increase of leaves after 7 hours' irradiance in a growth chamber. The effect of φ on DMP is examined by comparing DMP under various conditions with that under φ=0° condition, i.e. the enlargement of DMP. The φ conditions both from 10° to 30° and from -10° to -30° give positive effect on the DMP of the leaves. The maximum enlargement of 38% is obtained under the φ=20° condition.The enlargement of Po, which is similarily defined in the case of DMP, changes with φ, as same as those of DMP. The maximum value 9.9% of the enlargement of Po is obtained under the φ=20° condition. Transpiration rate Tr also changes with φ, in the same fashion as Po. The effects of φ conditions on the enlargements of DMP, Po, and Tr are agree fairly well with those estimated by the previous model experiments. These results confirm experimentally that the diffusion resistance of leaf boundary layer significantly affects the photosynthesis, transpiration, and hence the plant growth.As a consequence, followings are revealed from present results and those in the previous model experiments. The diffusion resistance of leaf-boundary layer takes smallest value at an attack angle between 20° and 30°, when a single leaf, whose aspect ratio is ranging from 0.3 to 2.5, is affected in a laminar flow with wind velocity up to 3m/s.
The relationships between CO2 assimilation rate, RuP2 carboxylase activity and sizes of the pools of ribulose 1,5-bisphosphate (RuP2) and 3-phosphoglyceric acid (PGA) were examined using a freeze clamp device to rapidly freeze sections of attached leaves of R. sativus which previously had gas-exchange measurements made on them.
At high irradiance and ambient partial pressures of CO2 and O2, RuP2 carboxylase was fully active in vivo. Activity was less at very low CO2 pressures and at high CO2 pressures, particularly when combined with low O2 pressures. In vivo RuP2 carboxylase activity and both RuP2 and PGA pool sizes increased with increasing irradiance. RuP2 pool sizes were high at low CO2 pressures and decreased at high CO2 pressures. PGA pool sizes, on the other hand, were low at low CO2 and high at high CO2 pressures.
A model of RuP2 carboxylase-oxygenase (Rubisco) kinetics is used to examine the quantitative relationship between in vivo RuP2 carboxylase activity, CO2 assimilation rate and RuP2 and PGA pools. The model predictions fit in vivo data, except at high CO2 pressures, if it is assumed that RuP2 does not bind tightly to the inactive enzyme form in vivo. It is shown that a large fraction of the RuP2 and PGA pools may be chelated by magnesium in the stroma and that the high RuP2 pools (e.g. at low irradiance) may represent an optimal concentration rather than be truly saturating. We conclude that RuP2 pool sizes above Rubisco site concentration do not necessarily indicate a Rubisco limitation of photosynthetic rate.