Photoassimilate Supply Research Articles

Effects of saline irrigation water and heat waves on potato production in an arid environment

Production of spring potato ( Solanum tuberosum L. cv. Désirée) on a deep sandy soil in the central highland of the Negev desert of Israel under drip irrigation with saline water (up to 6.2 dS m −1) was studied in the years 1992–1997. The objective of the study was to determine the effects of saline water irrigation on potato production in an arid environment with special focus on the interactions with weather conditions. Although yields were often high, salinity effects were evident in some years. Thus 1992 and 1996 yields were 6–7 kg m −2 and showed no significant effect of salinity, while a pronounced drop in yield with increasing salinity was observed in 1993 and 1994. Analysis of weather data for 1993–1994 suggests that the decline in yield was due to interactions between saline irrigation and prolonged heat wave events occurring during crop development. Further experimental work (1997) revealed that tuber yield was most sensitive to combined salt and heat stress when heat waves occurred at 40–60 days after emergence. The combined stress apparently leads to the collapse of mechanisms for avoiding salt accumulation in young expanding leaves, resulting in failure of vegetative growth recovery and a consequent reduction in the leaf area index and canopy functioning. The relationship between tuber sink demand and available photoassimilate supply at certain stages of plant development is discussed with reference to the ability of the potato plant to recover from the combined stress.

Quick and reversible inhibition of soybean root nodule growth by nitrate involves a decrease in sucrose supply to nodules.

The upper part of a nodulated soybean root hydroponically cultured in a glass bottle was monitored using a computer microscope under controlled environmental conditions, and the diameter of individual nodules was measured from 10-24 d after planting. The diameter of a root nodule attached to the primary root increased from 1 mm to 6 mm for 2 weeks under nitrogen-free conditions. The increase in diameter of the nodules was almost completely stopped after 1 d of supplying 5 mM nitrate, and was due to the cessation of nodule cell expansion. However, nodule growth quickly returned to the normal growth rate following withdrawal of nitrate from the solution. The reversible depression of nodule growth by nitrate was similar to the restriction of photoassimilate supply by continuous dark treatment for 2 d followed by normal light/dark conditions. In addition, the inhibitory effect of nitrate was partially alleviated by the addition of 3% (w/v) sucrose to the medium. Plant leaves were exposed to (11)C or (14)C-labelled carbon dioxide to investigate the effects of 5 mM nitrate on the translocation and distribution of photosynthates to nodules and roots. Supplying 5 mM nitrate stimulated the translocation rate and the distribution of labelled C in nitrate-fed parts of the roots. However, the (14)C partitioning to nodules decreased from 9% to 4% of total (14)C under conditions of 5 mM nitrate supply. These results indicate that the decrease in photoassimilate supply to nodules may be involved in the quick and reversible nitrate inhibition of soybean nodule growth.

Alternate coral–bryozoan competitive superiority during coral bleaching

Bleaching of corals results from the loss of their symbiotic algae (zooxanthellae) and/or pigments. The supply of photoassimilates provided by the zooxanthellae to the coral declines during bleaching and reduces the ability to activate energy-costly processes such as maintenance, growth and reproduction. In the present study we compared the competitive outcomes, expressed as overgrowth and changes in colony sizes of Oculina patagonica (an encrusting Mediterranean stony coral) and the bryozoan Watersipora sp., growing in contact with each other, during and between bleaching events. Year-round observations of tagged colonies showed alternating competitive outcomes: O. patagonica wins over Watersipora sp. between bleaching events, but loses during bleaching events. Using the 14C-point-labeling technique on coral tissue, we examined intra-colonial translocation of photosynthetic products from the point-tissue labeling towards interaction zones. In non-bleached O. patagonica, competition resulted in preferentially oriented translocation of 14C products to the interaction zone located up to 8 cm away from the tissue-labeling site. Sites opposite the interaction zone received significantly less labeled photoassimilates compared to the interaction zone. In bleached colonies (40–85% bleached surface area), such translocation did not occur, probably explaining the failure to compete with the encrusting neighbor Watersipora sp. Our findings demonstrate the importance of colonial integration and resource orientation for the competitive superiority of O. patagonica.

Bleaching effect on regeneration and resource translocation in the coral Oculina patagonica

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 234:119-125 (2002) - doi:10.3354/meps234119 Bleaching effect on regeneration and resource translocation in the coral Oculina patagonica M. Fine, U. Oren, Y. Loya* Department of Zoology, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel *Corresponding author. E-mail: yosiloya@post.tau.ac.il ABSTRACT: Bleaching of corals is the result of the loss of their symbiotic algae (zooxanthellae) and/or their pigments. The supply of photoassimilates provided by the zooxanthellae to the corals declines during bleaching and reduces their ability to activate energy-costly processes. In the present study we compared regeneration capabilities of unbleached Oculina patagonica colonies (an encrusting Mediterranean stony coral) with those of bleached and partly bleached colonies. Using the 14C point-labelling technique on coral tissue, we examined possible intra-colonial translocation of photosynthetic products from the site of tissue labelling to recuperating lesions in partly bleached versus unbleached intact colonies. The percentage recovery of 2 cm2 lesions inflicted on unbleached O. patagonica colonies was significantly higher than the percentage recovery of similar lesions within the bleached area of the partly bleached colonies. Totally bleached colonies showed no regeneration of lesions. Lesion regeneration in unbleached O. patagonica resulted in oriented intra-colonial translocation of 14C products towards recuperating lesions located up to 4-5 cm away. In partly bleached colonies (40 to 80%), such translocation did not occur, probably explaining the reduced recovery rates of lesions in these colonies. Our findings suggest a bleaching threshold of ca. 30% within O. patagonica colonies that determines the levels of colony integration and intra-colonial translocation of resources to regions of maximal demand. KEY WORDS: Coral bleaching · Lesion recovery · 14C-labelling · Colony integration · Resource translocation · Oculina patagonica Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 234. Online publication date: June 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Partitioning and conversion of 14C-photoassimilates in developing grains of wheat plants grown under field conditions in Cuba

Understanding the patterns of source–sink relationships in wheat under field conditions is crucial to determine the physiological factors limiting grain filling. The aim of this investigation was to assess the time-course of source–sink interactions in wheat after anthesis under Cuban field conditions. For this purpose, plants of wheat, cultivar Cuba C-204, were sown in the field at six dates with contrasting temperature conditions. The duration of grain filling, patterns of partitioning of 14C-photoassimilates between organs and incorporation of the label into starch in the kernels 24 h after feeding the flag leaf blade with 14CO 2, were investigated to evaluate source–sink balance. The proportion of 14C converted into starch in the kernels increased during the first 10 days of grain filling. Throughout the grain filling phase the proportion of total 14C fixed partitioned to the grains increased, whereas that allocated to the internodes decreased. At the beginning of grain filling, the patterns of 14C-photoassimilate partitioning differed among sowing dates. Additionally, the proportion of 14C incorporated into starch in the kernels correlated positively with the proportion of the label partitioned to the grains, but negatively with 14C accumulation in the internodes. This supports that the accumulation of 14C in the stem was closely related to the uptake capacity of the grains. The lack of such a relationship at later stages in grain development indicates a change from sink-limited to source-limited situation early after anthesis. In conclusion, the supply of photoassimilates under field conditions in Cuba limits grain filling, except the first days after anthesis, when it was sink-limited.

水耕培養液へのＮａＣｌ添加がキュウリ果実の糖濃度に及ぼす影響

Effects of 60 mM NaCl added into nutrient solution on the growth, concentrations of sugars, K, Na and Cl of cucumber (Cucumis sativus L. cv. Asomidori No.5) fruits were studied. In 1992 (Expt. 1) and 1993 (Expt. 2) springs, cucumber plants were solution-cultured in a greenhouse. The aeration rate of the nutrient solution was low and high in Expts. 1 and 2, respectively. When the 10th leaf was expanding, 60 mM NaCl was added to the solution. Both in Expts. 1 and 2, the initial growth of fruits was enhanced by NaCl. Thereafter, fruit growth at 60 mM NaCl was markedly suppressed in Expt. 1, whereas it was not much affected in Expt. 2. In Expt. 1, hexose concentration of fruits at 60 mM NaCI decreased with the suppression of fruit growth, whereas it was not much affected in Expt. 2. Possible involvement of the shortage of photoassimilate supply in the fruit growth suppression under salinity was discussed.

Diurnal changes in sucrose, nucleotides, starch synthesis and AGPS transcript in growing potato tubers that are suppressed by decreased expression of sucrose phosphate synthase

Sucrose export from leaves is high during the day and lower at night, when it depends on starch remobilisation. We have investigated the consequences of diurnal changes of photoassimilate supply for starch synthesis and other metabolic processes in growing potato tubers. Sucrose, the levels of the transcripts for SUS and AGPS, the levels of key metabolites, and the rate of synthesis of starch and other major end products, including protein and cell wall polysaccharides, increased twofold or more between the start and end of the light period. The stimulation of starch synthesis was accompanied by an increase of UDPglucose and ADPglucose, whereas glycolytic intermediates remained unaltered, revealing that sucrose synthase and ADP-glucose pyrophosphorylase are being co-ordinately regulated. Sucrose synthase is stimulated via an increase of its substrates, UDP and sucrose. UDP increases due to an increase of the overall uridine nucleotide pool, and a decrease of the UTP/UDP ratio that occurs in parallel with a decrease of the ATP/ADP ratio and adenylate energy charge when biosynthetic fluxes are high at the end of the day. Within the time frame of the diurnal changes, the changes in the SUS and AGPS transcript levels do not lead to significant changes in the encoded enzymes. Transformants with a progressive decrease of sucrose phosphate synthase expression, where diurnal changes in leaf sugar levels were damped, exhibited a progressive attenuation of the diurnal changes of sucrose, nucleotide sugars and nucleotides, and fluxes in their tubers. It is concluded that metabolic processes in tubers are tightly linked to the momentary supply of sucrose.

Cloning of a Picea abies monosaccharide transporter gene and expression – analysis in plant tissues and ectomycorrhizas

Ectomycorrhizas are formed between certain soil fungi and fine roots of woody plants. An important feature of this symbiosis is the supply of photoassimilates to the fungus. Hexoses, formed from sucrose in the common apoplast at the root/fungus interface, can be taken up by both plant and fungal monosaccharide transporters. Recently we characterised a monosaccharide transporter from the ectomycorrhizal fungus Amanita muscaria. This transporter was up-regulated in mycorrhizas, thus increasing the hexose uptake capacity of the fungal partner in symbiosis. In order to characterise host (Picea abies) root monosaccharide transporters, degenerate oligonucleotide primers, designed to match conserved regions from known plant hexose transporters, were used to isolate a cDNA fragment of a transporter by PCR. This fragment was used to identify a presumably full length clone (PaMST1) in a P. abies/A. muscaria mycorrhizal cDNA library. The entire cDNA code for an open reading frame of 513 amino acids, revealing best homology to H+/monosaccharide transporters from Ara- bidopsis, Saccharum and Ricinus. PaMST1 was highly expressed in the hypocotyl and in roots of P. abies seedlings, but not in needles. Mycorrhiza formation led to a slight reduction of PaMST1 expression. The results are discussed with special reference to carbon allocation in ectomycorrhizas.

The influence of growing season on fruit yield and quality of greenhouse melon (Cucumis melo L.) grown in nutrient film technique in a Mediterranean climate

SummaryThe influence of growing season on some physiological and biochemical variates related to fruit yield and quality was investigated in melon (Cucumis melo L.) plants cultivated in nutrient film technique in a greenhouse located at Pisa, Central Italy, from mid-March to mid-June, or from mid-July to mid-September. Compared with spring, the plants grown in summer exhibited faster growth and development, but produced fewer fruits of larger size and poorer quality due to reduced sucrose content. Growing season did not affect total leaf area, but dry-matter production and partitioning to the fruits was significantly lower in summer than in spring. Summer fruit ripened within 30–35 d after anthesis, about 14 d fewer than in spring. Higher average temperature was presumably responsible for earlier fruit maturation in summer, as in both seasons all melons were harvested after 450–500 degree-days (base temperature of 12°C) from anthesis. Fruit swelling did not account for the reduction of sucrose content in summer-grown fruits, which instead was due to shortage of photoassimilate supply and inadequate sucrose synthesis, as suggested by the rate of leaf gas exchange and the activity of sucrose phosphate synthase in the fruit flesh, as determined during the final stages of fruit development. Lower solar radiation was presumably responsible for the reduced leaf carbon assimilation in summer, as growing season did not affect leaf turgor, stomatal conductance, mineral status and chlorophyll content.

Photoassimilate Production and Distribution in Cherry

The two commercially important fruiting cherries in North America are Prunus cerasus L. (sour) and P. avium L. (sweet). Carbon partitioning between individual organs, reproductive and vegetative, is dependent upon photoassimilate supply from photosynthesis and storage carbohydrate, as well as the ability of the translocation system to deliver these resources to sinks (Layne and Flore, 1995). Sink strength of individual organs varies with time of year, and age of the plant, as the sink demands and seasonal development pattern of the plant changes with time. Herein we will review assimilate supply and storage, seasonal partitioning patterns, and the impact that biotic and abiotic stress has on supply, partitioning, and control.

Nitrogen deposition and ectomycorrhizas

As a result of increasing anthropogenic nitrogen deposition, N availability in many forest ecosystems, which are normally N‐limited, has been enhanced. We discuss the impacts of this increased N availability on the ectomycorrhizal (ECM) symbiosis which is generally regarded as an adaptation to nutrient limited conditions. Nitrogen deposition can influence fruit‐body formation by ECM fungi, the production and distribution of the extraradical mycelium in the soil and the formation of ectomycorrhizas.Available data from long‐term N deposition studies indicate that the most prominent effects might be discernible above‐ground (i.e. on the formation of fruit bodies). ‘Generalist’ species, forming a symbiosis with a wide range of tree species, seem to be less affected by increased N availability than ‘specialist’ species, especially those living in symbiosis with conifers. However, the importance of below‐ground investigations to determine the impacts of N deposition on the ECM symbiosis must not be underestimated. Culture experiments show an optimum N concentration for the formation of extraradical mycelium and mycorrhizas. Often, negative effects only become visible at comparatively high N concentrations, but the use of a few easily cultivated species of ECM fungi, which are adapted to higher N concentrations, undermines our ability to generalize.So far, N deposition experiments in the field have only shown minor changes in the below‐ground mycorrhizal population, as estimated from the investigation of mycorrhizal root tips. However, effects on the ECM mycelium, which is the main fungal component in terms of nutrient uptake, cannot be excluded and need further consideration.Because the photoassimilate supply from the plant to the fungal partner is crucial for the maintenance of the ECM symbiosis, we discuss the possible physiological implications of increasing N inputs on the allocation of C to the fungus. Together with ultrastructural changes, physiological effects might precede obvious visible changes and might therefore be useful early indicators of negative impacts of increasing N inputs on the ECM symbiosis.

Response of a hypernodulating soybean mutant to increased photosynthate supply

Growth chamber studies were conducted to determine if increased photoassimilate supply, through light enhancement and CO 2 enrichment, could reverse the deleterious plant growth and enhance nodule function traits of NOD1-3, a hypernodulating mutant of Williams. Both light enhancement and CO 2 enrichment increased nodule number, acetylene reduction activity plant −1 (but not specific activity) and dry matter accumulation in all tissues in both genotypes. Total biomass and specific nitrogenase activity were always less in the mutant than in Williams 82, indicating that the inferiority of the mutant may not be reversed by enhanced photoassimilate supply. Under all growth conditions, the mutant allocated relatively more photosynthate to nodules and less photosynthate to roots, compared to the control. Despite this, the decreased growth of the mutant relative to the control was not solely attributable to excessive nodulation of the mutant, since decreased growth was observed even on uninoculated plants. It is suggested that light enhancement and CO 2 enrichment may have stimulated nodulation through increased photosynthate supply, independent of the nodulation autoregulatory signal.

Relationship between Export Rate of Photoassimilates and Activation State of Sucrose Phosphate Synthase in Submerged Floating Rice.

Floating rice (Oryza sativa L.) responds to submergence by rapid internodal elongation. This stimulated growth needs an increase in the supply of photoassimilates. We investigated the effects of submergence on the export rate of photoassimilates determined by a 13C-tracer experiment and the activity of sucrose phosphate synthase (SPS), a key enzyme in the sucrose biosynthesis pathway, in floating rice. Plants at the 9.5 leaf stage were submerged up to the tip of the 7th leaf blade for 5 days. Export rates of the 13C-photoassimilates at the 9th (9L) and 10th (10L) leaf blades were much higher in the submerged plants than in the control. In order to analyze the activation state of SPS, the activities of SPS were assayed under saturated substrates (Vmax) and limiting substrates plus Pi (Vlimiting) . The Vlimiting of SPS of the 9L and 10L were higher in the submerged plants than in the control, whereas the Vmax did not differ between both plots. These results indicate that the SPS of the leaves in the upper position was more highly activated in the submerged plants. Moreover, the Vlimiting of. SPS was correlated positively with the export rate. These results suggest that an increase in the Vlimiting of SPS induced by a high activation state might accelerate the export rates of photoassimilates, and that this is probably one of the important factors that support the supply of photoassimilates required for the rapid growth of the sink organs in submerged floating rice.

Interspecific Competition among Phloem‐Feeding Insects Mediated by Induced Host‐Plant Sinks

The role of interspecific interactions among herbivorous insects is considered to be limited, especially in specialist communities. In the current study we report on exploitative interspecific interaction between two closely related phloem—feeding species of gall—forming aphids (Homoptera; Pemphigidae; Fordinae), mediated by the supply of photoassimilates from the host plant. Geoica sp. forms a spherical gall on the leaflet midrib of Pistacia palaestina (Anacardiaceae), while Forda formicaria forms crescent—shaped galls on the leaflet margin of the same host plant. Using 14C labeling, we were able to trace the food supply (assimilated carbohydrates) from the leaves to galls of each species. We found that Geoica galls are strong sinks. These galls divert the normal phloem transport of the plant and reduce the amount of assimilates imported by F. formicaria, especially when they are located on the same leaflet. By the end of the season Geoica caused death of 84% of F. formicaria galls that were located on the same leaflet, and reduced reproductive success in the surviving galls by 20%. This is because the presence of Geoica causes early senescence (but not abscission) of the leaflet it is on (whether or not F. formicaria is present). The interaction is asymmetrical: F. formicaria did not affect reproductive output of Geoica nor did it cause visible damage to the leaflets. To our knowledge, this it the first demonstration of exploitation competition for plant assimilates between two insect—induced sinks. This exploitative competition, mediated by manipulation of plant phloem transport, stands in contrast to the absence of interference competition for galling sites between the two aphid species. Although their spatial distributions partly overlapped, the niche breadth of each species (measured from gall positions on leaves along the shoot axis) was not affected by the presence of the other. Moreover, when both species were located on the same leaf, they formed galls independently on the same or different leaflets, and there was no indication of interference competition over galling sites.

Modification of Leaf Photosynthesis by Foliar Application of Aliphatic Alcohols

AbstractA decrease in net photosynthesis (Pn) in leaves of main shoot with age (7‐28 days after opening) at two stages of peanut (Arachis hypogaea L. var. M‐13) reproductive growth (70 and 90 DAS) is reported. Pn rate decreased with advance in age of leaves, the decline was rapid during peak period of pod filling (90 DAS). Foliar application with aliphatic alcohols, maintained the chlorophyll content and enhanced PEP carboxylase activity. The present studies suggest that aliphatic alcohols enhance functional capabilities of leaf for a larger duration and hence maintain the supply of photoassimilates to the developing kernels of peanut leading to increased yield.

Photosynthetic Rates and Solute Partitioning in Relation to Growth of Salt-Treated Pistachio Plants

Seedlings of Pistacia vera cv. Kerman were grown in a porous medium under glasshouse conditions and treated for up to 5 weeks with 0.5 (control), 100 or 175 mol m-3 Cl- in irrigation water, with accompanying cations Na+ , Ca2+ and Mg2+ in the molar ratio 6 : 1 : 1. Salinity treatments were raised to the respective levels by daily increments of 25 mol m-3 Cl-. Shoot growth was decreased by 100 mol m-3 CI- and ceased during treatment with 175 mol m-3 CI- . Turgor was maintained in expanding leaves during and after the period of incremental increase in salinity up to the maximum of 175 mol m-3 CI-. There was an increase in sucrose and reducing sugar concentrations on a tissue water basis in expanding leaves of plants after 2 days of treatment with 175 mol m-3 CI- but only a marginal increase in Cl- and Na+. Extension rates and final lengths of successively emerging leaves decreased with duration of treatment with 175 mol m-3 CI-. Emergence of new leaves eventually ceased on these plants. Bulk leaf Cl- and Na+ concentrations of expanded 42-day-old leaves after treatment of plants for 35 days with 175 mol m-3 CI- were 255 and 142 mol m-3, respectively, but despite these concentrations photosynthetic rates were not reduced. There was an increase in proline and a decrease in shikimic acid in these leaves. Rates of water vapour loss per unit leaf area of whole plants did not differ markedly between control and salt (175 mol m-3 CI-)-treated plants after 40 days of salt treatment. The salt- stressed plants accumulated higher sucrose and starch concentrations in the stem and higher concentrations of sucrose, reducing sugars and starch in the main root. Total carbohydrate concentrations were also marginally higher in the root tips of salt-treated plants. These data indicate that photosynthetic rate of expanded leaves, photoassimilate supply to, water relations of, and Cl- and Na+ concentrations of expanding leaves are unlikely to be responsible for the growth reduction of pistachio plants observed at relatively high salinities.

Lodging effects on high-yielding crops of irrigated semidwarf wheat

The effect of lodging caused by overturning in saturated soil was studied in high-yielding crops of irrigated semidwarf wheat ( Triticum aestivum L. and T. turgidum. L. var durum) during two years in northwest Mexico and one in southeast Australia. Plots lodged artificially or lodged naturally were compared to plots which never lodged, usually because of protection provided by supportive mesh. Culm lodging to an almost horizontal position caused grain yield to be reduced by 7–35%, with the greatest effect when lodging occurred in the first 20 days after anthesis. In one crop the yield reduction was clearly much less than in others with the same cultivar. Lodging just before anthesis was less deleterious, perhaps because the crop was able to right itself quickly by node bending. Kernel number per unit area tended to be reduced by early lodging, and kernel weight by later lodging accompanied by small increases in grain nitrogen percentage. Lodging during rainy ripening conditions increased grain sprouting. Lodging after anthesis reduced crop growth rate, and the adverse effect of lodging on grain yield is ascribed to this reduction in photoassimilate supply. The crop in which lodging had least effect on grain yield was characterized by a reduced degree of source limitation during grain filling.

Stomatal and Photosynthetic Limitations to Leaf Growth

Leaf area derives from the product of cell number and cell size within lamina tissues. Cell division thus dictates potential size, while cell enlargement is responsible for expression of that potential. These two facets of leaf growth differ in their substrate requirements under well nourished conditions and also show dissimilar sensitivity to environmental stress. Meristematic activity in terminal apices and subsequent emergence of successive leaves seem relatively insensitive to drought and salinity, but do appear limited by photoassimilate supply because CO2 input and radiation level exert strong effects. By contrast, lamina expansion depends more heavily on adequate supplies of water and nutrients, and is especially sensitive to environmental stresses. Impact of adverse conditions on leaf growth can be alleviated by physiological adjustments such as alteration of viscoelastic properties of lamina tissues, regulation of ionic balance and accumulation of organic osmotica. Effective adjustment becomes manifest as turgor maintenance, continuity of leaf growth and sustained gas exchange. This necessitates finely tuned water relations and draws upon energy and substrate of recent photosynthetic origin. Alleviation of drought and salinity stress is thus subject to stomatal and photosynthetic limitations.

Photosynthetic carbon dioxide exchange of bean plants grown at elevated carbon dioxide concentrations

Leaves of bean plants (Phaseolus vulgaris L. cv. Pure Gold Wax) grown in atmospheres enriched in CO2 (1400 μL L−1) showed a decrease in CO2 exchange capacity when compared with unenriched plants (340 μL L−1) measured at the same CO2 concentration. The decrease was not associated with changes in chlorophyll concentration or photorespiratory activity. The decrease was less evident in older leaves, in leaves maintained at low light intensity, and in those with reduced chlorophyll contents. Respiration rates in leaves of CO2-enriched plants increased only under conditions that caused a concurrent decrease in photosynthetic capacity. Enriched leaves had higher starch contents than unenriched leaves. The results were consistent with the idea that CO2 enrichment decreases photosynthetic capacity when photoassimilate supply exceeds sink demand.