Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

Effect of irradiance during growth of Glycine max on photosynthetic capacity and percent activation of ribulose 1,5-bisphosphate carboxylase

The initial (in vivo) and total (activity present after preincubation with CO₂ and Mg²⁺) activities of ribulose bisphosphate carboxylase were both assayed in extracts of leaves of soybean (Glycine max) plants which had been grown under 4 different irradiance levels. The total carboxylase activity per unit leaf area decreased with decreased irradiance during growth but was not different on a dry weight basis. The initial activity as a percentage of the total activity was unchanged (approximately 95%) except in leaves of plants grown at the lowest irradiance (74%). When the plants grown at the lowest irradiance were exposed to high irradiance, the initial activity was increased to 93% of the total. Light saturated rates of photosynthesis per unit leaf area were lower and saturated at lower irradiance for plants grown at lower irradiances. Initial carboxylase activity was correlated closely (r²=0.84) with leaf photosynthesis rate on a dry weight basis.     

Photosynthetic nitrogen-use efficiency of species that differ inherently in specific leaf area

Factors that contribute to interspecific variation in photosynthetic nitrogen-use efficiency (PNUE, the ratio of CO₂ assimilation rate to leaf organic nitrogen content) were investigated, comparing ten dicotyledonous species that differ inherently in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets at two irradiances (200 and 1000 μmol m^–2 s^–1). CO₂ and irradiance response curves of photosynthesis were measured followed by analysis of the chlorophyll, Rubisco, nitrate and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across species. High-SLA species had higher in situ rates of photosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUE_amb) was higher in those plants. Differences were somewhat smaller, but still present, when PNUE was determined at saturating irradiances (PNUE_max). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE. For plants grown under low irradiance, PNUE_amb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an overinvestment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic nitrogen to thylakoids and Rubisco, which further increased PNUE_amb. High-SLA species grown under high irradiance showed higher PNUE_amb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of N_org per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUE_max, differences between species in organic leaf nitrogen content per se were no longer important and higher PNUE_max of the high SLA species was due to a higher fraction of N in␣photosynthetic compounds (for low-light plants) and a higher Rubisco specific activity (for high-light grown plants). Received: 11 October 1997 / Accepted: 9 April 1998     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

Effects of Light, Carbon Dioxide, and Temperature on Photosynthesis, Oxygen Inhibition of Photosynthesis, and Transpiration in Solanum tuberosum

Individual leaves of potato (Solanum tuberosum L. W729R), a C₃ plant, were subjected to various irradiances (400-700 nm), CO₂ levels, and temperatures in a controlled-environment chamber. As irradiance increased, stomatal and mesophyll resistance exerted a strong and some-what paralleled regulation of photosynthesis as both showed a similar decrease reaching a minimum at about 85 neinsteins·cm⁻²·sec⁻¹ (about ½ of full sunlight). Also, there was a proportional hyperbolic increase in transpiration and photosynthesis with increasing irradiance up to 85 neinsteins·cm⁻²·sec⁻¹. These results contrast with many C₃ plants that have a near full opening of stomata at much less light than is required for saturation of photosynthesis.     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Leaf Factors Affecting Light-Saturated Photosynthesis in Ecotypes of Solidago virgaurea from Exposed and Shaded Habitats

Plants of Solidago virgaurea L. from exposed and shaded habitats differ with respect to the response of the photosynthetic apparatus to the level of irradiance during growth. An analysis was carried out on leaf characteristies which might be responsible for the differences established in the rates of Hght-saturated CO₂ uptake. The clones were grown in controlled environment chambers at high and low levels of irradiance. Light-saturated rates of photosynthesis and transpiration were measured at natural and lower ambient CO₂ concentrations. A low temperature dependence of light-saturated CO₂ uptake at natural CO₂ concentrations, and a strong response to changes in stomatal width, suggested that the rate of CO₂ transfer from ambient air towards reaetion sites in chloroplasts was mainly limiting the pholosynthetic rate. Resistances to transfer of CO₂ for different parts of the pathway were calculated. There was a weak but significant correlation between stomatal conductance and the product stomatal frequency ± pore length. Mesopbyll conductance and dry weight per unit area were highly correlated in leaves not damaged by high irradiance. This suggests that mesophyll conductance increases with increasing cross sectional area (per unit leaf area) of the pathways of CO₂ transfer in the mesophyll from cell surfaces to reaction sites. The higher light-saturated photosynthesis in clones from exposed habitats when grown at high irradiance than when grown at low irradiance was attributable mainly to a lower mesophyll resistance. In shade clones the effect upon CO₂ uptake of the increase in leaf thickness when grown at high irradiance was counteracted by the associated inactivation of the photosynthetic apparatus. The difference in CO₂ uptake present between clones from exposed and shaded habitats when preconditioned to high irradiance resulted from differences in both mesophyll and stomatal resistances. A few hybrid clones of an F₁-population from a cross between a clone from an exposed habitat and a clone from a shaded habitat reacted, on the whole, in the same way as the exposed habitat parent. When grown at high irradiance, the hybrid clones showed higher photosynthetic rates than either parent; this was largely attributable to the unusually low stomatal resistance of the hybrid leaves.     

Effects of phosphorus deficiency on the photosynthesis and respiration of leaves of sugar beet

Phosphorus deficiency was induced in sugar beet plants (Beta vulgaris L. var. F5855441), cultured hydroponically under standardized environmental conditions, by removal of phosphorus from the nutrient supply at the ten leaf stage 28 days after germination. CO₂ and water vapor exchange rates of individual attached leaves were determined at intervals after P cutoff. Leaves grown with an adequate nutrient supply attained net rates of photosynthetic CO₂ fixation of 125 ng CO₂ cm⁻² sec⁻¹ at saturating irradiance, 25 C, and an ambient CO₂ concentration of about 250 μl l⁻¹. After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic CO₂ uptake, photorespiratory evolution of CO₂ into CO₂-free air, and dark respiration, so that 30 days after cutoff these rates were about one-third of the control rates. The decrease in photosynthetic rates during the first 15 days after cutoff was associated with increased mesophyll resistance (r_m) which increased from 2.4 to 4.9 sec cm⁻¹, while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r_l′) from 0.3 to 0.9 sec cm⁻¹, as well as further increases in r_m to 8.5 sec cm⁻¹. Leaf diffusion resistance (r_l′) was increased greatly by low P at low but not at high irradiance, r_l′ for plants at low P reaching values as high as 9 sec cm⁻¹.     

Circadian rhythms in Kalanchoë: effects of irradiance and temperature on gas exchange and carbon metabolism

Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO₂ uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m^-2 and 24.0 h at 10 W m^-2. In bright light (80 W m^-2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO₂-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO₂ uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m^-2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEP phosphoenolpyruvate     

The influence of leaf area on the rooting physiology of leafy stem cuttings of Terminalia spinosa Engl.

Summary The effect of different leaf areas on the rooting of Terminalia spinosa Engl. cuttings in an non-mist propagation system in glasshouses at Edinburgh was investigated by trimming the leaves to 0, 7.5, 15 and 30 cm² before cuttings were severed from stockplants. Cuttings were taken to a standard length of 5 cm from the lateral shoots of previously pruned stockplants grown in a tropicalised glasshouse. During the rooting period, photosynthetic rate, stomatal conductance, water potential and relative water content of the cuttings were assessed at regular intervals. It was found that (i) removal of the entire leaf area prevented rooting; (ii) cuttings with a 7.5 cm², 15 cm² and 30 cm² leaf all achieved 80% rooting after 3 weeks; (iii) an increase in leaf area from 7.5 cm² to 30 cm² increased the rate of rooting and the length of the longest root after 2 weeks, but also increased the number of original leaves abscised after 6 weeks; and (iv) the greatest number of new leaves were produced by cuttings with 7.5 cm² and 15 cm² leaf area per cutting. All leafy cuttings actively photosynthesized during the propagation period, with a mean rate of 2 mol CO₂ m^-2 s^-1 with an irradiance of 100 mol m^-2 s^-1. Cuttings with 30 cm² leaf area had lower relative water contents, lower stomatal conductances and lower photosynthetic rates per unit leaf area than those with a 7.5 cm² and 15 cm² leaf. It was concluded that T. spinosa cuttings are easy to root, provided the cuttings have leaves to produce current assimilates.A member of the Edinburgh Centre for Tropical Forests     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

Shade adaptation of photosynthesis in Coffea arabica

The effect of irradiance on the rate of net photosynthesis was measured for mature leaves of coffee grown under five levels of radiation from 100% to 5% daylight. The rate of light-saturated photosynthesis per unit leaf area (P_Nmax) increased from 2 mol CO₂ m^-2 s^-1 under 5% daylight to 4.4 mol CO₂ m^-2 s^-1 under 100% daylight. The photon flux density (PAR, photosynthetically active radiation) needed for 50% saturation of photosynthesis, as well as the light compensation point, also increased with increasing levels of irradiation during growth. The quantum efficiency of photosynthesis (), measured by the initial slope of the photosynthetic response to increasing irradiance, was greater under shaded growth conditions. The rate of dark respiration was greatest for plants grown in full daylight. On the basis of the increase in the quantal efficiency of photosynthesis and the low light compensation point when grown under shaded conditions, coffee shows high shade adaptation. Plants adjusted to shade by an increased ability to utilize short-term increases in irradiance above the level of the growth irradiance (measured by the difference between photosynthesis at the growth irradiance, P_Ng, and P_Nmax).     

Novel characteristics of cassava,Manihot esculenta Crantz,a reputed C3-C4 intermediate photosynthesis species

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C₄-type photosynthesis in cassava leaves. In the present work we found no evidence for a C₄ metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, incorporation of ¹⁴CO₂ into C₄ organic acids in short time periods, 10 s, and the lack of ¹⁴C transfer from C₄ acids to other compounds in ¹²CO₂, ii) by the lack of C₄ enzyme activity changes during leaf development and the inability to detect C₄ acid decarboxylases, and iii) by leaf CO₂ compensation values between 49 and 65 l of CO₂ 1^–1 and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C₃ pathway of photosynthesis with no indication of a C₃-C₄ mechanism.However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with nearly 45% of the label in sucrose in about one min of ¹⁴CO₂ photosynthesis, contrasting with 34% in soybean (C₃) and 25% in pigweed (C₄). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 mol m^–2 s^–1. Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form fence-like arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.Abbreviations ADP adenosine diphosphate - Asp aspartate - BSA bovine serum albumin - CoA coenzyme A - DTT dithiothreitol - EDTA ethylenediaminetetraacetic acid - FBP fructose-1,6-biphosphate - Gly glycine - HEPES N-2-hydroxyethylpiperazine-N-2-ethansulfonic acid - Mal malate - NAD nicotinamide adenine dinucleotide (oxidized form) - NADH nicotinamide adenine dinucleotide (reduced form) - NADP nicotinamide adenine dinucleotide phosphate (oxidized form) - PAR photosynthetic active radiation (400–700 nm) - PEP phosphenolpyruvate carboxylase - p-FBPase plastid fructose-1,6-biphosphatase - PGA 3-phosphoglyceric acid - PMSF phenylmethylsulfonyl fluoride - PVP polyvinylpyrrolidone - Rubisco ribulose-1,5-biphosphate carboxylase/oxygenase - RuBP ribulose-1,5-biphosphate - Ser serine - sugar-P sugar-phosphates     

Photosynthetic characteristics of leaves developed at different irradiances and temperatures: an extension of the current hypothesis

Photosynthetic characteristics at high measurement irradiance were analyzed for single leaves of two C₃ and one C₄ species grown under twenty one combinations of irradiance level, irradiance duration, and air temperature in order to test the idea that photosynthetic characteristies developed by leaves in different environments are controlled by the daily amount of photosynthesis. Photosynthetic rates per unit area and mesophyll conductances at 25°C and air levels of CO₂ and O₂, and parameters for two photosynthesis models were used to characterize the photosynthetic properties of the leaves. Leaves with highest values of the photosynthetic parameters for each species were often developed in environments with irradiance levels below saturation for photosynthesis, and with only 12 hours of irradiance per day. Lower air temperature during growth increased the photosynthetic characteristics for a given irradiance regime. Photosynthetic characteristics had higher correlation coefficients with daily photosynthesis of mature leaves divided by 24-hour leaf elongation rates of young leaves, than with daily photosynthesis alone, indicating that photosynthetic characteristics may be related to a balance between photosynthesis and leaf expansion.     

Photosynthetic characteristics of leaves developed at different irradiances and temperatures: an extension of the current hypothesis.

Photosynthetic characteristics at high measurement irradiance were analyzed for single leaves of two C₃ and one C₄ species grown under twenty one combinations of irradiance level, irradiance duration, and air temperature in order to test the idea that photosynthetic characteristics developed by leaves in different environments are controlled by the daily amount of photosynthesis. Photosynthetic rates per unit area and mesophyll conductances at 25°C and air levels of CO₂ and O₂, and parameters for two photosynthesis models were used to characterize the photosynthetic properties of the leaves. Leaves with highest values of the photosynthetic parameters for each species were often developed in environments with irradiance levels below saturation for photosynthesis, and with only 12 hours of iradiance per day. Lower air temperature during growth increased the photosynthetic characteristics for a given irradiance regime. Photosynthetic characteristics had higher correlation coefficients with daily photosynthesis of mature leaves divided by 24-hour leaf elongation rates of young leaves, than with daily photosynthesis alone, indicating that photosynthetic characteristics may be related to a balance between photosynthesis and leaf expansion.     

THE EFFECT OF NUTRIENT LIMITATION AND ITS INTERACTION WITH LIGHT UPON THE PRODUCTS OF PHOTOSYNTHESIS IN MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

The metabolic fate of photosynthetically-fixed CO₂ was determined by labeling samples of Merismopedia tenuissima Lemmerman for 30 min with NaH¹⁴CO₃ and analyzing its incorporation into low molecular weight compounds, polysaccharide and protein. In N- and P-sufficient cultures, relative incorporation into protein increased as the irradiance used during the labeling period was decreased to 20 μE · m^-2 s^-1. This pattern was found for cells grown at irradiances of either 20 or 180 μE · m^-2· s^-1, although incorporation into protein was greater in cultures grown at the higher irradiance. In N-limited continuous cultures, relative incorporation into protein was low, independent of growth rate, and the same for samples tested at 20 or 180 μE · m^-2· s^-1 irradiance. In contrast, ¹⁴C incorporation into protein by P-limited cultures increased as growth rate increased, and at relative growth rates greater than 0.25, the incorporation was greater at 20 than at 180 μE · m^-2· s^-1. However, the total RNA content and maximum photosynthetic rate of the cultures was the same at all growth rates tested. The interaction between nutrient concentration and light intensity was studied by growing-limited continuous cultures at the same dilution rate, but different irradiances. Relative incorporation into protein was highest in cultures grown at 20 μE · m^-2· s^-1, in which the relative growth rate was 0.4. These results suggest that photosynthetic carbon metabolism may respond to relative growth rate μ/μ_max rather than to growth rate directly.     

Some ecophysiological features in sun and shade leaves of tall beech trees

Some ecophysiological features in sun and shade leaves of tall European beech trees (Fagus sylvatica L.) growing in a natural forest stand were investigated. Quantitative leaf characteristics were followed in the field and under controlled conditions. In the sun leaves significantly higher rates of photosynthesis, photorespiration and dark respiration, and also photosynthetic CO₂ fixation capacity, photosynthetic productivity, and saturating, adaptation and compensating irradiances were found. Specific leaf mass, mean leaf area, stomata density and size as well as the chlorophyll content per unit dry mass were also significantly different in both types of the leaves. Higher photosynthetic efficiency in the shade leaves allows them a better utilization of the lower irradiance for carbon dioxide uptake. The importance of these findings for annual carbon gain of the shade tolerant European beech species is also discussed.     

Responses of C4 grasses to atmospheric CO2 enrichment

Summary The growth and photosynethetic responses to atmospheric CO₂ enrichment of 4 species of C₄ grasses grown at two levels of irradiance were studied. We sought to determine whether CO₂ enrichment would yield proportionally greater growth enhancement in the C₄ grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1^-1 CO₂ and 1,000 or 150 mol m^-2 s^-1 photosynthetic photon flux density (PPFD). An increase in CO₂ concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO₂. Plants grown in CO₂-enriched atmosphere had lower photosynthetic capacity relative to the low CO₂ grown plants when exposed to lower CO₂ concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO₂ compensation point for photosynthesis.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with “antisense” rbcS

Tobacco (Nicotiana tabacum L.) plants transformed with antisense rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO₂ at 20°C at 100, 300 and 750 mol·m^–2·s^–1 irradiance, and at 28°C at 100, 300 and 1000 mol·m^–2·s^–1 irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C _infRubisco ^supA ) was very low (0.01–0.03) at low growth irradiance, and still fairly low (0.24–0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that C _infRubisco ^supA increases as incident irradiance rises, (iii) When low-light (100 mol·m^–2·s^–1)-grown plants are exposed to high (750–1000 mol·m^–2·s^–1) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750–1000 mol·m^–2·s^–1) are suddenly exposed to high and saturating irradiance (1500–2000 mol·m^–2·s^–1), C _infRubisco ^supA remained relatively low (0.23–0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in sun leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the light reactions and Rubisco; this was shown by the low value of C _infRubisco ^supA (see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) Antisense plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light.Abbreviations A rate of photosynthesis - C _infRubisco ^supA flux control coefficient of Rubisco for photosynthesis - c_i internal CO₂ concentration - qE energy-dependent quenching of chlorophyll fluorescense - qQ photochemical quenching of chlorophyll fluorescence - NADP-MDH NADP-dependent malate dehydrogenase - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose-1,5-bisphosphate This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137).     

Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.