Respiratory CO2 Fluxes in Photosynthesizing Leaves of C3 Species Varying in Rates of Starch Synthesis1

The rates of CO₂ fixation and respiratory CO₂ fluxes in six C₃ species, namely Solanum tuberosum, Nicotiana tabacum, Arabidopsis thaliana, Hordeum vulgare, Triticum aestivum, and Secale cereale, were determined under steady-state photosynthesis. The plants may be divided into two groups: (a) cereals with a low rate of starch synthesis (7–5% of true photosynthesis); (b) plants with a high rate of starch synthesis (45–35% of true photosynthesis). In the light, primary and stored photosynthates are consumed as substrates for both respiratory and photorespiratory pathways. In leaves of cereals, the total rate of respiratory and photorespiratory decarboxylations of stored photosynthates was higher in the light than in the dark, while, in starch-synthesizing species, stored photosynthates were consumed at a higher rate in the dark. Under normal environmental conditions, respiratory decarboxylation of stored photosynthates was suppressed by light in all species studied. The total rate of respiration as the sum of decarboxylation of stored and primary photosynthates was not affected by light in cereals, but suppressed in starch-accumulating plants. This suppression was not compensated for by the additional supply of respiratory substrates from primary photosynthates in the light.     

Carbon partitioning into sorbitol,sucrose, and starch in source and sink apple leaves as affected by elevated CO2

Experiments were conducted in controlled growth chambers to evaluate how increases in CO₂ concentration ([CO₂]) affected carbon metabolism and partitioning into sorbitol, sucrose, and starch in various ages of apple leaves. Apple plants (Malus domestica), 1 year old, were exposed to [CO₂] of 200, 360, 700, 1000, and 1600 μl l⁻¹ up to 8 days. Six groups of leaves (counted from the shoot apex): leaves 1–5 (sink), 6–7 (sink to source transition), 8–9 (sink to source transition), 10–11 (nearly-matured source), 21–22 (mid-age source), and 30–32 (aged source), were sampled at 1, 2, 4, and 8 days after [CO₂] treatments for carbohydrate analysis. Increases in [CO₂] from a sub-ambient (200 μl l⁻¹) to an ambient level (360 μl l⁻¹) significantly increased the concentrations of sorbitol, sucrose, glucose, and fructose tested in all ages of leaves. Continuous increase in [CO₂] from ambient to super-ambient levels up to 1600 μl l⁻¹ also increased sorbitol concentration by ≈50% in source leaves, but not in sink and sink to source transition leaves. Increases in [CO₂] from 360 to 1600 μl l⁻¹, however, had little effect on sucrose content in all ages of leaves. Starch concentrations increased in all ages of leaves as [CO₂] increased. Rapid starch increases (e.g. 5-, 6-, 20-, and 50-fold increases for leaf groups 1–5, 6–7, 10–11, and 21–22, respectively) occurred from 700 to 1600 μl l⁻¹ [CO₂] during which increases in sorbitol concentration either ceased or slowed down. Our results indicate that changes in carbohydrates were much more responsive to CO₂ enrichment in source leaves than in sink and sink to source transition leaves. Carbon partitioning was favored into starch and sorbitol over sucrose in all ages of leaves when [CO₂] was increased from 200 to 700 μl l⁻¹, and was favored into starch over sorbitol from 700 to 1600 μl l⁻¹ [CO₂].     

Photosynthesis and transpiration of tomato and CO2 fluxes in a greenhouse under changing environmental conditions in winter

Responses of tomato leaves in a greenhouse to light and CO₂ were examined at the transient stage at the end of winter, when both photoperiod and irradiance gradually increase. Additionally, CO₂ fluxes were calculated for a greenhouse without supplementary lighting and without CO₂ enrichment based on CO₂ sinks (plant photosynthesis) and CO₂ sources (plant and substrate respiration). In January, tomato leaves in the greenhouse showed low photosynthesis with a maximum assimilation of 6–8 μmol CO₂ m⁻² s⁻¹, a quantum yield of 0.06 μmol CO₂ μmol⁻¹ photosynthetic active radiation (PAR) and a low light compensation point of 26 μmol PAR m⁻² s⁻¹, a combination which classifies them as shade leaves. In February, tomato leaves increased their light compensation point to 39 μmol PAR m⁻² s⁻¹ and quantum yield to 0.08, the former indicating the adaptation to increased irradiance and photoperiod. These tomato leaves increased their transpiration from 0.4 to 0.9 in January to ∼2 mmol H₂O m⁻² s⁻¹ in February. Both photosynthesis and transpiration were primarily limited by light but neither by stomatal conductivity nor by CO₂. In January, light response of photosynthesis, dark respiration and transpiration were negligibly affected by increasing CO₂ concentrations from 600 to 900 ppm CO₂ under low light conditions, indicating no benefit of CO₂ enrichment unless light intensity increased. In February, tomato leaves were photoinhibited at inherent greenhouse CO₂ concentrations on the first sunny day; this photoinhibition was further enhanced by an increased CO₂ concentration of 1000 ppm. CO₂ fluxes in the greenhouse appeared strongly dependent on solar radiation. After exceeding the light compensation point in the morning, greenhouse CO₂ concentrations decreased by 58 or by 110 ppm CO₂ h⁻¹ on a sunny day in January or February and by 23 ppm on overcast days in both months. Calculated per overall tomato canopy, plant photosynthesis contributed 42–50% to the morning CO₂ depletion in the greenhouse. Dark respiration of tomato leaves was ∼2 μmol CO₂ m⁻² s⁻¹ in January and ∼3 μmol CO₂ m⁻² s⁻¹ in February. This dark respiration resulted in rises of 15 and 17 ppm CO₂ h⁻¹ at night in the greenhouse compartment and was identified as primary source of CO₂. Respiration of the substrate used to grow the plants, which produced 7.3 ppm CO₂ h⁻¹, was identified as secondary source of CO₂. The combined plant and substrate respiration resulted in peaks of up to 900 ppm CO₂ in the greenhouse before dawn.     

ASSIMILATION OF 14CO2 BY CATKINS OF CARYA ILLINOENSIS AND APPARENT TRANSLOCATION TO THE POLLEN

Carbon-14 dioxide and autoradiography were employed to determine if the catkins (staminate inflorescence) of Carya illinoensis Koch ‘Stuart’ are a sink for photosynthates produced by the current season's leaves; to observe ¹⁴CO₂ assimilation by the catkin at different stages of physiological development; and to determine if photosynthates produced by the catkins are utilized in pollen production. For a brief period in their development, the catkins were a sink for photosynthates produced by the leaves of the current season. The catkins assimilated ¹⁴CO₂ from time of emergence from the bud until senescence, and photosynthates produced by the catkins were incorporated into the pollen.     

Refixation of respiratory CO2 in the ears of C3 cereals

The spatial arrangement of tissues within the ears of cereals,and gas exchange measurements on intact ears of barley and durumwheat suggest that respiratory CO₂ associated with grain-fillingprocesses, may be refixed close to its site of evolution. Apparentrefixation of respiratory CO₂ in intact ears was compared withthat in flag leaves, by feeding both organs with ¹⁴C-labelledsucrose and trapping ¹⁴CO₂ released by respiration. Apparentrefixation in the ears was twice that measured in flag leafblades of durum wheat genotype Durelle. In ears, the capacityof refixation of respiratory CO₂ at 210 mmol mol^–1 O₂ranged from 55% in barley genotype Roxana to 75% in barley genotypeHatif, and 60% in duwm wheat genotype Bidi 17. A low O₂ concentrationincreased refixation of respiratory CO₂ by up to 90%, 92% and82%, respectively. The occurrence of CO₂ refixation in the field,in a set of 12 barley genotypes grown under irrigated and rainfedMediterranean field conditions, was consistent with observedcarbon isotope ratios (     

Changing sources of soil respiration with time since fire in a boreal forest

Radiocarbon signatures (Δ¹⁴C) of carbon dioxide (CO₂) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ¹⁴C of soil respiration and soil CO₂ in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ¹⁴C with Δ¹⁴C of CO₂ evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ¹⁴C of respired CO₂ indicated marked variation in respiration sources in space and time. The ¹⁴C signature of respired CO₂ respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ¹⁴C greater (averaging ～120‰) than autotrophic respiration. The Δ¹⁴C of autotrophically respired CO₂ in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO₂ respired by black spruce roots in stands >40 years old had Δ¹⁴C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ～50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO₂ had Δ¹⁴C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ¹⁴C of soil respiration in younger successional stands dropped below those of the atmospheric CO₂.     

Photosynthetic and Respiratory Activity of Fruiting Forms within the Cotton Canopy

The supply of photosynthates by leaves for reproductive development in cotton (Gossypium hirsutum L.) has been extensively studied. However, the contribution of assimilates derived from the fruiting forms themselves is inconclusive. Field experiments were conducted to document the photosynthetic and respiratory activity of cotton leaves, bracts, and capsule walls from anthesis to fruit maturity. Bracts achieved peak photosynthetic rates of 2.1 micromoles per square meter per second compared with 16.5 micromoles per square meter per second for the subtending leaf. However, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age, nor was their photosynthesis as sensitive as leaves to low light and water-deficit stress. The capsule wall was only a minor site of ¹⁴CO₂ fixation from the ambient atmosphere. Dark respiration by the developing fruit averaged −18.7 micromoles per square meter per second for 6 days after anthesis and declined to −2.7 micromoles per square meter per second after 40 days. Respiratory loss of CO₂ was maximal at −158 micromoles CO₂ per fruit per hour at 20 days anthesis. Diurnal patterns of dark respiration for the fruit were age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata on the capsule wall of young fruit were functional, but lost this capacity with increasing age. Labeled ¹⁴CO₂ injected into the fruit interior was rapidly assimilated by the capsule wall in the light but not in the dark, while fiber and seed together fixed significant amounts of ¹⁴CO₂ in both the light and dark. These data suggest that cotton fruiting forms, although sites of significant respiratory CO₂ loss, do serve a vital role in the recycling of internal CO₂ and therein, function as important sources of assimilate for reproductive development.     

The use of C14O2 canopy techniques for measuring carbon transfer through the plant-soil system

Summary Methods for labelling growing plants by exposing them to C¹⁴O₂ under a cellulose acetate-butyrate canopy have been developed for laboratory and field use. The length of labelling ranged from 2 to 33 days and the C¹⁴O₂ content of the atmosphere was automatically controlled. This made it possible to measure carbon assimilation by the plants, transfer of photosynthates beneath ground and respiration of the roots.In the laboratory, root respiration of wheat plants was measured by separating the above and beneath ground plant parts using a RTV rubber partition. Half to two thirds of the assimilated carbon was found above ground, 15 to 25 per cent in the roots and shoot bases below the partition and 17 to 25 per cent was lost by underground respiration. The variability of these proportions was related to the stage of maturity of the plants.On native grassland, the relative above and beneath ground productivity was 50 per cent. The time required for the photosynthates to reach the roots at various depths ranged from 1 to 5 days and the amount of material deposited in the roots changed with time and soil moisture content. The use of tubes inserted at various depths beneath the canopy permitted sampling of soil air for C¹⁴ and CO₂ measurements. The soil C¹⁴O₂ flux indicated that root respiration during 8 days accounted for 24 per cent of the labelled carbon translocated to the roots after a two days labelling period.     

Yellowing and photosynthetic decline of barley primary leaves in response to atmospheric CO2 enrichment

The photosynthetic response of barley (Hordeum vulgare L. cv. Brant) primary leaves was studied as a function of chlorosis induced by CO₂ enrichment. Leaf yellowing, measured as changes of chlorophyll a and b, was more extensive in controlled environments at elevated (680 ± 17 µl l^?1) than at ambient (380 ± 21 µl l^?1) CO₂. Stomatal conductance of primary leaves was decreased by growth in elevated CO₂ between 11 and 18 days after sowing (DAS) when measured at both 380 and 680 µl l^?1 CO₂. Internal leaf CO₂ concentration (C_i) was also lower for elevated- compared to ambient-CO₂-grown primary leaves between 11 and 14 DAS. Results suggest that non-stomatal factors were responsible for the decreased photosynthetic rates of elevated- compared to ambient-CO₂-grown primary leaves 18 DAS. Various photochemical measurements, including quantum absorptance (α), minimal (F₀), maximal (F_m), and variable (F_v) chlorophyll fluorescence, as well as the F_v/F_m ratio, were significantly decreased 18 DAS in the elevated- compared to ambient-CO₂ treatment. Photochemical (qP) and nonphotochemical (qN) chlorophyll fluorescence quenching coefficients of 18-day-old primary leaves did not differ between CO₂ treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated- compared to ambient-CO₂-grown primary leaves 18 DAS. Concentrations of α-amino N (i.e. free amino acids) in barley primary leaves were increased by CO₂ enrichment 10 DAS, but subsequently, α-amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated- than in ambient-CO₂-grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO₂-dependent yellowing of barley primary leaves.     

In situ CO2 gas-exchange in fruits of a tropical tree,Durio zibethinus Murray

We examined the in situ CO₂ gas-exchange of fruits of a tropical tree, Durio zibethinus Murray, growing in an experimental field station of the Universiti Pertanian Malaysia. Day and night dark respiration rates were exponentially related to air temperature. The temperature dependent dark respiration rate showed a clockwise loop as time progressed from morning to night, and the rate was higher in the daytime than at night. The gross photosynthetic rate was estimated by summing the rates of daytime dark respiration and net photosynthesis. Photosynthetic CO₂ refixation, which is defined as the ratio of gross photosynthetic rate to dark respiration rate in the daytime, ranged between 15 and 45%. The photosynthetic CO₂ refixation increased rapidly as the temperature increased in the lower range of air temperature T _c (T _c <28.5 °C), while it decreased gradually as the temperature increased in the higher range (T _c 28.5 °C). Light dependence of photosynthetic CO₂ refixation was approximated by a hyperbolic formula, where light saturation was achieved at 100 mol m^–2 s^–1 and the asymptotic CO₂ refixation was determined to be 37.4%. The estimated gross photosynthesis and dark respiration per day were 1.15 and 4.90 g CO₂ fruit^–1, respectively. Thus the CO₂ refixation reduced the respiration loss per day by 23%. The effect of fruit size on night respiration rate satisfied a power function, where the exponent was larger than unity.     

Apoplastic Transport of 14C-Photosynthates Measured under Drought and Nitrogen Supply

Using water infiltration of the plant and individual shoots with the subsequent intercellular liquid extraction by the pressure chamber, dynamics of the movement ¹⁴C-photosynthates from cell to apoplast, and ¹⁴C distribution among photosynthetic products in mesophyll cells and apoplast were studied. The relative quantity of ¹⁴C-photosynthetes in leaf apoplast depended on growing conditions; drought increased, and nitrate supply decreased it. When the middle leaves absorbed ¹⁴CO₂, photosynthates moving down in stem phloem appeared in intercellular space, where they were transported up by transpiration stream. ¹⁴C-photosynthates entering to the apex and young leaves were utilized a accumulated, and photosynthates transported to the mature leaves were reloaded into the phloem and reexported. Thus, photosynthates circulated through the plant and were redistributed to the plant organs according to their transpiration. In leaf apoplast photosynthetic sucrose was partly hydrolyzed to glucose and fructose. This increased under high nitrogen supply. The result indicate that apoplast sucrose hydrolysis is the basic cause of the reduction of photosynthate flux from leaves when the nitrate concentration in soil increases.     

Distribution and utilization of assimilated carbon in red clover during the first year of vegetation

Summary The pattern of distribution of¹⁴C labelled assimilates and translocation with time was measured in red clover during one reproductive cycle. Measurements were made on whole plants grown outdoors in pots by exposing the aerial parts to¹⁴CO₂ during one photoperiod. Simultaneously, root respiration and N₂ fixation were recorded.At the beginning of the vegetative period, 2/3 of the assimilates remained in the leaves (basal leaves), and 1/3 were directed to the root system. Then the development of branches required as much as 40% of the C and the root allocation decreased. Reproductive structures diverted 17% of the current photosynthates. Nitrogen fixation was optimal during the maximum extension of the basal leaves and decreased during the development of branches. During this period, C allocation to the nodulated roots was high with an estimated amount of 3.2 mg of C per mg of N fixed.With time, translocation occured within the foliage, from basal leaves to the leaves of the branches and to the new basal leaves developed after senescence of the branches. Remobilization to the reproductive structures remained minimal indicating that flower and seed growth was supported by current photosynthesis.     

Utilization of Photosynthetically Fixed 14CO2 into Alkaloids in Relation to Primary Metabolites in Developing Leaves of Catharanthus roseus

Partitioning of current photosynthates towards primary metabolites and its simultaneous incorporation in leaf alkaloids was investigated in developing leaves of medicinally important Catharanthus roseus. Of the total ¹⁴CO₂ assimilated, the leaves at positions 1–6 fixed 8, 22, 25, 19, 13, and 8 %, respectively, and stem 3 %. Leaf fresh mass, chlorophyll content, and CO₂ exchange rate increased up to the third leaf. The total alkaloid content was highest in young actively growing leaves, which declined with age. Total ¹⁴C fixed and its content in ethanol soluble fraction increased up to the third leaf and then declined. The ¹⁴C content in primary metabolites such as sugars and organic acids was also highest in the 3^rd leaf. The utilization of ¹⁴C assimilates into alkaloids was maximum in youngest leaf which declined with leaf age. Hence the capacity to synthesize alkaloids was highest in young growing leaves and metabolites from photosynthetic pathway were most efficiently utilized and incorporated into alkaloid biosynthetic pathway by young growing leaves.     

Respiration and photosynthesis in cones of Norway spruce (Picea abies (L.) Karst.)

Summary Dark respiration and photosynthetic carbon dioxide refixation in purple and green Picea abies cones were investigated from budbreak to cone maturity. The rate of dark respiration per unit dry weight and CO₂ refixation capacity decreased during cone maturation. At the beginning of the growing season, photosynthetic CO₂ refixation could reduce the amount of CO₂ released by respiration in green and purple cones by 50% and 40%, respectively. The seasonal performance of the components of the cone carbon balance was calculated using information on the seasonal course of respiration, refixation capacity and the light response curves of cone photosynthesis, as well as the actual light and temperature regime in the field. The daily gain of CO₂ refixation reached 28%–34% of respiration in green and 22%–26% in purple cones during the first month of their growth, but decreased later in the season. Over the entire growth period refixation reduced carbon costs of cone production in both cone colour polymorphs by 16%–17%.     

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO₂ concentration on root respiration rates during short-term CO₂ exposure, and on plant growth during long-term CO₂ exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO₂ on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO₂. Respiration rates of citrus roots were unaffected by the CO₂ concentration used during the respiration measurements (200 and 2000 μmol mol⁻¹), regardless of the soil CO₂, concentration during the previous month (600 and 20 000 μmol mol⁻¹). Bean plants were grown with their roots exposed to either a natural CO₂ diffusion gradient, or to an artificially maintained CO₂ concentration of 600 or 20 000 μmol mol⁻¹. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO₂ pretreatment and the CO₂ concentration used during the respiration measurements (200–2000 μmol mol⁻¹). We conclude that soil CO₂ concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.     

Effects of Benzyladenine on Bean Leaf Senescence and the Translocation of 14C-Assimilates

Treating primary leaves of bean plants with benzyladenine (BA) greatly increased the retention of photosynthetic assimilates in the treated leaves. Within 24 hours of treatment, the BA treated primary leaves retained 70 % of their assimilates and maintained this high level throughout the period studied. In contrast, the primary leaves of control plants retained 30 % at week 2, increased retention to 80 % between week 4 and 5 and dropped to 50 % during senescence at week 6. When the trifoliate leaves of 5 week old plants were fed ¹⁴CO₂, less than 1 % of the total activity was recovered from the BA treated leaves. It is concluded that the retardation of leaf senescence by BA on intact plants is not due to mobilization of metabolites from other plant parts, but is associated with a high retention of photosynthates.     

Metabolism of <Superscript>14</Superscript>C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of ¹⁴C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO₂ concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO₂), true photosynthesis (at 0.03% CO₂), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO₂ evolution resulting from metabolization of exogenous ¹⁴C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both ¹⁴C-glycine absorption and ¹⁴CO₂ evolution were lower in the light as compared with darkness, which is explained by CO₂ reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO₂ evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO₂ evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO₂ evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.     

Relationship between Respiration and CAM-Cycling in Peperomia camptotricha

Mature leaves of well-watered Peperomia camptotricha show Crassulacean acid metabolism (CAM). Young leaves show CAM-cycling in which CO₂ uptake occurs during the day concomitant with a marked diurnal fluctuation of organic acids as in CAM. Evidence is presented suggesting that respiration is the source of CO₂ for nocturnal acid synthesis in leaves exhibiting CAM-cycling. Respiratory quotients for these leaves were consistently much less than unity despite the fact that the leaves metabolize starch. The conservation of CO₂ by refixation into acids at night represents about 17% of the total photosynthetically fixed CO₂ and about 50% of the total respiratory CO₂.     

Partitioning of Photosynthetically Fixed 14CO2 Into Oil and Curcumin Accumulation in Curcuma Longa Grown Under Iron Deficiency

Changes in leaf growth, photosynthetic efficiency, and incorporation pattern of photosynthetically fixed ¹⁴CO₂ in leaves 1 and 2 from plant apex, in roots, and rhizome induced in Curcuma by growing in a solution culture at Fe concentration of 0 and 5.6 g m^–3 were studied. ¹⁴C was incorporated into primary metabolites (sugars, amino acids, and organic acids) and secondary metabolites (essential oil and curcumin). Fe deficiency resulted in a decrease in leaf area, its fresh and dry mass, chlorophyll (Chl) content, and CO₂ exchange rate at all leaf positions. The rate of ¹⁴CO₂ fixation declined with leaf position, maximum being in the youngest leaf. Fe deficiency resulted in higher accumulation of sugars, amino acids, and organic acids in leaves at both positions. This is due to poor translocation of metabolites. Roots and rhizomes of Fe-deficient plants had lower concentrations of total photosynthate, sugars, and amino acids whereas organic acid concentration was higher in rhizomes. ¹⁴CO₂ incorporation in essential oil was lower in the youngest leaf, as well as incorporation in curcumin content in rhizome. Fe deficiency influenced leaf area, its fresh and dry masses, CO₂ exchange rate, and oil and curcumin accumulation by affecting translocation of assimilated photosynthates.     

Evidence for Photorespiration in Tropical Grasses

Sugarcane leaves respired in full light and the CO₂ evolved could be detected in sorghum or miaze photosynthesizing in the same closed system. A combination of radiometric and infra-red gas analysis techniques allowed the estimation of photorespiration (total CO₂ evolution in light) and photosynthesis at increasing light intensities and of dark respiration. Rates of CO₂ evolution approaching those of temperate zone plants occurred at lower light intensities but rapidly decreased with higher light. Smaller but significant quantities of ¹⁴CO₂ were released even at intensities approximating full sunlight in leaves of maize, sorghum and sugarcane. Highly efficient CO₂ capture may explain the low rates of photorespiration at high light intensities.