Photosynthesis, Morphology, Leaf Anatomy, and Cytogenetics of Hybrids between C(3) and C(3)/C(4)Panicum Species

The Laxa group of the Panicum genus contains species which have CO₂ exchange and anatomical characteristics intermediate to C₃ and C₄ photosynthetic types (C₃/C₄), and also species characterized as C₃. Hybrids were made between two of the C₃/C₄ species and two C₃ species. Carbon dioxide exchange and morphological, leaf anatomical, and cytogenetic characteristics of F₁ hybrids between Panicum milioides Nees. ex Trin (C₃/C₄) and P. laxum Mez. (C₃), P. spathellosum Doell (C₃/C₄) and P. boliviense Hack. (C₃), and P. spathellosum and P. laxum were studied. There were no consistent differences in apparent photosynthesis, although two of the three hybrids had higher net CO₂ uptake than the C₃ parent. Values of inhibition of apparent photosynthesis by 21% O₂, CO₂ loss in the light, and CO₂ compensation concentration for the hybrids were between those of the parents. All three hybrids showed leaf anatomical traits, especially organelle quantities in the bundle sheath cells, between those of their respective parents. Linear regression of CO₂ compensation concentration on the percentage of mitochondria and chloroplasts in vascular bundle sheaths of the parents and hybrids gave correlation coefficients of −0.94. This suggests that the reduction in CO₂ loss in the C₃/C₄ species, and to a lesser degree in the F₁ hybrids, was due to development of organelles and perhaps a higher proportion of leaf photorespiration in bundle sheaths. The overall morphology of the hybrids was so different from the parents that they could be described as new taxonomic forms. The chromosomes in the hybrids were mainly unpaired or paired as bivalents indicating possible homology between some parental genomes.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Reduced Apparent Photorespiration by the C(3)-C(4) Intermediate Species, Moricandia arvensis and Panicum milioides

The CO₂/O₂ specificity factor of sucrose gradient purified ribulose 1,5-bisphosphate carboxylase/oxygenase from the C₃-C₄ intermediate plants Moricandia arvensis (79 ± 1) and Panicum milioides (89 ± 2) was similar to the respective values of the enzyme from the closely related C₃ species, Moricandia foetida (80 ± 5) and Panicum laxum (86 ± 2). Thus, the kinetic properties of this bifunctional enzyme do not explain the reduced rates of photorespiration exhibited by either of these intermediate species.     

Phosphoenolpyruvate carboxylase reduces photorespiration in Panicum milioides, a C3-C4 intermediate species.

Panicum milioides represents the first well-documented example of a higher plant species with reduced photorespiration and O₂ inhibition of photosynthesis. We have investigated the biochemical mechanism(s) involved in reducing O₂ sensitivity of photosynthesis in this species by parallel enzyme inhibitor experiments with thin leaf slices of P. milioides and C₃ and C₄Panicum species. The reduced O₂ sensitivity of net photosynthesis in P. milioides gradually increased with increasing concentrations of the phosphoenolpyruvate carboxylase (EC 4.1.1.31) inhibitors, maleate and malonate. At saturating levels of inhibitor, photosynthesis in 2% O₂ was decreased by about 18%, and the inhibitory effects of both 21% O₂ and 49% O₂ were identical to those observed with a C₃Panicum species in the absence or presence of inhibitor. A significant potential for C₄ photosynthesis in P. milioides, compared to its complete absence in a C₃Panicum species, was demonstrated on the basis of: (a) a coupling of leaf slice CO₂ fixation by phosphoenolpyruvate carboxylase with the C₃ cycle; (b) NAD-malic enzyme (EC 1.1.1.39)-dependent aspartate and malate decarboxylation in leaf slices; (c) a full complement of C₄ cycle enzymes in leaf extracts, including pyruvate, P_i dikinase (EC 2.7.9.1) and NAD-malic enzyme; and (d) Kranz-like leaf anatomy with numerous plasmodesmata traversing the mesophyll-bundle sheath interfacial cell wall. These data indicate that the reduced photorespiration and O₂ inhibition of photosynthesis in P. milioides is due to phosphoenolpyruvate carboxylase participation, possibly by creating a limited C₄-like CO₂ pump, rather than an altered ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39).     

Environmental Effects on Photorespiration of C(3)-C(4) Species : I. Influence of CO(2) and O(2) during Growth on Photorespiratory Characteristics and Leaf Anatomy

The possibility of altering CO₂ exchange of C₃-C₄ species by growing them under various CO₂ and O₂ concentrations was examined. Growth under CO₂ concentrations of 100, 350, and 750 micromoles per mole had no significant effect on CO₂ exchange characteristics or leaf anatomy of Flaveria pringlei (C₃), Flaveria floridana (C₃-C₄), or Flaveria trinervia (C₄). Carboxylation efficiency and CO₂ compensation concentrations in leaves of F. floridana developed under the different CO₂ concentrations were intermediate to F. pringlei and F. trinervia. When grown for 12 days at an O₂ concentration of 20 millimoles per mole, apparent photosynthesis was strongly inhibited in Panicum milioides (C₃-C₄) and to a lesser degree in Panicum laxum (C₃). In P. milioides, acute starch buildup was observed microscopically in both mesophyll and bundle sheath cells. Even after only 4 days exposure to 20 millimoles per mole O₂, the presence of starch was more pronounced in leaf cross-sections of P. milioides compared to those at 100 and 210 millimoles per mole. Even though this observation suggests that P. milioides has a different response to low O₂ with respect to translocation of photosynthate or sink activity than C₃ species, the concentration of total available carbohydrate increased in shoots of all species by 33% or more when grown at low O₂. This accumulation occurred even though relative growth rates of Festuca arundinacea (C₃) and P. milioides grown for 4 days at 210 millimoles per mole O₂, were inhibited 83 and 37%, respectively, when compared to plants grown at 20 millimoles per mole O₂.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: V. RESPONSE OF PANICUM MAXIMUM, PANICUM MILIOIDES, AND TALL FESCUE (FESTUCA ARUNDINACEA) TO NITROGEN NUTRITION

The response of apparent photosynthesis to N nutrition was studied in the C₃ grass, tall fescue (Festuca arundinacea Schreb.), in the C₄ species Panicum maximum Jacq., and in Panicum milioides Nees ex Trin., a species with characteristics intermediate between C₃ and C₄ photosynthetic types. Plants were grown in culture solution containing 1, 5, 50, and 200 milligrams N per liter. Apparent photosynthesis was measured on the youngest fully expanded leaves at 320 microliters of CO₂ per liter of air and 21% O₂. Leaf conductance was calculated from transpiration measurements, and CO₂ compensation concentrations were also estimated. Several leaf anatomical characteristics were studied on plastic-embedded material. Leaf N content was determined on leaves which were used in photosynthesis measurements.     

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

The capacity for C₄ photosynthesis in Panicum milioides, a specieshaving reduced levels of photorespiration, was investigatedby examining the activity of certain key enzymes of the C₄ pathwayand by pulse-chase experiments with ¹⁴CO₂. The ATP$P₁ dependentactivity of pyruvate,P₁ dikinase in the species was extremelylow (0.14–0.18 µmol mg chlorophyll^–1 min^–1).Low activity of the enzyme was also found in Panicum decipiensand Panicum hians (related species with reduced photorespiration)and in Panicum laxum (a C₃ species). The antibody to pyruvate,P₁dikinase caused about 70% inhibition of the ATP$P₁ dependentactivity of the enzyme in P. milioides. The activity of NAD-malicenzyme and NADP-malic enzyme in ^P. ^milioides was equally low(approximately 0.1–0.2 µmol mg chlorophyll^–1min^–1) and similar to the activity in P. decipiens, P.hians and P. laxum. Photosynthetic pulse-chase experiments underatmospheric conditions showed a typical C₃-like pattern of carbonassimilation including the labelling of glycine and serine asexpected during photorespiration. During the pulse with ¹⁴CO₂only about 1% of the labelled products appeared in malate and2–3% in aspartate. During a chase in atmospheric levelsof CO₂ for up to 6 min there was a slight increase in labellingin the C₄ acids. The amount of label in carbon 4 of aspartatedid not change during the chase, indicating little or no turnoverof the C₄ acid via decarboxylation. The results indicate thatunder atmospheric conditions P. milioides assimilates carbondirectly through the C₃ pathway. Photorespiration as indicatedby the CO₂ compensation point may be repressed in the speciesby a more efficient recycling of photorespired CO₂. (Received June 8, 1982; Accepted July 22, 1982)     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways : VI. DIFFERENTIAL EFFECTS OF TEMPERATURE AND LIGHT INTENSITY ON PHOTORESPIRATION IN C(3), C(4), AND INTERMEDIATE SPECIES

The effects of temperature and photosynthetically active radiation levels on photorespiration were investigated in Panicum milioides Nees ex Trin. and Panicum schenckii Hack., species known to have low photorespiration rates and other characteristics intermediate between C₃ and C₄ species. Comparisons were made with the C₃ grass species tall fescue (Festuca arundinacea Schreb.). An increase in temperature from 20 to 35 C raised photorespiration from 7.3 to 10.2 milligrams per square decimeter per hour in tall fescue, but the increase in P. schenckii was less than 1 milligram per square decimeter per hour. Increases in temperature caused much less change in CO₂ compensation concentration in P. milioides and P. schenckii than in tall fescue, values of 160 microliters per liter being obtained in tall fescue at 40 C compared to about 40 microliters per liter for P. milioides and P. schenckii. Photorespiration in P. schenckii increased by only about 1 milligram CO₂ per square decimeter per hour as the photosynthetically active radiation level was raised from 100 to 2,000 microEinsteins per square meter per second. Loss of CO₂ into CO₂-free air actually decreased from 2.2 to 1.0 milligrams per square decimeter per hour as the radiation level was raised from 100 to 1,100 microEinsteins per square meter per second but tended to rise again at 2,000 microEinsteins per square meter per second. In contrast, photorespiration in tall fescue tripled with radiation level over the same range, reaching a maximum value of 7.2 milligrams per square decimeter per hour as determined by extrapolation of the CO₂ response curves to zero CO₂. The CO₂ compensation concentration in tall fescue was nearly insensitive to photosynthetically active radiation above 140 microEinsteins per square meter per second but, in P. milioides and P. schenckii, it decreased from values of 69 and 62 microliters per liter, respectively, to values of 21 and 16 as the radiation level was increased from 50 to 1075 microEinsteins per square meter per second. Interpolation of CO₂-response curves showed that an increase in photosynthetically active radiation level from 100 to 2,000 microEinsteins per square meter per second reduced the CO₂ compensation value of P. schenckii from 38 to 19 microliters per liter. Data from these experiments indicate reduced photorespiration or a CO₂-recycling mechanism in P. milioides and P. schenckii which causes apparent photorespiration to be nearly insensitive to temperature in the 20 to 35 C range and to decrease at high radiation intensities.     

Photosynthetic/Photorespiratory Carbon Metabolism in the C(3)-C(4) Intermediate Species, Moricandia arvensis and Panicum milioides

The distribution of ¹⁴C in photosynthetic metabolites of two naturally occurring higher plants with reduced photorespiration, Moricandia arvensis and Panicum milioides, in pulse and pulse-chase ¹⁴CO₂ incorporation experiments was similar to that for the C₃ species, M. foetida and Glycine max. After 6 seconds of ¹⁴CO₂ incorporation, only about 6% of the total ¹⁴C fixed was in malate and aspartate in both M. arvensis and P. milioides. The apparent turnover of the C₄ acids was very slow, and malate accumulated during the day in M. arvensis. Thus, C₄ acid metabolism by M. arvensis and P. milioides had no significant role in photosynthetic carbon assimilation under the conditions of our experiments (310 microliters CO₂ per liter, 21% O₂, 1100 or 1900 micromoles photon per square meter per second, 27°C).

After a 36-second chase period in air containing 270 microliters CO₂ per liter, about 20% of the total ¹⁴C fixed was in glycine with M. arvensis, as compared to 15% with M. foetida, 14% with P. milioides, and 9% with G. max. After a 36-second chase period in 100 microliters CO₂ per liter, the percentage in glycine was about twice that at 270 microliters CO₂ per liter in the C₃ species and P. milioides, but only 20% more ¹⁴C was in glycine in M. arvensis. These data suggest that either the photorespiratory glycine pool in M. arvensis is larger than in the other species examined or the apparent turnover rate of glycine and the flow of carbon into glycine during photorespiration are less in M. arvensis. An unusual glycine metabolism in M. arvensis may be linked to the mechanism of photorespiratory reduction in this crucifer.

     

Biochemical components of the photosynthetic CO2 compensation point of higher plants.

Barley, Panicum milioides and Panicum maximum were exposed to ¹⁴CO₂ near their photosynthetic CO₂ compensation points and their respective ¹⁴C-products were determined. In short exposure times Panicum maximum had 100% of its ¹⁴C in malate and aspartate whereas Panicum milioides and barley had 16 and 3% of their respective ¹⁴C in C₄ organic acids. Near the respective CO₂ compensation points a linear relationship occurs in plotting the ratio of glycine, serine, and glycerate to C₄ organic acids. The ratio of ribulose 1,5-bisphosphate oxygenase to phosphoenolpyruvate carboxylase is linear with their CO₂ compensation points. The photosynthetic CO₂ compensation point apparently is controlled by the activity of enzymes producing photorespiration metabolites and the activity of phospheonolpyruvate carboxylase.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways : VIII. Ultrastructural Characteristics of Panicum Species in the Laxa Group

Ultrastructural studies of leaves of seven Panicum species in or closely related to the Laxa group and classified as C₃, C₄ or C₃-C₄ intermediate were undertaken to examine features associated with C₃ and C₄ photosynthesis. The C₃ species Panicum rivulare Trin. had few organelles in bundle sheath cell profiles (2 chloroplasts, 1.1 mitochondria, and 0.3 peroxisomes per cell section) compared to an average of 10.6 chloroplasts, 17.7 mitochondria, and 3.2 peroxisomes per bundle sheath cell profile for three C₃-C₄ species, Panicum milioides Nees ex Trin., Panicum decipiens Nees ex Trin. and Panicum schenckii Hack. However, two other C₃ species, Panicum laxum Sw. and Panicum hylaeicum Mez, contained about 0.7, 0.5, and 0.3 as many chloroplasts, mitochondria, and peroxisomes, respectively, as in bundle sheath cell profiles of the C₃-C₄ species. Chloroplasts and mitochondria in bundle sheath cells were larger than those in mesophyll cells for the C₄ species Panicum prionitis Griseb. and the C₃-C₄ species, but in C₃ species the organelles were similar in size or were smaller in the bundle sheath cells. The C₃-C₄ species and P. laxum and P. hylaeicum exhibited an unusually close association of organelles in bundle sheath cells with mitochondria frequently surrounded in profile by chloroplasts. The high concentrations in bundle sheath cells of somewhat larger organelles than in mesophyll cells correlates with the reduced photorespiration of the C₃-C₄ species.     

CO2 donation by malate and aspartate reduces photorespiration in Panicum milioides,A C3C4 intermediate species

Oxygen inhibition of leaf slice photosynthesis in Panicum milioides increased from 20% to 30% at 21% O₂ in the presence of maleate, a phosphoenolpyruvate carboxylase inhibitor. The increased O₂ sensitivity was completely reversed by the addition of malate and aspartate, the stable products of the phosphoenolpyruvate carboxylase reaction. The C₄ acids, malate and aspartate, also reduced O₂ inhibition of photosynthesis by isolated bundle sheath strands, but not mesophyll protoplasts. Similarly, only bundle sheath strands exhibited an active C₄ acid-dependent O₂ evolution. Compartmentation of C₄ cycle enzymes, with pyruvate, Pi dikinase in the mesophyll and NAD-malic enzyme in the bundle sheath, was demonstrated. It is concluded that reduced photorespiration in P. milioides is due to a limited potential for C₄ photosynthesis permitting an increase in pCO₂ at the site of bundle sheath ribulosebisphosphate carboxylase.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO₂ compensation concentrations than in C₃ species, lower CO₂ evolution in the light, and less response of apparent photosynthesis to O₂ concentration. The lower response to O₂ in P. milioides could be due to reduced O₂ competition with CO₂ for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO₂, or to an initial fixation of CO₂ by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C₄ species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C₃ species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO₂ exchange was measured at five CO₂ concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O₂. Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO₂ concentrations against apparent photosynthesis. In the C₄ species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O₂ concentration. At 21% O₂ mesophyll conductance of tall fescue was decreased 32% below the value at 2% O₂. Decreases in mesophyll conductance at 21% O₂ for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO₂ in CO₂-free air, estimated by extrapolating the CO₂ response curve to zero CO₂, was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O₂ was raised from 2-21%. Loss of CO₂ was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O₂. The results suggest that the reduced O₂ response in P. milioides and P. schenckii is due to a lower loss of CO₂ in the light rather than less inhibition of carboxylation by O₂, since the decrease in carboxylation efficiency at 21% O₂ was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O₂ in these three species at low light intensities was similar at 31 to 36% which also indicates similar O₂ effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O₂ in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO₂ in these species than in tall fescue.     

Photosynthetic Characteristics of the C(3)-C(4) Intermediate Parthenium hysterophorus

The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO₂ compensation concentration (20-25 microliters per liter at 30°C and 21% O₂) and by a reduced sensitivity of net photosynthesis to 21% O₂. In contrast, the related C₃ species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO₂ compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O₂. Furthermore, in P. hysterophorus the CO₂ compensation concentration is relatively less sensitive to changes in O₂ concentrations and shows a biphasic response to changing O₂, with a transition point at about 11% O₂. Based on these results, P. hysterophorus is classified as a C₃-C₄ intermediate. The activities of diagnostic enzymes of C₄ photosynthesis in P. hysterophorus were very low, comparable to those observed in the C₃ species P. incanum (e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to ¹⁴CO₂ (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial ¹⁴C products (77-84%), with ≤4% of the ¹⁴C-label in malate plus aspartate. These results indicate that in the C₃-C₄ intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C₄ photosynthesis.     

A Comparison of Dark Respiration between C(3) and C(4) Plants

Lower respiratory costs were hypothesized as providing an additional benefit in C₄ plants compared to C₃ plants due to less investment in proteins in C₄ leaves. Therefore, photosynthesis and dark respiration of mature leaves were compared between a number of C₄ and C₃ species. Although photosynthetic rates were generally greater in C₄ when compared to C₃ species, no differences were found in dark respiration rates of individual leaves at either the beginning or after 16 h of the dark period. The effects of nitrogen on photosynthesis and respiration of individual leaves and whole plants were also investigated in two species that occupy similar habitats, Amaranthus retroflexus (C₄) and Chenopodium album (C₃). For mature leaves of both species, there was no relationship between leaf nitrogen and leaf respiration, with leaves of both species exhibiting a similar rate of decline after 16 h of darkness. In contrast, leaf photosynthesis increased with increasing leaf nitrogen in both species, with the C₄ species displaying a greater photosynthetic response to leaf nitrogen. For whole plants of both species grown at different nitrogen levels, there was a clear linear relationship between net CO₂ uptake and CO₂ efflux in the dark. The dependence of nightly CO₂ efflux on CO₂ uptake was similar for both species, although the response of CO₂ uptake to leaf nitrogen was much steeper in the C₄ species, Amaranthus retroflexus. Rates of growth and maintenance respiration by whole plants of both species were similar, with both species displaying higher rates at higher leaf nitrogen. There were no significant differences in leaf or whole plant maintenance respiration between species at any temperature between 18 and 42°C. The data suggest no obvious differences in respiratory costs in C₄ and C₃ plants.     

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : I. Leaf Anatomy, Photosynthetic Responses to O(2) and CO(2), and Activities of Key Enzymes in the C(3) and C(4) Pathways

Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C₃-C₄ plants based on leaf anatomy, photosynthetic CO₂ compensation point, O₂ inhibition of photosynthesis, and activities of C₄ enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C₄ species F. trinervia, while the other C₃-C₄ intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO₂ compensation points of these intermediates at 30°C were very low relative to those of C₃ plants, ranging from 7 to 14 microliters per liter. In contrast to C₃ plants, net photosynthesis by the intermediates was not sensitive to O₂ concentrations below 5% and decreased relatively slowly with increasing O₂ concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O₂ varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C₃ species. The inhibition of carboxylation efficiency by 21% O₂ varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C₄ (2% for F. trinervia) and C₃ (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C₄ enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C₄ photosynthesis. It appears that these Flaveria species may be true biochemical C₃-C₄ intermediates.     

Photosynthetic efficiency of Panicum hians and Panicum milioides in relation to C3 and C4 plants

Panicum hians and Panicum milioides were found to have characteristicsintermediate to those of C₃ and C₄ species with respect to CO₂compensation point, percentage inhibition of photosynthesisby O₂ at various O₂/CO₂ solubility ratios, and water use efficiency.C₄ species have a higher carboxylation efficiency than eitherthe intermediate or C₃ species. During photosynthesis, evenunder 2.5% O₂, C₄ species have a higher affinity for intercellularCO₂ (Km 1.6 µM) apparently due to the initial carboxylationthrough PEP carboxylase. Under low O₂ the intermediate and C₃species had a similar affinity for intercellular CO₂ duringphotosynthesis (Km 5–7 µM) consistent with carboxylationof atmospheric CO₂ through RuDP carboxylase. There were considerablevariation in photosynthesis/unit leaf area at saturating CO₂levels in the species examined which in part is due to differencesin RuDP carboxylase /unit leaf area. The highest rates of photosynthesis/unitleaf area under CO₂-saturating conditions were with the C₃ specieswhich had a correspondingly high level of RuDP carboxylase/unitleaf area. Possibilities for the greater efficiency of P. hiansand P. milioides in comparison to C₃ species in utilizing lowlevels of CO₂ in the presence of atmospheric O₂ are discussed. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and theUniversity of Wisconsin Research Committee with funds from theWisconsin Alumni Research Foundation. (Received June 25, 1977; )     

Comparative growth analyses of panicum species with differing rates of photorespiration

Panicum milioides, a naturally occurring species with reduced photorespiration, P. bisulcatum, a C₃ species, and P. miliaceum, a C₄ species, were grown for 4 weeks at altered pO₂ and pCO₂ and several vegetative growth parameters were determined at weekly intervals. Compared to a pO₂ of 10%, a greater O₂ inhibition of the relative growth rate and dry matter production was observed for P. bisulcatum than for P. milioides at both 21% and 40% O₂, whereas little effect of O₂ was noted for P. miliaceum. Similarly, exposures to elevated pCO₂ of 500 and 1000 μ1 CO₂/liter resulted in a greater stimulation of vegetative growth for P. bisulcatum than for P. milioides, with little effect on P. miliaceum. The CO₂ compensation concentration of P. milioides was less than that of P. bisulcatum over a pO₂ range of 5 to 40%. At 5% O₂, the compensation concentration was relatively O₂-insensitive, whereas above 5% it increased with increasing pO₂. It is concluded that P. milioides represents the first well documented example of a C₃ plant with reduced photorespiration, based on both leaf CO₂ exchange parameters and growth analyses of dry matter production.     

Photosynthesis of F(1) Hybrids between C(4) and C(3)-C(4) Species of Flaveria

Photosynthetic characteristics were studied in several F₁ hybrids between C₄ and C₃-C₄ species of Flaveria. Stable carbon isotope ratios, O₂ inhibition of apparent photosynthesis, and phosphoenolpyruvate carboxylase activities in the hybrids were similar to the means for the parents. Values of CO₂ compensation concentrations were nearer to those of the C₄ parent and apparent photosynthesis was below that of both parents, being only 60 and 74% of that of the lowest (C₃-C₄) parent in two experiments. Reductions of CO₂ compensation concentration and O₂ inhibition of apparent photosynthesis as well as increases in carbon isotope ratios and phosphoenolpyruvate carboxylase activities compared to values in C₃-C₄ species suggest transfer of a limited degree of C₄ photosynthesis to the F₁ hybrids. However, the lower apparent photosynthesis of the hybrids suggests that transfer of C₄ characteristics to non-C₄ species is detrimental unless characteristics associated with C₄ photosynthesis are fully developed. There was a highly significant negative correlation (r = −0.90) between CO₂ compensation concentration and the logarithm of phosphoenolpyruvate carboxylase activity in the parents and hybrids, suggesting involvement of this enzyme in controlling the CO₂ compensation concentration. Although bundle-sheath cells were more developed in leaves of hybrids than in C₃-C₄ parents, they appeared to contain lower quantities of organelles than those of the C₄ parent. Reduced quantities of organelles in bundle-sheath cells could indicate incomplete compartmentation of partial pathways of the C₄ cycle in the hybrids. This may mean that the reduction of CO₂ compensation and O₂ inhibition of apparent photosynthesis relative to the C₃-C₄ parents is less dependent on fully developed Kranz anatomy than is increased apparent photosynthesis.     

Differences in drought sensitivities and photosynthetic limitations between co-occurring C3 and C4 (NADP-ME) Panicoid grasses

Background and Aims

The success of C₄ plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C₄ grasses are not necessarily less sensitive to drought than C₃ grasses and are proposed to respond with greater metabolic limitations, while the C₃ response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C₃ and C₄ NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response.

Methods

Six species of locally co-occurring grasses, C₃ species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C₄ (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO₂ concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types.

Key Results

Leaves of C₄ species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C₄ photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO₂ assimilation, whereas, in the C₃ species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C₄ plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C₃ plants which recovered a greater proportion of photosynthesis through increased stomatal conductance.

Conclusions

Within the Panicoid grasses, C₄ (NADP-ME) species are metabolically more sensitive to drought than C₃ species and recover more slowly from drought.