An Optimization Model of the Photosynthetic Leaf: The Model of Optimal Photosynthetic CO2 Fixation within Leaves of Mesophytic C3 Plants (original) (raw)
Related papers
Journal of Experimental Botany, 2005
The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO2 diffusion within a leaf is examined first. Because affinity of Rubisco for CO2 is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O2, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C3 plants to maintain the CO2 concentration in the chloroplast as high as possible. Since the internal conductance for CO2 diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.
Leaf Functional Anatomy in Relation to Photosynthesis
PLANT PHYSIOLOGY, 2011
Rubisco is a large enzyme with a molecular mass of approximately 550 kD. The maximum rate of CO 2 fixation (i.e. ribulose-1,5-bisphosphate [RuBP] carboxylation) at CO 2 saturation is only 15 to 30 mol CO 2 mol 21 Rubisco protein s 21 at 25°C. Affinity to CO 2 is also low, and the K m , K c , at 25°C in the absence of oxygen is comparable to the CO 2 concentration in water equilibrated with air containing 39 Pa CO 2 (approximately 390 mL L 21 ), 13 mM. Moreover, RuBP carboxylation is competitively inhibited by RuBP oxygenation, which is the primary step of the energy-wasting process, photorespiration. If the CO 2 concentration in the chloroplast stroma is low, the carboxylation rate will decrease while the oxygenation rate will increase. Under such conditions, light energy and other resources, including nitrogen and water, are all wasted, eventually leading to a decrement of fitness of the plants. From these data, we may consider that structural features of the leaf contributing to the maintenance of the high CO 2 concentration in the chloroplast stroma may have been selected during evolution.
Comparative ecophysiology of leaf and canopy photosynthesis
Plant, Cell and Environment, 1995
Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation ofthe light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.
Does Leaf Position within a Canopy Affect Acclimation of Photosynthesis to Elevated CO2
1998
Previous studies of photosynthetic acclimation to elevated CO 2 have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO 2 partial pressure of 55 Pa within a replicated field experiment using free-air CO 2 enrichment. Gas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-limited photosynthesis. Net photosynthetic CO 2 uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO 2 did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO 2 uptake remained higher under elevated CO 2 . Acclimation at elevated CO 2 was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO 2 led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO 2 therefore depended on both vertical position within the canopy and the developmental stage. ; fax 44 -1206 -873416.
Oxygen Exchange in Leaves in the Light
Plant Physiology, 1980
Photosynthetic 02 production and photorespiratory 02 uptake were measured using isotopic techniques, in the C3 species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO2 and normal 02,02 production increased linearly with light intensity. At low 02 or low C02, 02 production was suppressed, indicating that increased concentrations of both 02 and CO2 can stimulate 02 production. At the CO2 compensation point, 02 uptake equaled 02 production over a wide range of 02 concentrations. 02 uptake increased with light intensity and 02 concentration. At low light intensities, 02 uptake was suppressed by increased CO2 concentrations so that 02 uptake at 1,000 microliters per liter CO2 was 28 to 35% of the uptake at the CO2 compensation point. At high light intensities, 02 uptake was stimulated by low concentrations of CO2 and suppressed by higher concentrations of C02. 02 uptake at high light intensity and 1000 microliters per liter CO2 was 75% or more of the rate of 02 uptake at the compensation point. The response of 02 uptake to light intensity extrapolated to zero in darkness, suggesting that 02 uptake via dark respiration may be suppressed in the light. The response of 02 uptake to 02 concentration saturated at about 30% 02 in high light and at a lower 02 concentration in low light. 02 uptake was also observed with the C4 plant Amaranthus edulis, the rate of uptake at the CO2 compensation point was 20% of that observed at the same light intensity with the C3 species, and this rate was not influenced by the CO2 concentration. The results are discussed and interpreted in terms of the ribulose-1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of 02. Both 02 evolution and 02 uptake take place in leaves of C3 and C4 plants in the light (4, 8, 17, 21, 23, 24, 27, 28). 02 evolution is derived entirely from the water-splitting reaction of PSII, but three principal 02 uptake processes are presently recognized. These are: the oxygenase reaction of ribulose bisP carboxylase-oxygenase and the associated metabolism of P-glycolate (2, 4, 5, 18, 20); the Mehler reaction (22), which results in the direct photoreduction of 02 and may support ATP synthesis via pseudocyclic photophosphorylation (9, 13, 16); and the possibility that 02 uptake associated with mitochondrial respiration continues in the light (18). Volk and Jackson and their colleagues (17, 23, 24, 27, 28) have made substantial contributions to the study of 02 exchange in intact leaves, but only a limited range of conditions were em-This paper is Carnegie Institute of Washington publication No. 686.
New Phytologist, 2005
• Gas exchange is generally regarded to occur between the leaf interior and ambient air, i.e. in vertical (anticlinal) directions of leaf blades. However, inside homobaric leaves, gas movement occurs also in lateral directions. The aim of the present study was to ascertain whether lateral CO 2 diffusion affects leaf photosynthesis when illuminated leaves are partially shaded. • Measurements using gas exchange and chlorophyll fluorescence imaging techniques were performed on homobaric leaves of Vicia faba and Nicotiana tabacum or on heterobaric leaves of Glycine max and Phaseolus vulgaris. • For homobaric leaves, gas exchange inside a clamp-on leaf chamber was affected by shading the leaf outside the chamber. The quantum yield of photosystem II (Φ PSII) was highest directly adjacent to a light/shade border (LSB). Φ PSII decreased in the illuminated leaf parts with distance from the LSB, while the opposite was observed for nonphotochemical quenching. These effects became most pronounced at low stomatal conductance. They were not observed in heterobaric leaves. • The results suggest that plants with homobaric leaves can benefit from lateral CO 2 flux, in particular when stomata are closed (e.g. under drought stress). This may enhance photosynthetic, instead of nonphotochemical, processes near LSBs in such leaves and reduce the photoinhibitory effects of excess light.
Journal of Plant Research, 2001
Light-saturated rates of photosynthesis on leaf area basis (A) depend not only on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO2 diffusion from the substomatal cavity to the stroma is substantial, it is likely that mesophyll structure affects A through affecting diffusion of COZ in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional model of COZ diffusion in the leaf. When mesophyll thickness of the leaf is changed with the Rubisco content per unit leaf area kept constant, the maximum A occurs at an almost identical mesophyll thickness irrespective of the Rubisco contents per leaf area. On the other hand, with an increase in Rubisco content per leaf area, the mesophyll thickness that realizes a given photosynthetic gain per mesophyll thickness (or per leaf cost) increases. This probably explains the strong relationship between A and mesephyll thickness. In these simulations, an increase in mesophyll thickness simultaneously means an increase in the diffusional resistance in the intercellular spaces ria^), an increase in the total surface area of chloroplasts facing the intercellular spaces per unit leaf area (S,), and an increase in construction and maintenance cost of the leaf. Leaves can increase S, and decrease R h also by decreasing cell size. Leaves with smaller cells are mechanically stronger. However, actual leaves do not have very small cells. This could be because actual leaves exhibiting considerable rates of leaf area expansion, adequate heat capacitance, high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also discussed.