Selective Inhibition by Actinomycin D of the Synthesis in Photosynthetic and Non-photosynthetic Enzymes During the Greening of Etiolated Bean Leaves

The effect of actinomycin D on the synthesis of the photosynthetic apparatus during illumination of etiolated leaves of Phaseolus vulgaris was studied. The increase of chlorophyll content and of the activities of some photosynthetic enzymes (NADPH diaphorase, ferredoxin, NADP⁺ glyceraldehyde-3-phosphate dehydrogenase) was compared with simultaneous measurements of the level of other enzymes not considered associated with photosynthesis (ornithine transcarbamylase, glucose-6-phosphate dehydrogenase, NAD⁺ glyceraldehyde-3-phosphate dehydrogenase).     

Influence of leaf age on photosynthesis, enzyme activity, and metabolite levels in wheat

The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25°C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25°C) was limited by utilization of photosynthate.     

Effects of Polyploidy on Photosynthetic Rates, Photosynthetic Enzymes, Contents of DNA, Chlorophyll, and Sizes and Numbers of Photosynthetic Cells in the C(4) Dicot Atriplex confertifolia

Photosynthetic rates, chlorophyll content, and activities of several photosynthetic enzymes were determined per cell, per unit DNA, and per unit leaf area in five ploidal levels of the C₄ dicot Atriplex confertifolia. Volumes of bundle sheath and mesophyll protoplasts were measured in enzymatic digestions of leaf tissue. Photosynthetic rates per cell, contents of DNA per cell, and activities of the bundle sheath enzymes ribulose 1,5-bisphosphate carboxylase (RuBPC) and NAD-malic enzyme per cell were correlated with ploidal level at 99% or 95% confidence levels, and the results suggested a near proportional relationship between gene dosage and gene products. There was also a high correlation between volume of mesophyll and bundle sheath cells and the ploidal level. Contents of DNA per cell, activity of RuBPC per cell, and volumes of cells were correlated with photosynthetic rate per cell at the 95% confidence level. The mesophyll cells did not respond to changes in ploidy like the bundle sheath cells. In the mesophyll cells the chlorophyll content per cell was constant at different ploidal levels, there was less increase in cell volume than in bundle sheath cells with an increase in ploidy, and there was not a significant correlation (at 95% level) of phosphoenolpyruvate carboxylase activity or content and pyruvate,Pi dikinase activity with increase in ploidy. The number of photosynthetic cells per unit leaf area progressively decreased with increasing ploidy from diploid to hexaploid, but thereafter remained constant in octaploid and decaploid plants. Numbers of cells per leaf area were not correlated with cell volumes. The mean photosynthetic rates per unit leaf area were lowest in the diploid, similar in 4×, 6×, and 8×, and highest in the decaploid. The photosynthetic rate per leaf area was highly correlated with the DNA content per leaf area.     

Leaf phosphate status, photosynthesis, and carbon partitioning in sugar beet: I. Changes in growth, gas exchange, and calvin cycle enzymes

Sugar beets (Beta vulgaris L. cv F58-554H1) were cultured hydroponically for 2 weeks in growth chambers with two levels of orthophosphate (Pi) supplied in half strength Hoagland solution. Low-P plants were supplied with 1/20th of the Pi supplied to control plants. With low-P treatment, the acid soluble leaf phosphate and total leaf P decreased by about 88%. Low-P treatment had a much greater effect on leaf area than on photosynthesis. Low-P decreased total leaf area by 76%, dry weight per plant by 60%, and the rate of photosynthesis per area at light saturation by 35%. Low-P treatment significantly decreased the total extractable activity of phosphoglycerate kinase (by 18%) and NADP-glyceraldehyde-3-phosphate dehydrogenase (by 16%), but did not decrease the total activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) and ribulose-5-phosphate kinase. Low-P treatment decreased the initial activities of three rate-limiting Calvin cycle enzymes, but had no effect on the initial activity of RuBPCase. Furthermore, low-P treatment significantly increased the total extractable activities of fructose-1,6-bisphosphatase (by 61%), fructose-1,6-bisphosphate aldolase (by 53%), and transketolase (by 46%). The results suggest that low-P treatment affected photosynthetic rate through an effect on RuBP regeneration rather than through RuBPCase activity and that the changes in Calvin cycle enzymes with low-P resulted in an increased flow of carbon to starch.     

Photosynthesis, Leaf Anatomy, and Cellular Constituents in the Polyploid C(4) Grass Panicum virgatum

Photosynthetic gas exchange, activities of six key C₄ cycle enzymes, amounts of soluble protein, chlorophyll, and DNA, and various leaf anatomical and structural features were measured in naturally occurring tetraploid and octaploid plants of the NAD-malic enzyme type C₄ grass Panicum virgatum L. On a leaf area basis, the photosynthetic rate and concentrations of DNA, soluble protein, and chlorophyll were 40 to 50% higher, and enzyme activities 20 to 70% higher in the octaploid than in the tetraploid. Photosynthetic cells in the octaploid were only 17 to 19% larger in volume, yet contained 33 to 38% more chloroplasts than cells in the tetraploid. On a per cell basis the contents of DNA, soluble protein, and chlorophyll, activities of carboxylating photosynthetic enzymes, and carbon assimilation rate were all doubled in octaploid compared with tetraploid cells. Since cellular volume did not double with genome doubling, cellular constituents were more concentrated in the cells of the octaploid. The influences of polyploidy were balanced between mesophyll and bundle sheath cells since the changes in physical and biochemical parameters with ploidy level were similar in both cell types. We conclude that photosynthetic activity in these two polyploid genotypes of P. virgatum is determined by enzyme activities and concentrations of biochemical constituents, and that selection for smaller cell volume has led to higher photosynthetic rates per unit leaf area in the octaploid. The ratio of DNA content to cellular volume is a major factor determining the concentrations of gene products in cells. The number of chloroplasts, however, is controlled more by cellular volume than by the number of nuclear chromosomes.     

Limiting Factors in Photosynthesis: VI. Regeneration of Ribulose 1,5-Bisphosphate Limits Photosynthesis at Low Photochemical Capacity

Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO₂ concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.     

Limiting Factors in Photosynthesis: II. IRON STRESS DIMINISHES PHOTOCHEMICAL CAPACITY BY REDUCING THE NUMBER OF PHOTOSYNTHETIC UNITS

It has been proposed that Fe stress may be used in the study of limiting factors in photosynthesis as an experimental means of varying photochemical capacity in vivo (Plant Physiol 1980 65: 114-120). In this paper the effect of Fe stress on photosynthetic unit number, size, and composition was investigated by measuring P₇₀₀, cytochrome (Cyt) f, chlorophyll (Chl) a, and Chl b in sugar beet leaves. The results show that when Fe stress reduced Chl per unit area by 80% (from 60 to 12 micrograms per square centimeter), it decreased the number of P₇₀₀ molecules per unit area by 88% and Cyt f per unit area by 86%; over the same range the Chl to P₇₀₀ ratio increased by 37% but there was no significant change in the Chl to Cyt f ratio. These data suggest that Fe stress decreases photochemical capacity and Chl per unit area by diminishing the number of photosynthetic units per unit leaf area.     

Metabolic Activities in Extracts of Mesophyll and Bundle Sheath Cells of Panicum miliaceum (L.) in Relation to the C(4) Dicarboxylic Acid Pathway of Photosynthesis

The activities of certain enzymes related to the carbon assimilation pathway in whole leaves, mesophyll cell extracts, and bundle sheath extracts of the C₄ plant Panicum miliaceum have been measured and compared on a chlorophyll basis. Enzymes of the C₄ dicarboxylic acid pathway—phosphoenolpyruvate carboxylase and NADP-malic dehydrogenase—were localized in mesophyll cells. Carbonic anhydrase was also localized in mesophyll cell extracts. Ribose 5-phosphate isomerase, ribulose 5-phosphate kinase, and ribulose diphosphate carboxylase—enzymes of the reductive pentose phosphate pathway—were predominantly localized in bundle sheath extracts. High activities of aspartate and alanine transaminases and glyceraldehyde-3-P dehydrogenase were found about equally distributed between the photosynthetic cell types. P. miliaceum had low malic enzyme activity in both mesophyll and bundle sheath extracts.     

Growth and photosynthesis of pea plants under different iron supply

Soil conditions, leading to iron deficiency or toxicity, are widespread in nature. Our objective was to study the effect of Fe supply, ranging from complete deficiency to excess, on growth and on some photosynthetic indices of pea plants. Both iron deficiency and toxicity decreased shoot and root growth. Complete deficiency resulted in a lower shoot/root ratio and a higher content of dry biomass per unit of fresh biomass in roots, while iron excess led to higher content of dry biomass per unit of fresh biomass in shoot. Complete deficiency was also characterized by low chlorophyll and carotenoid content, elevated ratios of chlorophyll a/chlorophyll b and carotenoids/chlorophylls, a drop of photosynthetic rate per leaf area, and an increase of photosynthetic rate per chlorophyll. The stomatal resistance substantially increased, while the transpiration rate decreased. Smaller changes in stomatal resistance and transpiration rate, but not in photosynthetic rate per leaf area, were found under partial iron deficiency and under excess of iron. In the first case, the chlorophyll content decreased, while in the second it increased. The maximum efficiency of photosystem II was unaffected by iron supply. Even when no genetic or experimental differences existed, changes in growth, pigment content and photosynthesis due to variation of Fe supply depended on the type and severity of the imposed stress, as well as on the studied parameter. A combination of indices described better the effect of iron supply, especially when small differences were characterized.     

Limiting Factors in Photosynthesis: I. USE OF IRON STRESS TO CONTROL PHOTOCHEMICAL CAPACITY IN VIVO

The possibility of using Fe stress as an experimental tool in the study of limiting factors was explored. Results show that Fe stress decreased the chlorophyll (Chl) a, Chl b, carotene, and xanthophyll content of leaves of sugar beets (Beta vulgaris L.) and that the maximum rate of photosynthetic CO₂ uptake (P_max) per unit area was linearly related to Chl (a + b) per unit area. Measurements of noncyclic ATP formation by isolated chloroplasts at light saturation indicate that photosynthetic electron transport capacity decreased concomitantly with pigment content under Fe stress.     

Biochemical Changes that Occur during Senescence of Wheat Leaves : I. Basis for the Reduction of Photosynthesis

Changes in activities of photosynthetic enzymes and photochemical processes were followed with aging of vegetative and flag leaves of wheat (Triticum aestivum L. cv Roy). Activities of stromal enzymes began to decline prior to photochemical activities. In general, total soluble protein and the activities of ribulose-1,5-bisphosphate carboxylase and NADP-triose-phosphate dehydrogenase declined in parallel and at an earlier age than leaf chlorophyll (Chl), leaf photosynthesis, and photosynthetic electron transport activity. Leaves appeared to lose whole chloroplasts as opposed to a general degradation of all chloroplasts based on three lines of evidence: (a) electron transport activity calculated on an area basis declined much earlier than the same data expressed on a Chl basis; (b) Chl content per chloroplast was similar for mature and senescent tissue; and (c) the absorbance at 550 nanometers (light scattering) per unit of Chl remained essentially constant until the end of senescence. Chloroplasts did, however, undergo some modifications before they were lost (e.g. loss of stromal enzyme activities), but the reduction in leaf photosynthesis was apparently caused by a loss of whole chloroplasts.     

Changes in ribulosebisphosphate carboxylase/oxygenase and ribulose 5-phosphate kinase abundances and photosynthetic capacity during leaf senescence

The abundances of ribulose-1,5-bisphosphate carboxylate/oxygenase (Rubisco) and ribulose-5-phosphate (Ru5P) kinase in field-grown soybean (Glycine max L. Merr.) leaves were quantified by a Western blot technique and related to changes in chlorophyll and photosynthetic capacity during senescence. Even though the leaf content of Rubisco was approximately 80-fold greater than that of Ru5P kinase, the decline in the levels of these two Calvin cycle enzymes occurred in parallel during the senescence of the leaves. Moreover, the decrease in the content of Rubisco was accompanied by parallel decreases of both the large and small subunits of this enzyme but not by an accumulation of altered large or small subunit isoforms. With increasing senescence, decreases in abundances of Rubisco, Ru5P kinase and chlorophyll were closely correlated with the decline in photosynthetic capacity; thus, the specific photosynthetic capacity when expressed per abundance of any of these parameters was rather constant despite an 8-fold decrease in photosynthetic capacity. These results suggest that during senescence of soybean leaves the chloroplast is subject to autolysis by mechanisms causing an approximately 80-fold greater rate of loss of Rubisco than Ru5P kinase.Jointly supported by the United States Department of Agricultural Research Service and the Kentucky Agricultural Experiment Station, Lexington (paper No. 88 3 286).Mention of a commercial product does not constitute endorsement by the United States Department of Agriculture.     

Dependence of photosynthetic rates on leaf density thickness in deciduous woody plants grown in sun and shade

Comparisons of photosynthetic rates were made on leaves of ten species of woody dicotyledons grown in the field under full sun or under a canopy which transmitted approximately 18% of full light. Photosynthesis and dark respiration were measured and compared on various bases: area, chlorophyll, fresh weight of lamina, density thickness (fresh weight per unit area), and protein.

Light-saturated photosynthesis per unit area or unit chlorophyll was about 1.5 times greater in the sun leaves than in the shade leaves and essentially equal per unit fresh weight or unit protein. Sun leaves were thicker but the enzymes per unit fresh weight remained constant as thickness varied. Chlorophyll per unit area remained about constant; chlorophyll per unit fresh weight varied inversely with changes in leaf thickness. Thus, density thickness variation is important in photosynthetic adaptation to sun and shade. This is also shown by the relationship between light-saturated photosynthesis per unit area and density thickness.

     

Calvin-Benson Cycle Enzymes in Guard-Cell Protoplasts from Vicia faba L: Implications for the Greater Utilization of Phosphoglycerate/Dihydroxyacetone Phosphate Shuttle between Chloroplasts and the Cytosol

Activities of Calvin-Benson cycle enzymes were found in protoplasts of guard cells from Vicia faba L. The activities of NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPD) and ribulose-1,5-bisphosphate carboxylase (RuBPC) were 2670 and 52 micromoles per milligrams chlorophyll per hour, respectively. Activities of NADP-GAPD and RuBPC in guard cells were increased by red light illumination, and the light activations were inhibited completely by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. Enzymes related to the Calvin-Benson cycle such as 3-phosphoglycerate kinase (PGAK), triose phosphate (TP) isomerase, and fructose-1,6-bisphosphatase (FBPase) were shown to be present in guard-cell chloroplasts. From these results, we conclude that the photosynthetic carbon reduction pathway is present in guard-cell chloroplasts of Vicia faba. We compared these enzyme activities in guard cells with those in mesophyll cells. The activities of NADP-GAPD and PGAK were more than several-fold higher and that of TP isomerase was much higher in guard-cell chloroplasts than in mesophyll chloroplasts. In contrast, activities of RuBPC and FBPase were estimated to be roughly half of those in mesophyll chloroplasts. High activities of PGAK, NAD-GAPD, and TP isomerase were found in fractions enriched in cytosol of guard cells. Illumination of guard-cell protoplasts with red light increased the cellular ATP/ADP ratio from 5 to 14. These results support the interpretation that guard cells utilize a shuttle system (e.g. phosphoglycerate [PGA]/dihydroxyacetone phosphate [DHAP] shuttle) for an indirect transfer of ATP and reducing equivalents from chloroplasts to the cytosol.     

Does a terminal drought tolerance QTL contribute to differences in ROS scavenging enzymes and photosynthetic pigments in pearl millet exposed to drought?

The control of reactive oxygen species (ROS) and the stability of photosynthetic pigments under stress conditions are hypothesized to contribute to drought tolerance. Here we studied how ascorbic peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) isozyme activities and chlorophyll a, b (Chl a, b) and carotenoids (Car) contents responded to water stress and whether they related to presence of a terminal drought tolerance QTL in pearl millet. We used PRLT2/89-33 (QTL donor), H77/833-2 (sensitive), and near-isogenic lines (QTL-NILs) introgressed with the QTL in H77/833-2 background. Under water stress there was no significant change in the total APX activity; only the proportional APX5 activity increased, with higher band intensity in tolerant genotypes. There were no significant changes in total activities of CAT and SOD under water stress, with similar band intensities in all genotypes, and a new CAT isozyme was induced in all genotypes. The photosynthetic pigment content decreased under water stress, although not differently in any genotype. Under water stress, the activities of most APX, CAT and SOD isozymes were closely related to the total chlorophyll/carotenoids ratio. Overall, besides APX5, water stress did not lead to major changes in the profile of isoenzymes involved in ROS scavenging. Similarly, the pigment content under stress did not discriminate genotypes according to the presence/absence of the QTL. This absence of discrimination for the ROS scavenging enzymes and for the pigment content under stress suggests that these traits may not play a key role in terminal drought tolerance in pearl millet.     

Photosynthetic carbon metabolism in isolated maize bundle sheath strands

Photosynthetically active bundle sheath strands capable of assimilating up to 8 micromoles CO₂ per milligram chlorophyll per hour have been isolated from fully expanded leaves of Zea mays L. Mesophyll cell contamination of the preparations was negligible, as evidenced by light and electron microscopy and by a high ratio of chlorophyll a to chlorophyll b in the strands. Ribose 5-phosphate markedly stimulated the rate of photosynthetic ¹⁴CO₂ fixation by the isolated strands. In contrast, both pyruvate and phosphoenolpyruvate had a comparatively small stimulatory effect on bundle sheath ¹⁴CO₂ fixation. After 5 minutes of photosynthesis in ¹⁴C-bicarbonate, 95% of the incorporated ¹⁴C was found in compounds other than C₄-dicarboxylic acids, most notably in 3-phosphoglycerate and sugar phosphates. A similar distribution of ¹⁴C was observed in the presence of exogenous ribose 5-phosphate. Extracts of bundle sheath strands contained high specific activities of “malic” enzyme, phosphoglycolate phosphatase, hydroxypyruvate reductase, and ribulose 1,5-diphosphate carboxylase, whereas the specific activities of NADP⁺-malate dehydrogenase and phosphopyruvate carboxylase were extremely low. These results indicate that the Calvin cycle occurs in the bundle sheath cells of maize.     

CO(2) Assimilation and Activities of Photosynthetic Enzymes in High Chlorophyll Fluorescence Mutants of Maize Having Low Levels of Ribulose 1,5-Bisphosphate Carboxylase

Photosynthetic properties were examined in several hcf (high chlorophyll fluorescence 11, 21, 42 and 45) nuclear recessive mutants of maize which were previously found to have normal photochemistry and low CO₂ fixation. Mutants usually either died after depletion of seed reserves (about 18 days after planting), or survived with slow growth up to 7 or 8 weeks. Both the activity and quantity of ribulose 1,5-bisphosphate carboxylase (Rubisco) were low in the mutants (5-25% of the normal siblings on a leaf area basis) and the loss of Rubisco tended to parallel the reduction in photosynthetic capacity. The Rubisco content in the mutants was often marginal for photosynthetic carbon gain, with some leaves and positions along a leaf having no net photosynthesis, while other leaves had a low carbon gain. Conversely, the activities of C₄ cycle enzymes, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, NADP-malate dehydrogenase, and NADP-malic enzyme, were the same or only slightly reduced compared to the normal siblings. The mutants had about half as much chlorophyll content per leaf area as the normal green plants. However, the Rubisco activity in the mutants was low on both a leaf area and chlorophyll basis. Low Rubisco activity and lower chlorophyll content may both contribute to the low rates of photosynthesis in the mutants on a leaf area basis.     

Photosynthesis in relation to leaf characteristics of cotton from controlled and field environments

In situ and light-saturated net photosynthetic rates per unit leaf area were greater in cotton (Gossypium hirsutum L.) plants grown in pots in the field than in similar plants from a phytotron growth chamber. Light-saturated stomatal resistances did not differ in leaves of similar age and exposure on field and chamber plants; lower photosynthetic rates in chamber leaves were associated with greater mesophyll resistance. Differences in net photosynthetic rates were related to differences in leaf thickness. When the photosynthetic rates were expressed per unit of mesophyll volume or per unit chlorophyll differences between field and chamber plants were much less than when rates were expressed per unit leaf area. Characterization of the chloroplast lamellar proteins showed that the field leaves had smaller photosynthetic units than the chamber leaves. Since the field leaves also contained more chlorophyll per unit area, this resulted in a much larger number of photosynthetic units per unit area in the field leaves.     

Reduction in chlorophyll content without a corresponding reduction in photosynthesis and carbon assimilation enzymes in yellow-green oil yellow mutants of maize

The photosynthetic properties of a yellow lethal mutant, Oy/oy, and two yellow-green mutants of maize which are allelic (a homozygous recessive oy/oy and a heterozygous dominant Oy/+) were examined. Although Oy/oy had little or no chlorophyll or capacity for CO₂ fixation compared to normal siblings, it had 28% as much ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) activity, and from 40% to near normal activities of C₄ cycle enzymes.Both yellow-green mutants had only half as much chlorophyll per leaf area as normal green seedlings in greenhouse-grown plants in winter and spring. However, the absorbance of light by the mutants was relatively high, as their transmittance was only 5 to 8% greater than normal leaves. In winter-grown greenhouse plants, the activities of Rubisco and several C₄ cycle enzymes in the mutants were unaffected and similar to those of normal seedlings on a leaf area basis. After allowing for small differences in leaf absorbance, the light response curves for photosynthesis in the mutants were similar on a leaf area basis but much higher on a chlorophyll basis than those of the normal seedlings. In spring-grown greenhouse plants the enzyme activities and photosynthesis rates were about 30% lower per leaf area in the yellow-green mutant leaves compared to the wild type. The maximum carboxylation efficiency (measured under low CO₂ and 1000 mol quanta m^-2 s^-1) in the mutants and normal leaves was similar on a Rubisco protein basis. The results indicate that maize can undergo a 50% reduction in chlorophyll content without a corresponding reduction in enzymes of carbon assimilation, and still maintain a high capacity for photosynthesis.Abbreviations Chl chlorophyll - PEP phosphoenolypruvate - Rubisco ribulose-1,5-bisphosphate carboxylase oxygenase This research was supported by CSIRO and by USDA Competitive Grant 86-CRCR-1-2036.