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.     

Functional analysis of corn husk photosynthesis

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C(3) photosynthetic pathway, in contrast to its leaves, which use the C(4) photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO(2) compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO(2) assimilation rate in the husk was less than that in the leaves and did not saturate at high CO(2), indicating CO(2) diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO(2) compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C(4)-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C(4) photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.     

Effect of inorganic carbon supply on the photosynthetic physiology of Gracilaria tenuistipitata

Gracilaria tenuistipitata Zhang et Xia was cultured for 15 d at low, normal and high inorganic carbon concentrations under constant light, temperature and nutrient conditons. Carbonic anhydrase (CA; EC 4.2.1.1.) activity, ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco; EC 4.1.1.39) content, pigment content and C/N ratio were measured, and also the photosynthesis and growth rates. Both Rubisco content and CA activity increased under conditions of low inorganic carbon (C_i) but decreased at high C_i with respect to the control. The amount of pigments declined considerably at high C_i and was slightly higher at low C_i. The maximum rate of photosynthesis and the photosynthetic efficiency increased in low C_i and the opposite was found at high C_i concentration. The effects of C_i concentration on maximum rate of photosynthesis and photosynthetic efficiency are discussed in relation to the variation in pigment and Rubisco contents and CA activity. The data indicate that C_i may be an important factor controlling the photosynthetic physiology of G. tenuistipitata with regard, not only to the enzymes of C_i metabolism, but also to the pigment content.Abbreviations APS_max maximum apparent photosynthetic rate - CA carbonic anhydrase - Chl chlorophyll - C_i inorganic carbon - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work has been supported by grants No. PB91-0962 and No. MAR90-0365 from Spanish Direction for Science and Technology (DIGICYT). M.J. G-S holds a fellowship from the DIGICYT.     

两个品种烟草叶片发育过程中几种光合参数变化的比较

比较烟草2个品种‘NC89’和‘JYH’叶片发育过程中几个光合参数变化的结果表明,烟草叶片发育过程中光合速率变化表现为上升期、高值持续期（APD）和速降期,叶绿素含量变化经历上升期、相对稳定期（RSP）和速降期。光合功能衰退过程中,核酮糖．1,5-二磷酸羧化酶（RuBPCase）活性比电子传递活性下降快。可逆衰退阶段的2个品种类囊体膜多肽组分和‘NC89’的核酮糖-1,5-二磷酸羧化酶加氧酶（Rubisco）大亚基基本上无变化;不可逆衰退阶段的2个品种类囊体膜多肽组分、Rubisco大小亚基均快速降解,尤其是光系统Ⅱ（PSⅡ）复合体和Rubisco小亚基。‘JYH’的叶龄为10-40d的叶中各光合参数与‘NC89’的差别不大,但‘JYH’的光合功能期短,光合功能衰退过程中光合电子传递与碳同化失衡较严重,光合功能衰退比‘NC89’早而迅速。     

Changes in photosynthesis and other chloroplast traits in lanceolate leaflet isoline of soybean

Lanceolate leaflet soybean (Glycine max L. Merrill) has been known to photosynthesize more CO₂ per unit leaf area than normal leaflet soybean. The exact reason for this increase in photosynthetic rate is still unclear. The present study was undertaken to investigate the leaf photosynthetic rate and other physiological traits in relation to chloroplast of lanceolate leaflet soybean. Ontogenic changes in apparent photosynthesis (AP) were related primarily to variations in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area, and only secondarily to difference in specific activity of the said enzyme. Moreover, lanceolate leaflet consistently maintained a higher leaf AP, higher Rubisco activity, and more chloroplasts per unit leaf area basis than did normal leaflet soybean throughout leaf ontogeny. However, lanceolate soybean tended to have lower AP and Rubisco activity on a chloroplast basis. The superiority of leaf AP and other leaf physiological traits, expressed on a leaf area basis, in lanceolate leaflet soybean is associated with a corresponding increase in chloroplast number.     

Effects of growth light and nitrogen nutrition on the organization of the photosynthetic apparatus in leaves of a C4 plant, Amaranthus cruentus

Properties of C4 photosynthesis were examined in Amaranthus cruentus L. (NAD-malic enzyme (ME) subtype, dicot) grown under different light and nitrogen (N) conditions, from the viewpoint of N investment into their photosynthetic components. In low-light (LL) leaves, chlorophyll content per leaf area was greater and chlorophyll alb ratio was lower than in high-light (HL) leaves. These indicate that LL leaves invest more N into their light-harvesting systems. However, this N investment did not contribute to the increase in the quantum yield of photosynthesis on the incident photon flux density (PFD) basis (Qi) in LL leaves. N allocation to ribulose 1,5-bisphosphate carboxylasel oxygenase (Rubisco) was significantly higher in HL-high N (HN) leaves than in other leaves. On the other hand, N allocation to C4 enzymes [phosphoenolpyruvate carboxylase (PEPC) and pyruvate Pi dikinase (PPDK)] was unaffected by the growth conditions. Maximum photosynthetic rates (Pmax) per Rubisco content were similar irrespective of the growth light treatments. Carbon isotope ratios (delta13 C) in the leaf dry matter were more negative in LL leaves than in HL leaves (LL = -19.3% per hundred, HL = -16.0% per hundred) and independent of leaf N. Vein density was highest in HL-HN leaves, and leaf thickness was unaffected by the growth light treatments. From these results, we conclude that A. cruentus leaves would not acclimate efficiently to low growth light.     

Development of leaf photosynthetic parameters in Betula pendula Roth leaves: correlations with photosystem I density

The global modelling of photosynthesis is based on exact knowledge of the leaf photosynthetic machinery. The capacities of partial reactions of leaf photosynthesis develop at different rates, but it is not clear how the development of photoreactions and the Calvin cycle are co-ordinated. We investigated the development of foliar photosynthesis in the temperate deciduous tree Betula pendula Roth. using a unique integrated optical/gas exchange methodology that allows simultaneous estimation of photosystem I and II (PS I and PS II) densities per leaf area, interphotosystem electron transport activities, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. We combined these measurements with in vitro determinations of Rubisco, soluble protein and chlorophyll contents. We observed a strong increase in leaf photosynthetic capacity in developing leaves per leaf area, as well as per dry mass, that was paralleled by accumulation of leaf Rubisco. Enhanced mesophyll conductance was the outcome of increased carboxylation capacity and increased CO(2) diffusion conductance. However, Rubisco was only partly activated in the leaves, according to in vivo measurements of Rubisco kinetics. The amount of active Rubisco increased in proportion with development of PS I, probably through a direct link between Rubisco activase and PS I electron transport. Since the kinetics for post-illumination P700 re-reduction did not change, the synthesis of cytochrome b(6)f complex was also proportional to PS I. The synthesis of PS II began later and continued for several days after reaching the full PS I activity, but leaf chlorophyll was shared equally between the photosystems. Due to this, the antenna of PS II was very large and not optimally organized, leading to greater losses of excitation and lower quantum yields in young leaves. We conclude that co-ordinated development of leaf photosynthesis is regulated at the level of PS I with subordinated changes in PS II content and Rubisco activation.     

Regulation of Photosynthesis in Triazine-Resistant and -Susceptible Brassica napus

The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO₂ partial pressure (C_i), O₂ partial pressure, and leaf temperature (15-35°C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. Three categories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 35°C leaf temperatures with low C_i. When the carbon assimilation rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters were similar between the two biotypes. A second category, called feedback-limited photosynthesis, was evident at 15 and 25°C above 300 microbars C_i. The third category, photosynthetic electron transport-limited photosynthesis, was evident at 25 and 35°C at moderate to high CO₂. At low temperature, when the response curves of carbon assimilation to C_i indicated little or no electron transport limitation, the carbon assimilation rate was similar in the resistant and susceptible biotypes. With increasing temperature, more electron transport-limited carbon assimilation was observed, and a greater difference between resistant and susceptible biotypes was observed. These observations reveal the increasing importance of photosynthetic electron transport in controlling the overall rate of photosynthesis in the resistant biotype as temperature increases. Photochemical quenching of chlorophyll fluorescence (q_P) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q_P, but not when it had less than 60% q_P.     

Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content and Activity in Wheat, Rye and Triticale

Photosynthetic parameters were measured in triticale and its parents wheat and rye. Soluble protein content in leaves, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content per fresh mass, total chlorophyll content, biomass yield, leaf area, leaf mass and specific leaf mass were higher but Rubisco content expressed as percentage of soluble protein, carboxylase activity, photosynthetic rate and stomatal conductance were significantly lower in rye than in wheat. Native-PAGE of Rubisco revealed that rye carboxylase was different from that of wheat. The difference was not related to either the small or large subunit of Rubisco but, may be, to the ionic and/or other properties of the Rubisco protein moiety. Triticale Rubisco was similar to wheat. For most of the studied physiological parameters, triticale showed much more similarity with wheat than with rye.     

Regulation of Photosynthetic Rate of Two Sunflower Hybrids under Water Stress

The effect of short-term water stress on photosynthesis of two sunflower hybrids (Helianthus annuus L. cv Sungro-380 and cv SH-3622), differing in productivity under field conditions, was measured. The rate of CO₂ assimilation of young, mature leaves of SH-3622 under well-watered conditions was approximately 30% greater than that of Sungro-380 in bright light and elevated CO₂; the carboxylation efficiency was also larger. Growth at large photon flux increased assimilation rates of both hybrids. The changes in leaf composition, including cell numbers and sizes, chlorophyll content, and amounts of total soluble and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein, and in Rubisco activity and amount of ribulose-1,5-bisphosphate (RuBP) were determined to assess the factors regulating the differences in assimilation of the hybrids at high and low water potentials. The amounts of chlorophyll, soluble protein, Rubisco protein and the initial activity of Rubisco and its activation state did not differ significantly between hybrids. However, unstressed leaves of SH-3622 had more, smaller cells per unit area and 60% more RuBP per unit leaf area than that of Sungro-380. Water stress developing over 4 days decreased the assimilation of both hybrids similarly. Changes in the amounts of chlorophyll, soluble and Rubisco protein, and Rubisco activity and activation state were small and were not sufficient to explain the decrease in photosynthesis; neither was decreased stomatal conductance (or stomatal “patchiness”). Reduction of photosynthesis per unit leaf area from 25 to 5 micromoles CO₂ per square meter per second in both hybrids was caused by a decrease in the amount of RuBP from approximately 130 to 40 micromoles per square meter in SH-3622 and from 80 to 40 micromoles per square meter in Sungro. Differences between hybrids and their response to water stress is discussed in relation to control of RuBP regeneration.     

Physiological N response of field-grown maize hybrids (Zea mays L.) with divergent yield potential and grain protein concentration

Fertilizer N availability impacts photosynthesis and crop performance, although cause–effect relationships are not well established, especially for field-grown plants. Our objective was to determine the relationship between N supply and photosynthetic capacity estimated by leaf area index (LAI) and single leaf photosynthesis using genetically diverse field-grown maize (Zea mays L.) hybrids. We compared a high yield potential commercial hybrid (FR1064 x LH185) and an experimental hybrid (FR1064 x IHP) with low yield potential but exceptionally high grain protein concentration. Plant biomass and physiological traits were measured at tassel emergence (VT) and at the grain milk stage (R3) to assess the effects of N supply on photosynthetic source capacity and N uptake, and grain yield and grain N were measured at maturity. Grain yield of FR1064 x LH185 was much greater than FR1064 x IHP even though plant biomass and LAI were larger for FR1064 x IHP, and single leaf photosynthesis was similar for both hybrids. Although photosynthetic capacity was not related to hybrid differences in productivity, increasing N supply led to proportional increases in grain yield, plant biomass, LAI, photosynthesis, and Rubisco and PEP carboxylase activities for both hybrids. Thus, a positive relationship between photosynthetic capacity and yield was revealed by hybrid response to N supply, and the relationship was similar for hybrids with a marked difference in yield potential. For both hybrids the N response of single leaf CER and initial Rubisco activity was negative when expressed per unit of leaf N. In contrast, PEP carboxylase activity per unit leaf N increased in response to N availability, indicating that PEP carboxylase served as a reservoir for excess N accumulation in field-grown maize leaves. The correlation between CER and initial Rubisco activity was highly significant when expressed on a leaf area or a total leaf basis. The results suggest that regardless of genotypic yield potential, maize CER, and potentially grain yield, could be improved by increasing the partitioning of N into Rubisco.     

Photosynthesis in the basal growing zone of barley leaves

Cell proliferation, elongation, determination and differentiation mainly take place in the basal 5 mm of a barley leaf, the so-called basiplast. A considerable portion of cDNAs randomly selected from a basiplast cDNA library represented photosynthetic genes such as CP29, RUBISCO-SSU and type I-LHCP II. Therefore, we became interested in the role of the basiplast in establishing photosynthesis. (1) Northern blot analysis revealed expression of photosynthetic genes in the basiplast, although at a low level. Analysis of basiplasts at different developmental stages of the leaves revealed maximal expression of photosynthetic genes during early leaf development. The activity of these genes shows that plastid differentiation involves the development of the photosynthetic apparatus even at this early state of leaf cell expansion. (2) This conclusion was supported by the fact that chlorophylls and carotenoids are synthesized in the basiplast. The qualitative pattern of pigment composition was largely similar to that of fully differentiated green leaves. (3) The transition from proplastids to chloroplasts progressed in the basal 5 mm of the leaf, so that the number of grana lamellae per thylakoid stack increased with distance from the meristem from zero to about five. (4) Photosynthetic function was studied by chlorophyll a-fluorescence measurements. In dark-adapted 8-day-old primary leaves, the fluorescence ratio (F_P-F_o)/F_P was little decreased in basiplasts as compared to leaf blades. During steady state photosynthesis, the ratio (F_M-F_o)/F_M was high in leaf blade (0.5), but low in the sheath (0.25) and in the basiplast (0.18), indicating the existence of functional, albeit low light-adapted chloroplasts in the basiplast. (5) Further on, chlorophyll a fluorescence analysis in relation to seedling age revealed efficient photosynthetic performance in the basiplast of 3- to 6-day-old seedlings which later-on differentiates into leaf blade as compared to the basiplast of 7- to 12-day-old seedlings which develops into leaf sheath and finally ceases to grow. The leaf age dependent changes in basiplast photosynthesis were reflected by changes in pigment contents and LHCP II expression both of which also revealed a maximum in the basiplast of 4-day-old seedlings.Abbreviations bas 1 basiplast-associated gene 1 encoding a peroxide reductase - cab chlorophyll a/b binding protein - CP 29 29 kDa chlorophyll binding protein - DIG digoxigenin - EMIP epidermal major intrinsic protein - LHCP II light harvesting complex of Photosystem II - LSU large subunit of Rubisco - NPQ non photochemical chlorophyll a fluorescence quenching - PSI/PS II Photosystem I/II - PQ photochemical chlorophyll a fluorescence quenching - Rubisco Ribulose-1,5-bisphosphate carboxylase - SSU small subunit of Rubisco     

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

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

Effects of strontium on photosynthetic characteristics of oilseed rape seedlings

The threat of environmental strontium pollution led to an increased interest to elucidating the mechanisms of this metal toxicity in the organism. To investigate strontium effects on vital photosynthesis characteristics, three-leaf stage oilseed rape seedlings (Brassica napus L., cv. Mianyou No.15), raised in the hydroponic culture, were provided with a nutrient solution containing 0, 10, 20, and 40 mM SrCl₂. Strontium uptake and distribution in oilseed rape plants and its effect on various aspects of photosynthesis were investigated after 0, 7, 14, and 21 days of strontium treatment. Oilseed rape seedlings demonstrated a strong ability of strontium accumulation. Strontium absorbed by roots was primarily transferred to leaves and accumulated there. The leaf photosynthetic oxygen evolution rate, chlorophyll content, and Rubisco (EC 4.1.3.9) and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activities declined progressively with increasing concentration of applied strontium and also with increasing the duration of exposure time. These results indicate that strontium accumulated in leaves damaged various processes of photosynthesis, such as energy absorption, energy transfer, and photosynthetic carbon assimilation.     

Carbon Fixation Gradients across Spinach Leaves Do Not Follow Internal Light Gradients

In situ measurements of 14C-CO2 incorporation into 40-[mu]m paradermal leaf sections of sun- and shade-grown spinach leaves were determined. Chlorophyll, carotenoid, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content in similar 40-[mu]m paradermal leaf sections was also measured. The carbon fixation gradient did not follow the leaf internal light gradient, which decreases exponentially across the leaf. Instead, the 14C-CO2 fixation was higher in the middle of the leaf. Contrary to expectations, the distribution of carbon fixation across the leaf showed that the spongy mesophyll contributes significantly to the total carbon reduced. Approximately 60% of the carboxylation occurred in the palisade mesophyll and 40% occurred in the spongy mesophyll. Carbon reduction correlated well with Rubisco content, and no correlation between chlorophyll and carotenoid content and Rubisco was observed in sun plants. The correlation among chlorophyll, carotenoids, Rubisco, and carbon fixation was higher in shade leaves than in sun leaves. The results are discussed in relation to leaf photosynthetic and biochemical measurements that generally consider the leaf as a single homogeneous unit.     

In vivo temperature response functions of parameters required to model RuBP-limited photosynthesis

The leaf model of C₃ photosynthesis of Farquhar, von Caemmerer & Berry (Planta 149, 78–90, 1980) provides the basis for scaling carbon exchange from leaf to canopy and Earth‐System models, and is widely used to project biosphere responses to global change. This scaling requires using the leaf model over a wider temperature range than that for which the model was originally parameterized. The leaf model assumes that photosynthetic CO₂ uptake within a leaf is either limited by the rate of ribulose‐1,5‐bisphosphate (RuBP) regeneration or the activity of RuBP carboxylase‐oxygenase (Rubisco). Previously we reported a re‐parameterization of the temperature responses of Rubisco activity that proved robust when applied to a range of species. Herein this is extended to re‐parameterizing the response of RuBP‐limited photosynthesis to temperature. RuBP‐limited photosynthesis is assumed to depend on the whole chain electron transport rate, which is described as a three‐parameter non‐rectangular hyperbolic function of photon flux. Herein these three parameters are determined from simultaneous measurement of chlorophyll fluorescence and CO₂ exchange of tobacco leaves, at temperatures from 10 to 40 °C. All varied significantly with temperature and were modified further with variation in growth temperature from 15 to 35 °C. These parameters closely predicted the response of RuBP‐limited photosynthesis to temperature measured in both lemon and poplar and showed a significant improvement over predictions based on earlier parameterizations. We provide the necessary equations for use of the model of Farquhar et al. (1980) with our newly derived temperature functions for predicting both Rubisco‐ and RuBP‐limited photosynthesis.     

Reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase content by antisense RNA reduces photosynthesis in transgenic tobacco plants

A complementary DNA for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was cloned from tobacco (Nicotiana tabacum) and fused in the antisense orientation to the cauliflower mosaic virus 35S promoter. This antisense gene was introduced into the tobacco genome, and the resulting transgenic plants were analyzed to assess the effect of the antisense RNA on Rubisco activity and photosynthesis. The mean content of extractable Rubisco activity from the leaves of 10 antisense plants was 18% of the mean level of activity of control plants. The soluble protein content of the leaves of anti-small subunit plants was reduced by the amount equivalent to the reduction in Rubisco. There was little change in phosphoribulokinase activity, electron transport, and chlorophyll content, indicating that the loss of Rubisco did not affect these other components of photosynthesis. However, there was a significant reduction in carbonic anhydrase activity. The rate of CO₂ assimilation measured at 1000 micromoles quanta per square meter per second, 350 microbars CO₂, and 25°C was reduced by 63% (mean value) in the antisense plants and was limited by Rubisco activity over a wide range of intercellular CO₂ partial pressures (p_i). In control leaves, Rubisco activity only limited the rate of CO₂ assimilation below a p_i of 400 microbars. Despite the decrease in photosynthesis, there was no reduction in stomatal conductance in the antisense plants, and the stomata still responded to changes in p_i. The unchanged conductance and lower CO₂ assimilation resulted in a higher p_i, which was reflected in greater carbon isotope discrimination in the leaves of the antisense plants. These results suggest that stomatal function is independent of total leaf Rubisco activity.     

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 μmol photons m⁻² s⁻¹). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO₂ (35 μM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO₂-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.     

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

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