Light-dependent inhibition of photosynthetic electron transport by zinc

The effects of zinc concentrations up to 400 μ M were examined on three photosynthetic electron transport reactions of thylakoids isolated from Pisum sativum L. cv. Meteor. Zinc (400 μ M ) had no effect on photosystem I mediated electron transport from reduced N,N,N',N'-tetramethyl- p -phenylenediamine to methyl viologen, but inhibited uncoupled electron flow from water to methyl viologen by ca 50% and to 2,6-dichlorophenol-indophenol (DCPIP) by ca 30% at saturating light levels. Zinc inhibition of DCPIP photoreduction was independent of the light intensity to which thylakoids were exposed. Decreasing the photon flux density below 400 μmol m⁻² s⁻¹ produced a logarithmic reduction in the zinc-induced inhibition of methyl viologen photoceduction; a stimulation of this reaction was observed below 80 μmol photons m⁻² s⁻¹. Increasing light intensity decreased the amount of zinc tightly bound to the thylakoid membranes, but increased the weakly associated zinc which could be removed by washing the membranes with buffer containing Mg². The results suggest that zinc acts on the photosynthetic electron transport system at two sites. Site 1 is on the oxidizing side of photosystem 2 and the inhibition by zinc is independent of the light intensity. Site 2 is between photosystems 1 and 2 and the electron flow can be positively or negatively affected by zinc depending on the light intensity.     

Endogenous abscisic acid levels are linked to decreased growth of bush bean plants treated with NaCl

This paper studies the relative importance of endogenous ABA and ion toxicity in the leaf growth inhibition caused by NaCl in salt-adapted and unadapted bush beans. Adaptation to salt-stress was achieved by germination of seeds in 75 m M NaCl, while unadapted plants were germinated in tap water. The adaptation process caused a transitory increase in leaf ABA, Na⁺ and Cl⁻ concentrations, while leaf expansion was inhibited. However, when grown for 8 or 13 days in 75 m M NaCl-containing nutrient solution, primary and first trifoliolate leaves of salt-adapted plants had greater areas than those of unadapted plants. Concentrations of ABA, Na⁺ and Cl⁻ in these leaves were lower in adapted plants, and a strong negative correlation between leaf expansion growth and either leaf Na⁺, Cl⁻ or ABA concentrations could be established. However, in the second trifoliolate leaves only the ABA, but not the Na⁺ or Cl⁻, concentrations were significantly correlated with leaf expansion. Our results suggest that salt-induced inhibition of leaf expansion growth in bush beans is mediated by ABA rather than Na⁺ or Cl⁻ toxicity. Moreover, the increase of ABA, induced by the salt-pretreatment, seems to play an important role in limiting the accumulation of Na⁺ and Cl⁻ in the leaves, leading to adaptation of bush beans to salt-stress.     

Tricolorin A, a potent natural uncoupler and inhibitor of photosystem II acceptor side of spinach chloroplasts

Tricolorin A, (11 S )-11-hydroxyhexadecanoic acid 11- O - α - l - rhamnopyranosyl-(1↠3)- O - α - l -{2- O -(2 S -methylbutanoyl)-4- O -(2 S -methylbutanoyl)}-rhamnopyranosil-(1↠2)- O - β - d -glucopyranosil-(1↠2)- β -fucopyranoside-(1,3'-lactone), the major phytogrowth inhibitor isolated from Ipomoea tricolor Cav. (Convolvulaceae) was found to be a potent uncoupler (U₅₀=0.33 μ M ) of photophosphorylation in spinach chloroplasts. Tricolorin A inhibited H⁺-uptake and adenosine 5'-triphosphate (ATP) synthesis, and stimulated basal and phosphorylating electron flows. Using a combination of two well-known fluorescent ΔpH probes, 9-aminoacridine and 9-amino-6-chloro-2-methoxyacridine, the uncoupling behavior of tricolorin A was also demonstrated for submitochondrial particles. Polarographic data showed that high concentrations (20 μ M ) of tricolorin A inhibited photosystem II (PSII) electron flow at the level of plastoquinone B (Q_B). Chlorophyll (Chl) a fluorescence analysis showed that tricolorin A induced accumulation of Q_A⁻ and strongly decreased the electron transport capacity, suggesting that the target of this molecule was located at the Q_B level. The macrocyclic lactone-type structure of this allelopathic agent proved to be an important structural requirement for uncoupling activity since its hydrolysis caused loss of the inhibitory potential.     

The contribution of electrostatic forces to the process of adherence of Candida albicans yeast cells to substrates

Abstract Thermoanaerobacter thermohydrosulfuricus Rt8.B1 catabolized xylose by the pentose phosphate pathway, and xylose isomerase and xylulokinase were inducible. The uptake of xylose was by two low-affinity, inducible systems. Both systems were resistant to the protonophore, tetrachlorosalicylanilide, the F₁F₀-ATPase inhibitor, N , N -dicyclohexylcarboiimide, and the sodium/proton antiporter, monensin. The high capacity system (100 nmol min⁻¹ (mg protein)⁻¹) was only expressed when the bacterium was grown with a high concentration of xylose (50 mM). It took more than 60 mM xylose to saturate the high capacity system. When T. thermohydrosulfuricus was grown with a low concentration of xylose (5 mM), xylose uptake was saturated by as little as 10 mM xylose (18 nmol min⁻¹ (mg protein)⁻¹). Cells grown with 50 mM xylose could not transport glucose, and high capacity xylose transport was not inhibited by glucose or non-metabolizable glucose analogues. Cells grown with 5 mM xylose transported glucose at a rapid rate (30 nmol min⁻¹ (mg protein)⁻¹), and low capacity xylose uptake was competitively inhibited by either glucose or 2-deoxy-glucose. Because the glucose uptake of cells grown on 5 mM xylose was competitively inhibited by xylose, it appeared that the low capacity xylose uptake system was a glucose/xylose carrier.     

The role of the chloroplast coupling factor in the inactivation of thylakoid membranes at low temperatures

Thylakoids isolated from spinach leaves ( Spinacia oleracea L. cv. Monatol) were exposed to variable low temperatures under non-freezing conditions. After incubation, changes in the activities of several photochemical reactions and physical properties of the membranes were measured at room temperature.
Cyclic photophosphorylation was strictly dependent on the temperature and the electrolyte concentration: decrease in temperature and increase in NaCl concentration enhanced membrane damage. Inactivation of photophosphorylation was accompanied by stimulation of non-cyclic electron transport, increase in proton permeability and decrease in δpH. When dicyclohexylcarbodiimide was added, the proton gradient became completely restored. The temperature- and salt-dependent breakdown of photophosporylation was closely related to the release of the chloroplast coupling factor (CF₁) from the membranes. The addition of Mg²⁺, very low concentrations of ATP or ADP, or higher concentrations of low-molecular-weight polyols prior to temperature treatment prevented thylakoid damage.
The data indicate that inactivation of photophosphorylation of thylakoids at low temperatures is determined to a considerable extent by the cold lability of the CF₁. As a consequence, it must be concluded that damage of biomembranes caused by freezing is not due solely to changes resulting from the ice formation but additionally by temperature-dependent alterations of cold-labile proteins. Moreover, the data explain the mechanism of non-colligative cryoprotection of isolated thylakoid membranes.     

Effect of gibberellic acid on K+ (Rb+) uptake and transport in sunflower roots

Four-week-old sunflower plants ( Helianthus annuus L. cv. Halcón), grown in different nutrient solutions, were used to study the effects of gibberellic acid (GA₃) on K⁺ (Rb⁺) uptake by roots or transport to the shoot. Gibberellic acid application to the nutrient solution did not affect the exudation process of excised roots. When GA₃ was sprayed on leaves 2 to 6 days before excising the roots, the rate of exudation and the K⁺ flux increased. When the exudation study was done keeping the roots in a nutrient solution in which Rb⁺ replaced K⁺, the GA₃ effects were evident also on Rb⁺ uptake and transport. In intact plants, GA₃ increased the Rb⁺ transported to the shoot but did not affect Rb⁺ accumulation in the root. It is suggested that these GA₃ effects can be explained if it is assumed that GA₃ acts on the transport of ions to the xylem vessels.     

Effect of potassium status on K+ (Rb)+ uptake and transport in sunflower roots

Young sunflower plants ( Helianthus annuus L. cv. Halcón), grown in nutrient solution at two K⁺ levels (0.25 and 2.5 m M ) were used to study the effect of K⁺ content in the root on uptake and transport of K⁺ to the exuding stream of decapitated plants. Roots of plants grown in low K⁺ gave higher exudation flux, higher K⁺ concentration in exudate and higher K⁺ flux than high K⁺ roots. After 6 h of uptake the K⁺ flux in low K⁺ roots was about three times that in high K⁺ roots. When the roots were kept in a nutrient solution in which Rb⁺ replaced K⁺, low K⁺ roots exuded much more Rb⁺ than K⁺ after the first 2 h, whereas high K⁺ roots exuded about similar amounts of K⁺ and Rb⁺. In intact plants grown at three different K⁺ levels (0.1, 1.0 and 10.0 m M ), there was an inverse relationship between the K⁺ level in the nutrient solution and the Rb⁺ accumulated in the roots or transported to the shoot. The results suggest that the transport of ions from xylem parenchyma to stele apoplast may be controlled by ions coming down from the shoot in sieve tubes.     

Heterosis and ion transport in hexaploid varieties of rye-wheat (triticale) compared to the parental species

Important winter and spring varieties of hexaploid rye-wheat (triticale cvs. 6048 and 5004) were selected for study of heterotic effects on growth and ion transport in the hybrids compared to the parental species rye ( Secale cereale L. cvs. MT 77 and Sv 6970) and wheat (Triticum aestivum L. cvs. Starke II and Sonett). After 3 days germination, seedlings were grown 11 days in water culture on a complete nutrient solution diluted to 1, 25 and 50%. Intracellular influx and transport to shoots of K⁺, Ca²⁺, sulphur and phosphate were determined by using radioactive tracers (⁸⁶Rb (for K⁺), ⁴⁵Ca, ³⁵S and ³²P). Varietal differences in the parameters studied were generally small compared to differences between species. The heterotic effect on growth of rye-wheat was mainly localized to the shoots at high ionic strengths (25% and 50%). There were no heterotic effects on ion influx or transport to the shoots. Ion influx and transport characteristics in rye-wheat appear to be inherited mainly from wheat. Growth of all species on 1% medium was severely reduced. At the low ionic strength ion influx was inhibited similarly for all species, except influx of K⁺ (⁸⁶Rb) which was higher in rye-wheat and wheat than in rye. Ion influx and transport in rye-wheat and wheat and in rye differed especially for 25% and 50% media. Rye had the highest ion influx and transport and the highest shoot/root fresh weight ratio at the high ionic strengths. To feed a comparatively large shoot, rye may compensate for a relatively small root system by efficient ion transport mechanisms.     

Influence of some metal chelators and light regimes on bioaccumulation and toxicity of Cd2+ in duckweed (Lemna gibba)

A factorial culture experiment was designed to investigate the influence of light regimes and of some metal chelators on the accumulation of cadmium by Lemna gibba L. The plants were grown in a complete nutrient solution containing Cd²⁺ concentrations ranging from 0 to 27 μ M with or without EDTA, ethylenediamine-N,N'- bis -( o -hydroxyphenylacetic acid) (EDDHA) or salicylic acid. Each experiment was run for eight days in 18 h:6 h light:dark or continuous light. An increase in the Cd²⁺ concentration in plants and a simultaneous drop in accumulation efficiency (ratio of Cd²⁺ concentration in plants to the initial Cd²⁺ concentration in the nutrient solution) with increasing ambient Cd²⁺ levels was best represented by regression power curves. At the lowest Cd²⁺ concentration which caused a significant decrease in the relative growth rate of duckweed, there was a decrease in manganese and zinc and an increase in the iron level in the plants. EDDHA and EDTA protected in some cases against the toxic action of cadmium without preventing its uptake by plants. It was thus observed that 9 μ M or higher levels of Cd²⁺ were toxic to Lemna gibba depending on the chelator and light regime. Duckweed grown in continuous light produced, in general, more dry matter and hence accumulated more cadmium.     

Effects of cadmium on growth of Helianthus annuus seedlings: physiological aspects

Sunflower seedlings ( Helianthus annuus hybrid Select) were grown in a complete nutrient solution in the absence or presence of Cd²⁺ (10 and 20 μM). Analyses were performed to establish whether there was a differential effect of Cd²⁺ on mature and young leaves. After 7 d the growth parameters as well as the leaf area had decreased in both mature and young leaves. Accumulation of Cd²⁺ in the roots exceeded that in the shoots. Seedlings treated with Cd²⁺ exhibited reduced contents of chlorophyll and CO₂ assimilation rate, with a greater decrease in young leaves. The photochemical efficiency of photosystem II (PSII) was not altered by Cd²⁺ treatment in either mature or young leaves, although during steady-state photosynthesis in young leaves there was a significant alteration in the following parameters: quantum yield of electron transport by PSII (Φ_PSII), photochemical quenching ( q _P), non-photochemical quenching ( q _NP), and excitation capture efficiency of PSII (Φ_exc).     

Chlorophyll fluorescence and photoacoustic characteristics in relationship to changes in chlorophyll and Ca2+ content of a Cu-tolerant Silene compacta ecotype under Cu treatment

The effect of Cu toxicity on photosynthetic function, chlorophyll and Ca²⁺ content of Cu-tolerant Silene compacta plants grown in nutrient solution was studied. Since, in plants grown under 8 μ M Cu, the chlorophyll and Ca²⁺ concentration as well as the photosystem II (PSII) photochemistry were increased, compared to the control, the development of an adaptive mechanism of the Cu-tolerant ecotype of S. compacta to 8 μ M Cu is suggested. Increased Cu tolerance of the S. compacta ecotype reflects modulation of the photosynthetic apparatus to optimize photosynthesis. However, exposure of plants to 160 μ M Cu resulted in a marked increase of the fraction of closed PSII centres and decreased quantum yield of PSII electron transport (Φ_PSU) which was accompanied by a significant decline of relative quantum yield for O₂ evolution (A_ox/A_pt). The concentration of chlorophyll and Ca²⁺ in leaves also decreased significantly under 160 μ M Cu treatment. Photochemical quenching (q_p) displayed a reduction as a result of perturbation of the photosynthetic electron transfer chain, while non-photochemical quenching (q_N) increased. High Cu treatment reduced photosynthetic productivity of S. compacta plants which can be attributed, in part, to pertubation of photosynthetic process and photosynthetic pigments as well as to Ca²⁺ loss.     

Direct and indirect effects of Cd2+ on photosynthesis in sugar beet (Beta vulgaris)

Sugar-beet plants ( Beta vulgaris L. cv. Monohill) were cultivated for 4 weeks in a complete nutrient solution. Indirect effects of cadmium were studied by adding 5, 10 or 20 μ M CdCl₂ to the culture medium while direct effects were determined by adding 1, 5, 20, 50 or 2 000 μ M CdCl₂ to the assay media. The photosynthetic properties were characterized by measurement of CO₂ fixation in intact plants, fluorescence emission by intact leaves and isolated chloroplasts, photosystem (PS) I and PSII mediated electron transport of isolated chloroplasts, and CO₂-dependent O₂ evolution by protoplasts. When directly applied to isolated leaves, protoplasts and chloroplasts. Cd²⁺ impeded CO₂ fixation without affecting the rates of electron transport of PSI or PSII or the rate of dark respiration. When Cd²⁺ was applied through the culture medium the capacity for, and the maximal quantum yield of CO₂ assimilation by intact plants both decreased. This was associated with: (1) decreased total as well as effective chlorophyll content (PSII antennae size), (2) decreased coupling of electron transport in isolated chloroplasts, (3) perturbed carbon reduction cycle as indicated by fluorescence measurements. Also, protoplasts isolated from leaves of Cd²⁺-cultivated plants showed an increased rate of dark respiration.     

Effect of low growth temperature on coupling between electron transport and proton flux in Vicia faba thylakoids

Coupling between electron transport and proton flux has been compared in chloroplasts from Vicia faba (cv. Windsor) plants grown at 20 and 5°C. Proton uptake by warm-grown thylakoids was sensitive to external pH and stimulated by micromolar adenine nucleotide above pH 7.0. Electron transport was modulated by pH, adenine nucleotide and energy transfer inhibitors (triphenyltin and Hg²⁺). By contrast, proton uptake by cold-grown thylakoids was generally lower and was insensitive to micromolar ATP. The rate of non-phosphorylating electron flow in cold-grown thylakoids was relatively insensitive to pH and Hg²⁺ and was not modulated by adenine nucleotides or triphenyltin. Stimulation of electron transport by phosphorylating conditions in cold-grown thylakoids was generally lower and insensitive to pH. It is concluded that the control of proton efflux through CF₀-CF₁ differs in thylakoids of V. faba grown at warm and cold temperatures.     

Characterization of Na+ exclusion mechanisms of salt-tolerant reed plants in comparison with salt-sensitive rice plants

Salt-tolerant reed plants ( Phragmites communis Trinius) and salt-sensitive rice plants ( Oryza sativa L. cv. Kinmaze) were grown in salinized nutrient solutions up to 50 m M NaCl, and growth, Na⁺ contents and kinetics of ²²Na⁺ uptake and translocation were compared between the species to characterize the salt tolerance mechanisms operating in reed plants. When both plants were grown under the same salinity, Na⁺ contents of the shoots were lower in reed plants, although those of the roots were quite similar. The shoot base region of both species accumulated Na⁺ more than the leaf blades did. Sodium-22 uptake and pulse-chase experiments suggested that the lower Na⁺ transport rate from root to shoot could limit excessive Na⁺ accumulation in the reed shoot. There was a possibility that the apparently lower ²²Na⁺ transport rate to the shoot of reed plants was due to net downward Na⁺ transport from shoot base to root.     

NO2‐induced nitrate reductase activity in needles of Norway spruce (Picea abies) under laboratory and field conditions

The induction of activity of the enzyme nitrate reductase (NR, EC 1.6.6.1, 1.6.6.2) in needles of Norway spruce ( Picea abies [L.] Karst.) by nitrogen dioxide (NO₂) was studied under laboratory and field conditions. In fumigation chambers an increase in nitrate reductase activity (NRA) was detected 4 h after the start of the NO₂ treatment. During the first 2 days with 100 µg NO₂ m⁻³, NRA reached a constant level and did not change during the following 4 days. At the same level of NO₂, NRA was lower in needles from trees grown on NPK‐fertilized soil than on non‐fertilized soil. After the transfer of spruce trees from fertilized soil to NPK‐rich nutrient solution, NRA was transiently increased. This effect was assigned to root injuries causing nitrate transport to the shoot and subsequent induction of NRA. Neither trees on fertilized soil nor trees transferred to NPK‐poor nutrient solution had increased NRA unless NO₂ was provided. The NO₂ gradient in the vicinity of a highway was used to test the long‐term effect of elevated levels of NO₂ on needle NRA of potted and field‐grown spruce trees. Compared with less polluted sites, permanently increased NRAs were detected when NO₂ concentrations were above 20 µg m⁻³. Controls of field measurements some 10 years after the introduction of catalytic converters in cars showed no significant change neither in NO₂ levels nor in the decreasing NRA of spruce needles with the distance from the highway.     

Influence of photoinhibition on electron transport and photophosphorylation of isolated chloroplasts

The effects of a photoinhibition treatment (PIT) on electron transport and photophosphorylation reactions were measured in chloroplasts isolated from triazine-resistant and susceptible Chenopodium album plants grown under high and low irradiance. Electron transport dependent on photosystem I (PSI) alone was much less affected by PIT than that dependent on both photosystem II (PSII) and PSI. There was a smaller difference in susceptibility to PIT between the photophosphorylation activitity dependent on PSI alone and that dependent on both PSII and PSI. Because in all cases photophosphorylation activity decreased faster upon PIT than the rate of electron transport, we conclude that photoinhibition causes a gradual uncoupling of electron transport with phosphorylation. Since the extent of the light-induced proton gradient across the thylakoid membrane decreased upon PIT, it is suggested that photoinhibiton causes a proton leakiness of the membrane. We have found no significant differences to PIT of the various reactions measured in chloroplasts isolated from triazine-resistant and susceptible plants. We have also not observed any significant differences to PIT of the photophosphorylation reactions in chloroplasts of plants grown under low irradiance, compared with those grown under high irradiance. However, the electron transport reactions in chloroplasts from plants grown under low irradiance appeared to be somewhat less sensitive to PIT than those grown under high irradiance.     

In vivo and in vitro effects of boron on the plasma membrane proton pump of sunflower roots

The effect of boron excess and deficiency on H⁺ efflux from excised roots from sunflower ( Heliarahus annuus L. cv. Enano) seedlings and on plasma membrane H⁺-ATPase (EC 3.6.1.35) in isolated KI-washed microsomes has been investigated. When seedlings were grown in media with toxic levels of H₃BO₃ (5 m M ) or without added boron and exposed to light conditions, an inhibition of the capacity for external acidification by excised roots was observed as compared to roots from seedlings grown with optimal H₃BO₃ concentration (0.25 m M ). Toxic and deficient boron conditions also inhibited the vanadate-sensitive H⁺-ATPase of microsomes isolated from the roots. The mechanism of boron toxicity was investigated in vitro with microsorne vesicles. A strong effect of boron on the vanadate-sensitive, ATP-dependent H⁺ transport was found, but the vanadate-sensitive phospho-bydrolase activity was not affected. These results suggest that boron could exert an effect on the plasma membrane properties, directly or indirectly regulating, proton transport.     

Photosynthesis, growth and structural characteristics of holm oak resprouts originated from plants grown under elevated CO2

The physiological characteristics of holm oak ( Quercus ilex L.) resprouts originated from plants grown under current CO₂ concentration (350 μl l⁻¹) (A-resprouts) were compared with those of resprouts originated from plants grown under elevated CO₂ (750 μl l⁻¹) (E-resprouts). At their respective CO₂ growth concentration, no differences were observed in photosynthesis and chlorophyll fluorescence parameters between the two kinds of resprout. E-resprouts appeared earlier and showed lower stomatal conductance, higher water-use efficiency and increased growth (higher leaf, stem and root biomass and increased height). Analyses of leaf chemical composition showed the effect of elevated [CO₂] on structural polysaccharide (higher cellulose content), but no accumulation of total non-structural carbohydrate on area or dry weight basis was seen. Four months after appearance, downregulation of photosynthesis and electron transport components was observed in E-resprouts: lower photosynthetic capacity, photosystem II quantum efficiency, photochemical quenching of fluorescence and relative electron transport rate. Reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) activity, deduced from the maximum carboxylation velocity of RuBisCo, accounts for the observed acclimation. Increased susceptibility of photosynthetic apparatus to increasing irradiance was detected in E-resprouts.     

Effects of di-n-butyl phthalate on growth and photosynthesis in algae and on isolated organelles from higher plants

Di- n -butyl phthalate (DBF) is widely used as a plasticizer and has been found in all types of ecosystems. It inhibits growth and photosynthesis of green algae ( Chlorella emersonii CCAP strain 211/8 h and Selenastrum capricornutum CCAP strain 278/4) at concentrations higher than 10-⁵ M . The IC₅₀ value for CO₂-dependent oxygen evolution in algae was 3 × 10^-4M. The CO₂-reduction in isolated protoplasts prepared from barley ( Hordeum vulgare L. cv. Simba) was also inhibited by phthalate. The IC₅₀ value was 2 × 10^-4 M . The electron transport in isolated thylakoids prepared from spinach was inhibited with an IC₅₀ value of 3 × 10^-4 M . The IC₅₀ value for uncoupled electron transport extrapolated to zero chlorophyll concentration was 2.5 × 10^-5 M . The effect of di-n-butyl phthalate was localized to reactions in photosystem II. Di-n-butyl phthalate could thus be a pollutant which affects growth and photosynthesis of plants. The reported IC₅₀ values may be underestimated since di- n -butyl phthalate can attach to surfaces. The results are discussed in relation to observed effects of di- n -butyl phthalate on other organisms.     

Effect of plant density on the uptake and distribution of 14C in the field bean, Vicia faba

Whole bean plants, ev. Cockfield, grown in pots crowded or well-spaced (50 or 10 plants m², respectively) were treated with ¹⁴CO₂ at the pod-fill stage (25 modes) and the radioactivity in each leaf was determined after 30 min. With spaced plants the uptake was greatest in the mid-stem leaves and was proportional to leaf area. In contrast, 70% of the total assimilation took place in the upper six leaves of crowded plants and there was a steady decrease down the stem.
When ¹⁴CO₂ was fed to single leaves of similar crowded plants the resultant distribution of labelled assimilates varied with the position of the treated leaf. After 6 h, 67% of the ¹⁴C fixed by a mid-stem leaf (node 13) was recovered from the beans, whereas 76% of that from an upper leaf (node 23) had accumulated along the stem. Due to the shading of mid-stem leaves at the higher planting densities, seed yield becomes increasingly dependent upon re-distribution of assimilates from stem to beans.