Peroxidase activity in Linum usitatissimum L.

Summary Crosses were made, in all combinations, between 2 parental genotypes of Linum and their reciprocal F ₁ hybrids. The parents and progeny obtained were grown in controlled environmental conditions and sampled at 35 and 70 days after germination to determine, on an individual plant basis, total plant fresh weight and peroxidase activity of main stem tissue. Peroxidase activity required transformation to a log₁₀ scale, whereas the original linear scale was satisfactory for plant weight. There was no correlation between plant weight and corresponding peroxidase activity. Pronounced heterosis appeared in the F ₁'s for both characters at sample 1, but this heterosis had declined at sample 2 and in the F ₂'s. Heterosis operated in a positive direction for plant weight and in a negative direction for peroxidase activity. No consistent differences were found amongst the variances of segregating or non-segregating generations for either character.     

Study of the effects produced by gamma-irradiation of common wheat F1 seeds using gliadins as genetic markers

A study of the effects produced by gamma-irradiation of dry F₁ seeds of common wheat at 200 Gy is described in the work. The experimental material was hybrids produced from near-isogenic lines based on Bezostaya 1. Irradiation led to a significant decrease in the productivity traits of F₁ plants but did not affect the survival of plants under the given growth conditions. It has been found that one of the effects of the F₁ seed irradiation was a relative, compared to the control, increase in the frequency of male gametes with the 1BL/1RS translocation that participated in the production of F₂ grains. The irradiation induced mutations at gliadin loci with a frequency of 7.4% (against 0.5% in the control).     

Exploitation of wild Cicer species for yield improvement in chickpea

 Chickpea (Cicer arietinum L.) ranks third in the world, and first in the Mediterranean basin, for production among pulses. Despite its importance as a crop and considerable research effort, traditional breeding methods have so far been unable to produce cultivars with a large impact on chickpea production. Interspecific hybridization is known to improve yield in many crops. Therefore, an attempt was made to increase the seed yield in chickpea through the introgression of genes from wild relatives at the International Center for Agricultural Research in the Dry Areas (ICARDA), Syria, from 1987 to 1995. Four crosses, ILC 482 (C. arietinum)×ILWC 179 (C. echinospermum) and ILC 482×ILWC 124 (C. reticulatum) and their reciprocals, were made. Pedigree selection was used to advance the material. Heterosis was recorded visually in F₁s, and single plant measurements for seed yield were recorded in F₂ populations. Promising and uniform progenies were bulked in the F₅ generation. Out of 96 F₆ lines, 22 were selected on the basis of seed yield and other agronomic characters, and evaluated in a replicated trial for seed yield and 14 agronomical, morphological and quality characters. A high level of heterosis was observed in F₁s. Several F₂ plants produced two to three times more seed yield than the best plant from the cultigen. Nine F₇ lines out-yielded the cultigen parent by up to 39%. Over 2 years, 12 lines had a higher yield than the cultigen parent. These lines were not only high yielding but also free of any known undesirable traits from the wild species, such as spreading growth habit, pod dehiscence, and non-uniform maturity. Quality traits, such as seed shape, type, colour, weight, and testa texture, protein content, cooking time and an organoleptic test of a Middle East dish, Homos Bi-Tehineh, were also similar to the cultigen parent. Both C. echinospermum and C. reticulatum contributed towards the increased yield. Received: 11 July 1996 / Accepted: 15 November 1996     

The cytogenetic effect of thermal neutrons inLens esculenta (Moench)

The first information concerning the cytogenetic efficiency of thermal neutrons in lentil are presented in this paper. The range of cytologically effective dosages of thermal neutrons in lentil was determined. This determination enables us to compare the efficiency with the mutagenic effect of thermal neutrons and with their effect on the growth and development of plants of M₁ generation. These effects were already evaluated in previous communications. Thermal neutron irradiation significantly affected all the characters studied. A linear dependence of the effect on the dose of the neutron radiation was found for most of the analyzed characters. From a sample of scored cells, whose nuclei were in the anaphase or early telophase, 9.0 to 72.0% of them had chromosomal bridges and fragments after irradiation with dosages from 3.3×10¹¹ n cm^?2 to 4.5×10¹² n cm^?2. The highest number of rearrangements per one cell reached 2.16 after irradiation with 4.25×10¹² n cm^?2 while the lowest dosage used, 3.3×10¹¹ n cm^?2, induced 0.17 of chromosomal rearrangements per one cell. Irradiation with thermal neutrons is capable of inducing a large number of very complicated chromosome rearragements.     

Effect of fast electrons on the reproductive biology of the grain miteAcarus siro (Acari: Acaridae)

Irradiation of the adult grain mite,Acarus siro L., with substerilizing doses of fast electrons significantly reduced fecundity and egg viability. The number of spermatophores found in the bursa of treated females was also considerably reduced. However, no change in the shape of the spermatophores was observed. It is not known whether the decrease in fecundity and egg viability was the result of the drop in spermatophore production or to the death of sperm cells.Fast electrons had less effect upon, fecundity than the same dose of gamma radiation. However, irradiation by fast electrons had a similar effect on egg viability (expressed in percent), number of spermatophores produced per individual, and the number of adult F₁ progeny.Fast-electron doses above 0.4 kGy caused sterility ofAcarus siro.     

Photoinhibition and xanthophyll cycle activity in bayberry (Myrica rubra) leaves induced by high irradiance

The effect of high irradiance (HI, photosynthetically active photon flux density of 1 300 μmol m⁻² s⁻¹) on net photosynthetic rate (P _N), chlorophyll fluorescence parameters, and xanthophyll cycle components were studied in fruit tree bayberry leaves. HI induced the photoinhibition and inactivation of photosystem 2 (PS2) reaction centres (RCs), which was characterized by decreased P _N, maximum yield of fluorescence after dark adaptation (F_m), photochemical efficiency of PS2 (F_v/F_m) and quantum yield of PS2 (Φ_PS2), and increased reduction state of Q_A (1-q_P) and non-photochemical quenching (NPQ). Initial fluorescence (F₀) showed a decrease after the first 2 h, and subsequently increased from the third hour exposure to HI. Furthermore, a greater increase in the ratio (F_i-F₀)/(F_p-F₀) which is an expression of the proportion of the Q_B non-reducing PS2 centres, whereas a remarked decrease in the slope of F_i to F_p which represents the rate of Q_A reduction was observed in leaves after HI exposure. Additionally, HI caused an increase in the pool size of the xanthophyll cycle pigments and sustained elevated contents of zeaxanthin (Z), antheraxanthin (A), and de-epoxidation state (DES) at the end of the irradiation period. During HI, decreased F_m, F_v/F_m, Φ_PS2, NPQ, slope of F_i to F_p, V+A+Z, and DES, and increased F₀, 1-q_P, ratio (F_i-F₀)/(F_p-F₀), and V were observed in dithiothreitol (DTT)-fed leaves compared to control ones under the same conditions. Hence photoinhibition caused by HI in bayberry was probably attributed to inactivation of PS2 RCs, and photoprotection from photodamage were mainly related to the xanthophyll cycle-dependent heat dissipation in excess photons.     

Efficiency of early generation selections for yield and related characters in safflower (Carthamus tinctorius L.)

The efficiency of early generation selection for yield and related characters in safflower (Carthamus tinctorius L.) was studied in the F₂, F₃ and F₄ generations. Twenty-five F₂ progenies derived from various crosses were studied. In the F₂ generation, number of capitula per plant (CNSP), number of seeds per capitulum (SPSP), test weight (SWSP), and seed yield (SYSP) were the criteria used for single plant selection. The analysis of variance showed significant differences for all of the characters in the F₂, F₃, and F₄ generations. The analysis of variance in each of the selection classes showed highly significant genotypic differences. A large number of selections in the CNSP and SYSP classes showed greater yield than the check variety. In each class the mean for that particular character showed a positive shift. The observed F₃ and F₄ means for seed yield per plant was higher in SYSP, indicating the effectiveness of single plant selection for yield. Correlated response showed that selection for number of capitula per plant was effective for improvement of yield.     

Organisation of Photosynthetic Apparatus of Triticale in Relation to Productivity

Chlorophyll (Chl) content, photochemical activity of chloroplasts as well as photosynthetic and crop productivity were studied in different winter hexaploid Triticale (xTriticosecale Witt.) lines and their F₁ hybrids. Heterosis enhanced Chl content, photosynthetic potential, photosynthetic productivity, and grain yield only in several F₁ hybrids of Triticale. Indication in some genotypes of close correlations among morphological structure, Chl content, photochemical activity of chloroplasts, photosynthetic potential, and plant productivity may be used in breeding practice of Triticale. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reduction of QA in the dark: Another cause of fluorescence Fo increases by high temperatures in higher plants

Increases in the chlorophyll fluorescence F_o (dark level fluorescence) during heat treatments were studied in various higher plants. Besides the dissociation of light-harvesting chlorophyll a/b protein complexes from the reaction center complex of PS II and inactivation of PS II, dark reduction of Q_A via plastoquinone (PQ) seemed to be related to the F_o increase at high temperatures. In potato leaves or green tobacco cultured cells, a part of the F_o increase was quenched by light, reflecting light-induced oxidation of Q_A ^- which had been reduced in the dark at high temperatures. Appearance of the F_o increase due to Q_A reduction depended on the plant species, and the mechanisms for this are proposed. The reductants seemed to be already present and formed by very brief illumination of the leaves at high temperatures. A ndhB-less mutant of tobacco showed that complex I type NAD(P)H dehydrogenase is not involved in the heat-induced reduction of Q_A. Quite strong inhibition of the Q_A reduction by diphenyleneiodonium suggests that a flavoenzyme is one of the electron mediator to PQ from the reductant in the stroma. Reversibility of the heat-induced Q_A reduction suggests that an enzyme(s) involved is activated at high temperatures and mostly returns to an inactive form at room temperature (25 °C).This revised version was published online in October 2005 with corrections to the Cover Date.     

Peculiarities of the effect of low-dose-rate radiation simulating high-altitude flight conditions on mice in vivo

In the present work, the effect of a low-dose rate of high-LET radiation in polychromatic erythrocytes of mice bone marrow was investigated in vivo. The spectral and component composition of the radiation field used was similar to that present in the atmosphere at an altitude of about 10 km. The dose dependence, adaptive response, and genetic instability in the F₁ generation born from males irradiated under these conditions were examined using the micronucleus test. Irradiation of the mice was performed for 24 h per day in the radiation field behind the concrete shield of the Serpukhov accelerator. Protons of 70 GeV were used over a period of 15–31 days, to accumulate doses of 11.5–31.5 cGy. The experiment demonstrated that irradiation of mice in vivo in this dose range leads to an increase in cytogenetic damage to bone marrow cells, but does not induce any adaptive response. In mice pre-irradiated with a dose of 11.5 cGy, an increase in sensitivity was observed after an additional irradiation with a dose of 1.5 Gy. The absence of an adaptive response suggests existence of genetic instability.     

Serial analysis of combining ability in diallel and fractional diallel crosses in linseed

Summary An analysis of the nature of the gene action for seven characters influencing productivity and wide adaptation in Linum usitatissimum L. was undertaken in this investigation over three seasons based on diallel and fractional diallel crosses, among a set of ten genetically diverse parents. Estimation of combining ability and the nature of geno-type-environment interactions for the above characters was also examined.A comparison of the means of the parents and hybrids over three environments showed positive heterosis for height, number of tillers, fruit-bearing branches and capsules per plant while negative heterosis was observed for flowering time. The remaining two characters viz., height at branching and seeds per five capsules showed little differences due to mutual cancellation of the components of heterosis.A serial analysis of combining ability over three seasons using Griffing's model (1956) has revealed significant sea-sonal effects on the estimates of combining ability effects. Additive gene action was found to be predominant in most of the characters although non-additive variation due to s.c.a. was equally prominent for fruit-bearing branches and capsules per plant. On the basis of g.c.a. for all the characters, M. 10 and N. 55, both Peninsular types were the best followed by N.P. (R.R.) 45 and Mohaba Local.In the fractional diallel analysis the relative estimates g.c.a. and s.c.a. were similar for s=9 and 7. However, there was overestimation in favour of s.c.a. when s was reduced to 3. The serial analysis of fractional diallel had confirmed the results of full diallel. Significant genotype x environment interactions for flowering time, height, height at branching, fruit-bearing branches and capsules per plant were observed in this analysis also. The results have indicated that reduction of the diallel to a fraction with s=n/2 would vitiate the order and precision of estimates of combining ability.The analysis of genotype x environment indicated that the pattern of interactions among the parents of different geographical groups was not similar but varied with the character under study. Considerable interactions were observed for capsules per plant, fruit-bearing branches and flowering time in that order respectively. A comparison of the parental and hybrid populations for the degree of interaction for all the characters had revealed that heterozygosis might not at all be directly related to the degree of homeostasis. Actually, heterozygotes were found to show more interactions than homozygotes for flowering time, height at branching and tiller number and lower interactions for the other four characters. Significant seasonal effect on the degree of heterosis for vegetative as well as reproductive characters was observed in a majority of F₁'s with variable degree of dominance.

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Zusammenfassung Über einen Zeitraum von drei Vegetationsperioden wurde mit einem Material von 10 genetisch verschiedenen Eltern anhand von vollständigen und unvollständigen Diallelkreuzungen die Genwirkung bei 7 die Ertrags- und Anpassungsfähigkeit von Linum usitatissimum L. beeinflussenden Merkmalen geprüft. Außerdem wurde die Kombinationseignung dieser Merkmale und die Genotyp-Umwelt-Interaktion untersucht.Ein Vergleich der Durchschnittswerte der Eltern und Hybriden an drei verschiedenen Anbauorten ergab positive Heterosis bezüglich der Merkmale Höhe, Anzahl der Triebe, samentragende Zweige und Kapseln je Pflanze, negative Heterosis wurde dagegen bezüglich des Merkmals Blütezeit beobachtet. Die restlichen zwei Merkmale, Höhe der Verzweigung und Samenansatz von je 5 Kapseln, zeigten infolge gegenseitiger Aufhebung der Heterosiskomponenten geringe Unterschiede.Eine dreijährige Reihenanalyse hinsichtlich der Kombinationseignung nach dem Modell von Griffing (1956) ergab einen signifikanten Einfluß des Anbaujahres auf die Schätzung der Kombinationseignung. Bei den meisten Merkmalen konnte auf additive Wirkung der Gene geschlossen werden, obgleich nichtadditive Wirkung als Folge von spezieller Kombinationseignung bei den Merkmalen fruchtende Sprosse und Kapseln je Pflanze ebenso ausgeprägt war. Hinsichtlich der allgemeinen Kombinationseignung für alle untersuchten Merkmale erwiesen sich die beiden Halbinsel-Sippen, M 10 und N 55, als die besten, gefolgt von P.P. (R.R.) 45 und Mohaba Local.In den unvollständigen diallelen Analysen waren die relativen Schätzungen für allgemeine und spezielle Kombinationseignung für s=9 und 7 ähnlich. Wenn s jedoch auf 3 reduziert wurde, ergab sich eine Überschätzung der speziellen Kombinationseignung. Die Reihenanalysen des unvollständigen DiallelVersuchs bestätigten die Ergebnisse des vollständigen Diallel-Versuchs. Auch in diesem Fall wurden signifikante Genotyp-Umwelt-Interaktionen bezüglich Blütezeit, Höhe, Verzweigungshöhe, fruchtende Sprosse und Kapseln je Pflanze beobachtet. Die Ergebnisse deuteten an, daß eine Verringerung des Diallel-Versuchs auf einen Teil mit s=n/2 die Reihenfolge und Genauigkeit der Schätzung der Kombinationseignung beeinträchtigen würde.Die Analyse der Genotyp-Umwelt-Interaktion zeigte, daß die Art der Interaktion bei den Eltern verschiedener geographischer Gruppen nicht gleich war, sondern je nach dem untersuchten Merkmal variierte. Beträchtliche Interaktionen wurden für die Merkmale Kapseln je Pflanze, fruchtende Sprosse und Blütezeit — in dieser Reihenfolge — beobachtet. Ein Vergleich der Eltern- und Hybridenpopulationen bezüglich des Grades der Interaktion hat bei allen Merkmalen ergeben, daß Heterozygotie durchaus nicht unmittelbar mit dem Grad der Homeostasis in Beziehung zu stehen braucht. Tatsächlich wurde gefunden, daß Heterozygote bezüglich Blütezeit, Verzweigungshöhe und Zahl der Triebe größere Interaktionen und für die anderen 4 Merkmale geringere Interaktionen als Homozygote zeigen. Bei der Mehrzahl der F ₁-Nachkommen wurde sowohl für die vegetativen wie reproduktiven Eigenschaften ein signifikanter Einfluß des Anbaujahres mit verschiedenem Dominanzgrad auf den Grad der Heterosis beobachtet.

     

Nigericin insensitive post-illumination reduction in fluorescence yield in Dunaliella tertiolecta (chlorophyte)

Cells of the green alga Dunaliella tertiolecta grown in a light/dark cycle were exposed to high light for about 15 min. In light, energy-dependent quenching reduced fluorescence emission and decreased PS II efficiency. Within 3 minutes after darkening fluorescence quenching largely relaxed. However, PS II fluorescence emission decreased again after further darkening. F_o and F_m decreased to the same relative extent and the PS II efficiency was not reduced. This Reduction in Fluorescence yield in Darkness, termed RFD for the purpose of this paper, lasted about 20 min. The deepoxidation state of xanthophylls remained unchanged during and after the 15-min exposure to high light. We show that RFD is insensitive to the uncoupler nigericin and thus unrelated to energy-dependent quenching. RFD correlated with a reduction of the PQ pool after darkening and low levels of far red or blue light (430 nm more than 460 nm) prevented RFD. This is in contrast to observations in higher plants, where a post-illumination reduction of the PQ pool causes and increase in F_o (Groom et al. (1993) Photosynth Res 36: 205–215). Changes in the adenylate energy charge were not correlated with RFD. Antimycin A and cyanide, both inhibitors of the PQ-oxidase, caused an increase in RFD whereas SHAM, an inhibitor of the chloroplastic glycolate-quinone oxidoreductase, caused a decrease. Low CO₂ concentrations, known to increase the oxygenase activity of Rubisco and to generate glycolate and P-glycolate in light, caused an increase in RFD. We propose that accumulated glycolate and P-glycolate reduce the PQ pool in darkness, leading to the formation of RFD. During RFD, 77 K fluorescence emission from PS II was more reduced than that from PS I, thus resembling a state I, state II transition. However, the reduction in fluorescence yield during RFD is much larger than the reduction previously attributed to state transitions and it is unclear whether RFD and state transitions are identical. The formation and relaxation of RFD increased with higher temperatures and the extent of RFD was largest at the growth temperature (25°C). RFD has to be taken into account when fluorescence is measured after darkening as it may be mistaken for energy-dependent quenching.Abbreviations F_o fluorescence, measured when PS II traps are open - F_o difference between F_o and F_o - F_m fluorescence, measured when PS II traps are temporarily closed - F_m difference between F_m and F_m - FR far red - PFD photosynthetically active photon flux density - PQ plastoquinone - RFD reduction in fluorescence in darkness - SHAM salicylhydroxamic acid - Q_A primary quinone acceptor of PS II     

Heterosis × nutrition interaction in Drosophila melanogaster

Summary The relationship between heterozygosity and the expression of heterosis at two different nutrition levels was investigated using Drosophila melanogaster. Average daily egg production and egg hatchability were measured in two parental strains and in F₁, F₂ and reciprocal backcross generations. Heterosis was more pronounced in the poor nutritional conditions. Two electrophoretic markers used to estimate the level of heterozygosity in F₂ and backcrosses revealed an excess of heterozygous genotypes. Quantitative genetic effects (an additive line effect and individual and maternal heterosis) were estimated for both traits in the two environments. Although this model gave a reasonable fit in most cases, some epistatic interaction would have to be invoked in order to explain fully the results.     

Dioxygen uptake by isolated thylakoids from lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> L.): simultaneous measurements of dioxygen uptake,pH change of the medium and chlorophyll fluorescence parameters

The setup has been elaborated for the simultaneous measurements of dioxygen uptake, pH changes, and chlorophyll a fluorescence parameters of an isolated thylakoid suspension. Using this equipment we have found at least three kinetically distinguishable components in the response of dioxygen uptake and pH increase to light intensity in the range of 0–1600 μE m⁻² s⁻¹. The pH changes were not observed in the presence of uncouplers (2 μM valinomycin plus 2 μM nigericin) while O₂ uptake increased by about 10% and F _v /F _m ratio appeared to be unaffected by this treatment. Treatment with DNP-INT, an inhibitor of plastoquinol oxidation, led to a significant reduction of pH increase and O₂ consumption whereas F _v /F _m was impaired only to 71% of the control. Incubation with catalase (580 U/ml) caused a total inhibition of oxygen uptake, while the pH increased and the F _v /F _m ratio decreased to about 60% and 85% of the control, respectively. The addition of catalase after the irradiation period led to an evolution of the same amount of dioxygen as was consumed during the light period. These results show that hydrogen peroxide was formed in the investigated system and accumulated during illumination. On the basis of the obtained data, three sites of dioxygen reduction within isolated thylakoid membranes and the dependence of dioxygen uptake on the photosystem activities were discussed.     

Influence of different Inocula of Meloidogyne arenaria on plant growth parameters and nematode multiplication in balsam (Impatiens balsamina)

Pathogenic effect of root-knot nematode Meloidogyne arenaria was studied on balsam (Impatiens balsamina) by inoculating the different inoculum levels of root-knot nematode. It was observed that the inoculum levels up to 2000 J₂ of root-knot nematode did not show significant reduction in plant growth characters as compared to control. Although the significant reduction in plant growth characters was recorded at and above 3000 J₂ of root-knot nematode, progressive increase in the host infestation as indicated by the number of galls as well as the population of root-knot nematode was recorded with an increase in the level of inoculum. However, the rate of nematode multiplication was reduced with the increase in the inoculum density of M. arenaria. It can be concluded from these results that the damaging threshold level of M. arenaria on balsam was found to be as 3000 J₂/plant.     

Diallel analysis for some quantitative characters in Petunia hybrida Hort

Summary Flowering time, plant height and flower size in Petunia hybrida Hort. (multiflora type) have been genetically analysed by means of a 5 × 5 diallel cross. The results indicated that: (1) the three characters are controlled by additive-dominance polygenic systems. The contribution of the additive gene actions to the genetic variance of flowering time was relatively higher than that of dominance. The reverse situation was found for plant height and flower size. (2) Dominance is ambi-directional for the three characters. Ratios of average dominance were in the range of partial for flowering-time, complete for plant height and overdominance for flower size. (3) Number of genes (or gene groups) controlling the characters are about 3, 3 and 5 for flowering time, plant height and flower size: respectively, (4) Heritability estimates are 0.84, 0.88 and 0.89 in the broad-sense and 0.40, 0.49 and 0.37 in the narrow-sense, for flowering time, plant height and flower size; respectively. (5) Heterosis as percent increase of the mean F₁-hybrid above the higher parent, or decrease below the lower parent, was observed for flowering time (+ 9.7% to +13.3%), for plant height (–13.6% to –20.3%) and for flower size (+2.5% to +16.0%).     

Growth control by phytochrome of de-etiolated bean hypocotyls mediated by chlorophyll fluorescence

The elongation of hypocotyls excised from de-etiolated seedlings of beans (Phaseolus vulgaris L. cv. British Wax) is inhibited by light, blue and red irradiations being equally effective. Conditions which decrease chlorophyll fluorescence, such as CO₂-free air, abolish the inhibitory effect of blue irradiation and enhance the inhibition by red light. Conversely, conditions which increase chlorophyll fluorescence, such as a N₂ atmosphere or irradiation through a chlorophyll filter, abolish the inhibitory effect of red light and enhance the inhibition by blue irradiation. The inhibitory effect of blue light is reversible by red irradiation under increased fluorescence as well as by far red. We propose that the chlorophyll fluorescence excited by blue and red irradiations in λ_F > 660 nm and λ_F > 720 nm, respectively, is responsible for the inhibitory effect of blue light and the reduction of the inhibitory effect of non fluorescing red light. Both red and blue wavelengths seem, therefore, to control hypocotyl elongation through phytochrome.     

Leaf structure and patterns of photoinhibition in two neotropical palms in clearings and forest understory during the dry season

We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphic, and more strongly affected by drought than HL-Scheelea. F_v/F_m (the ratio of variable to maximum chlorophyll fluorescence) and t_½ (the half-rise time of F_m) was lower in HL-leaves of both species, indicating photoinhibition. In HL-Scheelea, the light-induced reduction of F_v/F_m was much less than in HL-Socratea, and F_v/F_m recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in F_o indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of F_v/F_m in HL-plants relative to LL-plants on both leaf surfaces, particularly on the lower surface, due to a decrease of F_m in both surfaces and an increase in F., of lower surface. For both species, F_o for the lower surfaces of HL-plants was higher and t_½ was markedly lower than for the upper surface, as is typical for shade-adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought-enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.     

Influence of Manganese Toxicity on Photosynthesis in Ricebean (Vigna Umbellata) Seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) were reduced with increasing Mn concentration in nutrient solution. The reduction in g _s and E was more pronounced at 6 d of Mn treatment. However, P _N declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (F_v/F_m). A reduction in photochemical quenching (q_P) and excitation capture efficiency of open PS2 (F_v′/F_m′) with a concomitant increase in q_N was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in q_N) and limited electron transport (decreased q_P). This revised version was published online in August 2006 with corrections to the Cover Date.