Correlations among leaf CO2-exchange rates,areas and enzyme activities among soybean cultivars

Soybean (Glycine max (L.) Merr.) genotypes varying in area per nodal unit (usually a trifoliolate) and maturity class were grown in plots at the University of Illinois experimental farm. Leaf CO₂-exchange rates per unit area (CER) were measured under sunlight on intact plants. In addition to previously reported correlations with specific leaf weight and chlorophyll, CER was positively correlated with ribulose bisphosphate carboxylase (RuBPcase) activity, specific activity, and soluble protein, and was negatively correlated with area per leaf unit. The CER: chlorophyll correlation was destroyed by high CER values in 2 chlorophyll-deficient lines. CER values for 27 of the 35 lines tested fell within the range of those for isolines of cultivar Clark varying in leaf characteristics. The CER values were highest for fully expanded leaves during rapid pod fill. These results suggested that photoperiod (maturity) genes and genes for leaf area growth interact with genes controlling photosynthetic CO₂-exchange to produce the major differences in CER values among soybean genotypes.     

Response of leaf ontogeny and photosynthetic activity to reproductive growth in cotton

This study was conducted to determine if reproductive growth in cotton (Gossypium hirsutum L.) affects concurrent leaf development. Apparent photosynthesis (AP), stomatal conductance (Cs), soluble protein (SP), ribulose bisphosphate carboxylase (RuBisCO), and chlorophyll (Chl) were monitored in four main-stem cotton leaves which emerged at approximately 2 week intervals. The leaf which emerged during vegetative growth (48 days after planting) had higher AP, SP, and RuBisCO levels than that present in any leaves which emerged during fruit development. The last leaf studied (89 days after planting) was still present after boll maturation was completed and exhibited a rejuvenation in AP, SP, RuBisCO, and Chl starting at 30 days after leaf emergence. At 96 days after planting, the P700 Chl a-protein complex (PSI) was virtually absent from the leaves that emerged at 48 and 62 days after planting. The light harvesting Chl a/b complex was still present in these leaves, indicating greater degradation of PSI. The data emphasize the influence of developing fruit on concurrently developing leaves, an effect which was alleviated after boll maturation was completed. The declining AP per unit leaf area and smaller leaf size at the top of the plant results in a reduced photosynthetic potential of successively later emerging leaves. This reduction in leaf AP is consistent with earlier reported seasonal canopy photosynthesis patterns.     

Effects of cotton rootstock on endogenous cytokinins and abscisic acid in xylem sap and leaves in relation to leaf senescence

Leaf senescence varies greatly among cotton cultivars, possiblydue to their root characteristics, particularly the root-sourcedcytokinins and abscisic acid (ABA). Early-senescence (K1) andlate-senescence (K2) lines, were reciprocally or self-graftedto examine the effects of rootstock on leaf senescence and endogenoushormones in both leaves and xylem sap. The results indicatethat the graft of K1 scion onto K2 rootstock (K1/K2) alleviatedleaf senescence with enhanced photosynthetic (Pn) rate, increasedlevels of chlorophyll (Chl) and total soluble protein (TSP),concurrently with reduced malondialdehyde (MDA) contents inthe fourth leaf on the main-stem. The graft of K2 scion ontoK1 rootstock enhanced leaf senescence with reduced Pn, Chl,and TSP, and increased MDA, compared with their respective self-graftedcontrol plants (K1/K1 and K2/K2). Reciprocally grafted plantsdiffered significantly from their self-grafted control plantsin levels of zeatin and its riboside (Z+ZR), isopentenyl andits adenine (iP+iPA), and ABA, but not in those of dihydrozeatinand its riboside (DHZ+DHZR) in leaves in late season, whichwas consistent with variations in leaf senescence between reciprocallyand self-grafted plants. The results suggest that leaf senescenceis closely associated with reduced accumulation of Z+ZR, andiP+iPA rather than DHZ+DHZR, or enhanced ABA in leaves of cotton.Genotypic variation in leaf senescence may result from the differencein root characteristics, particularly in Z+ZR, iP+iPA, and ABAwhich are regulated by the root system directly or indirectly. Key words: Abscisic acid, cotton, cytokinins, grafting, leaf senescence Received 23 October 2007; Revised 17 January 2008 Accepted 23 January 2008     

Profiles of Leaf Nitrogen and Light in Reproductive Canopies of Cotton (Gossypium hirsutum)

During vegetative growth, the vertical profile of leaf nitrogen(N) often parallels the profile of light distribution withinthe canopy. This is more advantageous in terms of canopy photosynthesisthan a uniform distribution of leaf N. We investigated the influenceof both reproductive growth and N supply on the profiles ofN and light in canopies of irrigated cotton crops (Gossypiumhirsutum L.). Regular samplings were made from soon after theonset of reproductive growth until crop maturity. Every 2 weeks,a 1 m²sample of the canopy was cut in four successive verticallayers of equal thickness. Leaf area and N concentration (%)in each layer were measured. The vertical N gradient becamemore marked with ongoing reproductive development. It is hypothesizedthat because of the high rate of growth after the onset of reproductivedevelopment and the long duration of this phase compared toother species, the whole canopy photosynthetic benefit thatwould accrue from maintaining the N gradient is likely to beaccentuated. The rate of decline in leaf N concentration ina layer was not related to either the initial concentrationin the leaves nor the boll load within the layer.Copyright 2001Annals of Botany Company Gossypium hirsutum, leaf nitrogen, light profile, nitrogen, nitrogen distribution, remobilization, reproductive growth     

Nitrogen recycling and remobilization are differentially controlled by leaf senescence and development stage in Arabidopsis under low nitrogen nutrition

Five recombinant inbred lines (RILs) of Arabidopsis (Arabidopsis thaliana), previously selected from the Bay-0 x Shahdara RIL population on the basis of differential leaf senescence phenotypes (from early senescing to late senescing) when cultivated under nitrogen (N)-limiting conditions, were analyzed to monitor metabolic markers related to N assimilation and N remobilization pathways. In each RIL, a decrease of total N, free amino acid, and soluble protein contents with leaf aging was observed. In parallel, the expression of markers for N remobilization such as cytosolic glutamine synthetase, glutamate dehydrogenase, and CND41-like protease was increased. This increase occurred earlier and more rapidly in early-senescing lines than in late-senescing lines. We measured the partitioning of (15)N between sink and source leaves during the vegetative stage of development using (15)N tracing and showed that N remobilization from the source leaves to the sink leaves was more efficient in the early-senescing lines. The N remobilization rate was correlated with leaf senescence severity at the vegetative stage. Experiments of (15)N tracing at the reproductive stage showed, however, that the rate of N remobilization from the rosettes to the flowering organs and to the seeds was similar in early- and late-senescing lines. At the reproductive stage, N remobilization efficiency did not depend on senescence phenotypes but was related to the ratio between the biomasses of the sink and the source organs.     

Photosynthesis and transpiration of the flag leaf in four spring-wheat cultivars

Co₂ exchange and transpiration rates of the flag leaves of four spring wheat (Triticum aestivum L.) cultivars, namely Glenlea, Neepawa, Opal and Kolibri, were compared using infra-red gas-analysis technique. The plants were grown in a controlled environment under an 18-h photoperiod, with day and night temperatures of 20 and 15° C, respectively. The time course of the CO₂-exchange rate (CER) of the flag leaf differed among cultivars. CER began to decrease rapidly some 2 weeks after ear emergence in Glenlea, Neepawa and Kolibri, but only after 4 weeks in Opal. The decline in CER of Glenlea, Neepawa and Opal was continuous throughout the period of grain development whereas in Kolibri CER was maintained at a constant level between the 4th and 6th weeks after ear emergence. The transpiration rates of the flag leaves of the 4 cultivars did not change markedly until 6–7 weeks after ear emergence, indicating that the reduction in CER was not primarily a response to increased stomatal resistance to the diffusion of CO₂. Removing the ear of the main shoot of intact plants failed to depress CER of the subtending flag leaf until 5 weeks after ear removal. Removing the ears of all the tillers of plants in which all but 3 tillers had been removed at ear emergence did not depress CER until 4 weeks after ear emergence, but removal of the ear of the main shoot of plants where all the tillers had been removed at ear emergence reduced the CER of the flag leaf 2 weeks after ear removal. Removal of tillers at ear emergence had a marked effect on the time course of CER and transpiration rates of the flag leaf. Both CER and transpiration rates of a 4-tiller plant were maintained at a higher level throughout ear development as compared to those of a one-tiller plant. The transpiration rate of the flag leaf of Glenlea increased during the later part of the life of the leaf even for one-tiller plants with no ear, indicating that such a stomatal response may be part of the normal course of leaf aging and not a response to a feedback stimulus from the ear.     

Population dynamics ofHeloniopsis orientalis C. Tanaka (Liliaceae) in natural forests — Annual life cycle

Annual changes in the leaves and reproductive organs ofHeloniopsis orientalis C. Tanaka (Liliaceae), a perennial evergreen herb, were studied from 1991 to 1997 in two areas of South Korea, Namhansanseong and Maranggol. The period for active growth in the leaves was from mid-March to early June. Average leaf angle was 70° in early June, decreasing to 50° in late October. From December until June of each following year, leaf angle was maintained a 0° to horizontal. The specific leaf area (SLA) value was 185 cm².g^-1 early in the growing season, increasing to 332 cm²g^-1 in early June. By the end of October, SLA had decreased to 159 cm²g^-1, after which it increased again from March to June. Because the SLA curve had two peaks, it was inferred thatH. orientalis possesses two means for survival: 1) an anti-freezing mechanism by which its leaves thicken during the winter, and 2) a reallocation of energy from old leaves to new leaves or to reproductive organs.H. orientalis flowered in a semi-enclosed state in late March. Blooming out of the bract, the front of the flower faced the ground. Growth of the peduncle ended in early June, at which point it was 60 cm long. At that time, the fruit was oriented so that the seeds were dispersed upward. Therefore one can see thatH. orientalis has two physiological features that enhance long-distance seed dispersal — a rather long peduncle relative to overall plant size and an upward seed-dispersal mechanism. In the Namhansanseong area, energy from the roots and old leaves was translocated to new leaves early in the growing season (from late March to early May). However, after mid-May, energy was re-translocated from new leaves to the roots. Moreover, the leaves on flowering plants grew more slowly than on non-flowering plants because energy was translocation to the reproductive organs. Therefore, new leaf growth depended on energy stores of the roots and the biomass of old leaves early in the growing season.     

Variation in photosynthetic components among photosynthetically diverse cotton genotypes

Photosynthesis is an important component of upland cotton (Gossypium hirsutum L.) yield, but little has been done to increase the photosynthetic performance within the cotton germplasm pool. Part of this dilemma is due to the multi-component aspect of this process and also to lack of information on genetic variation among such components. The objectives of this research were to identify genetic variability in photosynthetic components for six cotton genotypes previously shown to differ in leaf CO₂-exchange rates (CER) and to determine if an afternoon decline in photosynthesis altered genotypic differences in CER. CO₂-exchange rates were measured at several internal CO₂ levels (C_i) to generate CER vs. C_i curves for each genotype and thereby isolate some of the components of photosynthesis. Ribulose 1,5 bisphosphate carboxylase-oxygenase (Rubisco), hydroxypyruvate reductase, malate dehydrogenase, and catalase activities were assayed on leaves used to generate the CER vs. C_i curves. Ambient CER and chlorophyll (Chl) fluorescence measurements were taken before and after solar noon to test for an afternoon decline in photosynthesis. Dixie King, a low ambient CER genotype, exhibited a greater CO₂ compensation point, lower carboxylation efficiency, and reduced Photosystem II (PS II) activity than the other genotypes. The carboxylation efficiency of DES 119 was 13% greater than STV 508 and 29% greater than Dixie King, but not different from the other genotypes in 1994. Pee Dee 3 had greater maximum assimilation rate (A) than all other genotypes except STV 213 in 1993. Although no significant genotype by time of day interaction was detected, CER and Chl fluorescence variable to maximum ratio (Fv/Fm) were reduced 8% and 39%, respectively, in the afternoon as compared to the morning. This study demonstrates genetic variations in many of the components of photosynthesis. However, the narrow range of variation in such components for superior photosynthesizing genotypes explains why difficulties are encountered when breeding for increased photosynthesis.     

Phosphorus nutrition influence on leaf senescence in soybean

Remobilization of mineral nutrients from leaves to reproductive structures is a possible regulatory factor in leaf senescence. The relationship between P remobilization from leaves of soybean (Glycine max [L.] Merr. cv McCall) during reproductive development and leaf senescence was determined by utilizing soil P treatments that supplied deficient, optimum, and supraoptimum soil P levels. The soil P treatments simulated field conditions, being initiated at the time of planting with no subsequent addition or removal of P. It was hypothesized that P deficiency would accelerate leaf senescence and that supraoptimum P nutrition would delay the timing or rate of leaf senescence relative to plants grown with optimum P. Supraoptimum soil P led to a two- to fourfold increase in leaf P concentration compared with optimum P, and during senescence there was no net P remobilization from leaves for this treatment. Leaf P concentration was similar for plants grown at optimum or deficient soil P, and there was significant net P remobilization from leaves of both treatments in one of the two experiments. As indicated by changes in leaf N, carbon dioxide exchange rate, ribulose 1,5-bisphosphate carboxylase/oxygenase activity, and chlorophyll concentration, leaf senescence patterns were similar for all soil P treatments. Thus, it can be concluded that leaf senescence was not affected by either P deficiency or enhanced leaf P concentration resulting from supraoptimum soil P. The results suggest that P nutrition in general, and specifically P remobilization from leaves, does not exert any regulatory control on the process of leaf senescence.     

Influence of Potassium Deficiency on Photosynthesis,Chlorophyll Content,and Chloroplast Ultrastructure of Cotton Plants

In cotton (Gossypium hirsutum L.) grown in controlled-environment growth chamber the effects of K deficiency during floral bud development on leaf photosynthesis, contents of chlorophyll (Chl) and nonstructural saccharides, leaf anatomy, chloroplast ultrastructure, and plant dry matter accumulation were studied. After cotton plants received 35-d K-free nutrient solution at the early square stage, net photosynthetic rate (P _N) of the uppermost fully expanded main-stem leaves was only 23 % of the control plants receiving a full K supply. Decreased leaf P _N of K-deficient cotton was mainly associated with dramatically low Chl content, poor chloroplast ultrastructure, and restricted saccharide translocation, rather than limited stomata conductance in K-deficient leaves. Accumulation of sucrose in leaves of K-deficient plants might be associated with reduced entry of sucrose into the transport pool or decreased phloem loading. K deficiency during squaring also dramatically reduced leaf area and dry matter accumulation, and affected assimilate partitioning among plant tissues.     

Transgenic rice with low sucrose-phosphate synthase activities retain more soluble protein and chlorophyll during flag leaf senescence

We investigated whether changes in sucrose-phosphate synthase (EC 2.4.1.14, SPS) activity could alter N remobilization during leaf senescence. Transgenic rice (Oryza sativa L. cv. Nipponbare) with low SPS activities and wild-type rice plants were grown with basal N (1.0 mM NH₄NO₃) until the late vegetative stage. Subsequently, half of the plants were transferred to a low N (0.1 mM NH₄NO₃) condition to accelerate leaf senescence, and the others were continuously grown with basal N. With low N supply, the amounts of chlorophyll and soluble protein in flag leaf blades decreased after anthesis in both the low SPS plants and wild-type plants, although the decrease was less in the low SPS plants. Panicle weights were significantly lower in the low SPS plant than in the wild-type plant. These results suggest that the remobilization of N from flag leaves was diminished by suppressing the development of reproductive sinks in the low SPS plant.     

SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO₂ Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.     

Postflowering photoperiod regulates vegetative growth and reproductive development of soybean

Soybean development is controlled by environmental factors, primarily photoperiod and temperature. To date, photoperiod effects on flowering have been well studied but the performances and mechanism of postflowering photoperiod responses have not been fully understood, especially for the photoperiod effects on vegetative growth after flowering. In the present study, the responses of vegetative growth and reproductive development in soybean to different postflowering photoperiod regimes were investigated in four separate experiments. Three varieties of different maturity groups (MG) including the early (Dongnong 36, MG 000), medium (Dandou 5, MG IV), and late (Zigongdongdou, MG IX) were exposed to two photoperiods, short (10, 12 h) and long (15, 16 or 18 h). The results showed that postflowering photoperiod not only regulated reproductive development but also affected vegetative growth. Even when flowers and pods were removed, short-day (SD) treatment promoted leaf senescence. The onset of leaf senescence among varieties tested appeared to be dependent on photoperiod sensitivity. Leaf senescence of the late-maturing variety of Zigongdongdou (sensitive to photoperiod) was delayed more significantly than that of the medium and early-maturing varieties (less sensitive to photoperiod). Long-day (LD) treatments delayed leaf senescence and seed maturation in the late-maturing variety of Zigongdongdou plants with only the SD-induced leaves produced before flowering. LD treatments imposed from the beginning bloom, beginning pod setting or beginning seed filling delayed leaf senescence and seed maturation of late-maturing soybean variety (Zigongdongdou). Results of night-break with red (R) and far-red (FR) light demonstrated that postflowering photoperiod responses of soybean were R/FR reversible reactions and the phytochromes seemed to be functional as receptors of photoperiod signals even after flowering. It was proposed that the regulation of photoperiod on development of soybean was effective from emergence through maturation, and the postflowering photoperiod signals were also mediated by phytochromes similar to those before flowering. The flowering reversion in late-MG soybean varieties under LD was a direct result of LD and was not due to secondary effect of abscission of pods and flowers. Soybean leaves not only received SD signals but also LD signals; furthermore, the LD effects reversed the SD effects and vice versa.     

In vitro flowering of indica rice (Oryza sativa L. spp. indica)

Nodal explants of rice cultivar Pathumthani 1 (PT1; short-day photoperiod insensitive) were collected, surface-disinfected, and cultured on modified MS medium under in vitro conditions for 90 d. A total of 60% nodal explants generated flowering plantlets (with one inflorescence per cluster). The net photosynthetic rate was greater, and soluble sugars (including glucose, fructose, and sucrose) accumulated to higher levels in the leaves of flowering as compared to non-flowering plants. In contrast, chlorophyll a, chlorophyll b, total chlorophyll, and total carotenoid content were enriched to a greater degree in the leaves of non-flowering as compared to flowering plants. Also, growth performance parameters, including plant height, number of leaves per plant, leaf area, fresh weight, and dry weight of plantlets derived from seedlings were superior to those of plantlets derived from nodal explants. In addition, the protocol proved to successfully induce flowering in KDML 105, a short-day photoperiod-sensitive rice cultivar.     

竹柏2种颜色叶片的光合特性研究

为了解竹柏(Podocarpus nagi)的光合特性,以3 a生全绿叶和花叶竹柏为材料,测定其光合色素含量和气体交换参数。结果表明,全绿叶竹柏叶片的叶绿素a、叶绿素b、类胡萝卜素、叶绿素a+b、叶绿素a/b和叶绿素a+b/类胡萝卜素均显著高于花叶竹柏;全绿叶竹柏叶片的初始量子效率、最大光合速率和暗呼吸速率均显著高于花叶,而光饱和点和光补偿点均显著低于花叶;全绿叶竹柏叶片的初始羧化效率、光合速率、CO2饱和点和光呼吸速率均高于花叶,而CO2补偿点低于花叶。2种颜色叶片的气孔导度、蒸腾速率和水分利用效率均随着光合有效辐射的增大而增大,且均表现为全绿叶花叶,而胞间CO2浓度则相反,表现为花叶全绿叶。因此,全绿叶竹柏利用弱光的能力强于花叶竹柏,而花叶竹柏利用强光的能力更强,在园林绿化配置中,可根据2种颜色叶片的光合特性合理配置。     

Abscisic acid and cytokinins in the root exudates and leaves and their relationship to senescence and remobilization of carbon reserves in rice subjected to water stress during grain filling

The possible regulation of senescence-initiated remobilization of carbon reserves in rice (Oryza sativa L.) by abscisic acid (ABA) and cytokinins was studied using two rice cultivars with high lodging resistance and slow remobilization. The plants were grown in pots and either well-watered (WW, soil water potential = 0 MPa) or water-stressed (WS, soil water potential = -0.05 MPa) from 9 days after anthesis until they reached maturity. Leaf water potentials of both cultivars markedly decreased at midday as a result of water stress but completely recovered by early morning. Chlorophyll (Chl) and photosynthetic rate (Pr) of the flag leaves declined faster in WS plants than in WW plants, indicating that the water deficit enhanced senescence. Water stress accelerated starch remobilization in the stems, promoted the re-allocation of pre-fixed (14)C from the stems to grains, shortened the grain-filling period and increased the grain-filling rate. Sucrose phosphate synthase (SPS, EC 2.4.1.14) activity was enhanced by water stress and positively correlated with sucrose accumulation in both the stem and leaves. Water stress substantially increased ABA but reduced zeatin (Z) + zeatin riboside (ZR) concentrations in the root exudates and leaves. ABA significantly and negatively, while Z+ZR positively, correlated with Pr and Chl of the flag leaves. ABA, not Z+ZR, was positively and significantly correlated with SPS activity and remobilization of pre-stored carbon. Spraying ABA reduced Chl in the flag leaves, and enhanced SPS activity and remobilization of carbon reserves. Spraying kinetin had the opposite effect. The results suggest that both ABA and cytokinins are involved in controlling plant senescence, and an enhanced carbon remobilization is attributed to an elevated ABA level in rice plants subjected to water stress.     

Control of PAR-saturated CO2 exchange rate in some C3 and CAM plants

We measured PAR-saturated CO₂ exchange rate (CER), and leaf N, P and chlorophyll (Chl) concentrations in 21 plant species, selected to encompass as broad a range in specific leaf area (SLA) as possible, and encompassing non-succulent C3 as well as succulent CAM plants. We worked with plants growing under uniform conditions in the facilities of a biological research station to ensure that any correlations found were due to inherent, genetically controlled, relationships between the measured parameters and not due to variations in resource availability in different habitats. We found CER to be strongly correlated to SLA, leaf N concentration and Chl concentration. CER increased much faster with increasing leaf N concentration (CER ≈ N^3.1) than with increasing SLA (CER ≈ SLA^1.2). CER also increased much faster with leaf N concentration than with increasing Chl concentration (CER ≈ Chl^1.3), indicating the photosynthetic N-use efficiency (NUE) to be higher for plants with high N concentration than for plants with low N concentration (NUE ≈ N^2.1). Analysis of covariance showed that these relationships exist even when comparing plants of widely different growth forms - succulent or non-succulent, and of different photosynthetic pathways, as the C3 and CAM plants compared here. Testing against scaling coefficients calculated using dimensional analysis, showed that the scaling of N, Chl and CER against SLA was not merely a result of diluting N and Chl with carbon in thicker leaves but that SLA, probably through influencing light absorptio and/or CO2 diffusion pathway, played an independent role in controlling CER. This revised version was published online in July 2006 with corrections to the Cover Date.     

Consequences of the Elimination of Old Leaves upon Spring Phenological Events and the New Leaves Nutrient Concentration in a Wintergreen Woody Species in the Southern Hemisphere

Previous studies analyzed the importance of old leaves conservancy for wintergreen species plant growth only after early spring old leaves elimination. However, carbon and nutrient resources for growth could have already been translocated from old leaves to shoots during autumn. In this work, the effect of old leaves absence on the leaf mass per area (LMA, g m⁻²) and nutrient concentration of new spring leaves, shoot growth, and flowering was studied in Aristotelia chilensis, an Andean Patagonic woody wintergreen species of Argentina. Plants were studied after autumn defoliation (AD) or late winter defoliation (WD) and results were compared to those of undamaged control plants (CO). The new leaves LMA and mineral nutrient (N, P, K, and Mg) concentration values did not decrease in AD or WD compared to CO plants. Conversely, CO plants showed higher flowering intensity and shoot lengthening compared to AD or WD plants. There were not remarkable differences regarding the defoliation time, though non-flowering shoots grew in a lesser degree than the flowering shoots in WD plants. It was concluded that A. chilensis old leaves cohort is an important source to shoot growth and flowering but their absence does not affect the new leaves structure or nutritional status from early spring in either AD or in WD plants. New leaves formation probably is guaranteed by resources (carbon and nutrients) previously stored in stems or even in the buds containing the preformed leaves since March, by the end of summer. Provided the availability of complete resources for the new leaf flush independently of the old leaves A. chilensis would restore the carbon balance as soon as possible to resume the growth of heterotrophic tissues at normal rates. Endogenous response to counterbalance the old leaves absence on non-flowering shoots was more effective when there was greater lag time between defoliation and shoot growth resume. Flowering and non-flowering shoots compete for the available resources when A. chilensis have not yet expanded leaves and shoots supporting reproductive structures were stronger sinks compared to non-flowering shoots in WD plants.     

Nitrogen from senescing lower leaves of common bean is re-translocated to nodules and might be involved in a N-feedback regulation of nitrogen fixation

The objective of the present study was to elucidate whether remobilized N from lower leaves is involved in causing the drop in N(2) fixation during pod-filling in common bean (Phaseolus vulgaris L). Moreover, we addressed the question of whether remobilized N from lower leaves would reach the nodules. Nodulated common bean plants were grown in a growth chamber in quartz sand. During a 2-week period, at vegetative and at reproductive growth, 50% of the leaves (lower part) were either excised or individually darkened, thereby removing the same photosynthetic capacity yet allowing N to be remobilized from the darkened leaves. Moreover, at the vegetative growth period, three lower leaves per plants were (15)N labelled by applying (15)NH(4)NO(3) prior to imposing the darkening treatment. Leaf darkening at vegetative growth induced N remobilization as well as reduced N(2)-fixation rates and growth. Leaf excision at reproductive growth enhanced N(2) fixation. Changes in N(2)-fixation rates were in all cases the result of altered growth rates, while the % N in the whole plant and in various plant parts remained conserved. Directly after leaf labelling, but also at the end of the vegetative growth period, substantial amounts of (15)N from the leaves could be recovered in nodules in the control, and in higher amounts in the leaf-darkening treatment. It is proposed that nitrogen from leaves circulates within the plant via nodules, and that the strength or composition of this circular flow may be the signal for a putative N-feedback effect.     

激光共聚焦显微镜观察玉米茎尖形态学变化

以玉米光敏感自交系CML288和不敏感自交系黄早4为实验材料,采用长日照15 h、短日照9 h的不同光周期处理,利用激光扫描共聚焦显微镜(laser scanning confocal microscope, LCSM)观察了不同叶龄期玉米茎尖分生组织的形态学变化.结果表明,短日照能促进玉米开花,促进茎端分生组织向生殖生长转化,黄早4和CML288分别在6叶期和7叶期完成茎尖分生组织的生殖转化;而长日照则明显延迟开花,延迟茎尖分生组织向生殖生长转化,黄早4和CML288分别在8叶期和11叶期完成茎尖分生组织的生殖转化;因此光周期诱导玉米开花因光照条件和品种有一定差异,短日照条件下,光敏感和不敏感的玉米自交系开花提前,花期更接近,而长日照条件下光敏感玉米自交系开花延迟要比不敏感自交系明显得多.