Reduced Leaf Size Research Articles

What is the functional significance of leaf size reduction in phosphate impoverished habitats?

What is the functional significance of leaf size reduction in phosphate impoverished habitats?

Assessment of Disturbances in Growth and Physiology of Carrot Caused by Chromium Stress

ABSTRACT To see the deleterious effects of excess chromium (Cr) on carrot (Daucus carota L.) the cv. ‘Pusa Keshari’ was grown in refined sand under controlled glasshouse conditions with a complete nutrient solution (without Cr) for 48 days. On the 49th day, pots with two plants each were separated into three sets. One set served as a control. In each of the other two sets, Cr was supplied as dichromate at 0.1 and 0.5 mM to the basal nutrient solution. At 0.5 mM Cr toxicity symptoms appeared at d 52 (4 days after Cr supply) as reduction in growth and leaf size and loss of turgor. Old leaves became chlorotic and wilted. Chlorosis intensified and turned necrotic in another few days. These symptoms spread to next upper leaf. The development of chlorosis in leaves was delayed in plants receiving 0.1 mM Cr. At excess Cr (0.5 mM) in carrot, the biomass, concentration of chlorophylls a and b, iron (Fe), sulfur (S), and phosphorus (P) in shoots, and activity of catalase (CAT) in leaves decreased whereas the concentration of Cr and the activity of peroxidase (POX), superoxide dismutase (SOD), ribonuclease (RNAse), and acid phosphatase (A P) in leaves increased.

Ultraviolet Radiation as a Limiting Factor in Leaf Expansion and Development

Reductions in leaf growth are a commonly observed response to ultraviolet radiation, but the underlying mechanisms remain poorly defined. This study examined the response of leaves exposed to a UV environment across a range of organizational scales, including leaf expansion rate, epidermal cell size and number, biomechanical properties, leaf-water relations and activity of cell-wall peroxidases. Two experimental approaches were used; Lettuce (Lactuca sativa L.) plants were propagated under (a) supplementary UV-B (9 kJ m(-2) day(-1)) in controlled environment (CE) conditions, and (b) field conditions, where plants were placed under three horticultural films with differing UV transmissions. In both experiments, UV-B caused the greatest reductions in leaf expansion and final leaf size, with some reductions attributable to UV-A wavelengths. In supplementary UV-B conditions, adaxial cell size was reduced, while in field plants, both cell size and cell number were lower in an increased UV environment, as was the case with abaxial cells in CE plants. Although leaf turgor and leaf extensibility were not affected by UV wavelengths, breaking strain of leaf tissue was decreased under supplementary UV-B. Cell-wall peroxidase activity was increased in both supplementary UV conditions and in the field, where only a zero UV environment showed no upregulation of cell-wall peroxidase.

Chromium phytotoxicity alters metabolism in radish

ABSTRACT Industrialization has posed an increasing threat to the environment. Effluents from different industries ultimately find their way into water used to irrigate crops. There is a risk that heavy metals accumulating in these agricultural soils may eventually contaminate the human food chain. Radish, an important vegetable consumed by humans, was selected as the test plant. The cv. Jaunpuri was grown in refined sand supplemented daily for 50 days with complete nutrient solution. On the 51st day pots with plants were separated into three lots, one lot served as the control and received only the nutrient solution, and in each of two other lots chromium (Cr) as dichromate was added at 0.1 and 0.5 mM to the basal nutrient solution. At 0.5 mM Cr, toxicity symptoms were observed 5–8 days after treatment expressed as reductions in leaf size accompanied by loss of turgor, and old leaves became chlorotic and wilted. Treatment with Cr reduced growth, biomass, concentration of chlorophyll (a and b), relative wa...

Foliar morphological variation in the white oak Quercus rugosa Née (Fagaceae) along a latitudinal gradient in Mexico: Potential implications for management and conservation

Quercus rugosa Nee (section Quercus) has a distribution from the southern United States to Honduras. This study characterized leaf variation across the whole distribution of the species in Mexico. Ten foliar morphometric characters were scored in each of 241 individuals from 25 localities. A principal component analysis resolved four principal components (PCs) that explained 76.4% of the total variation. A nested analysis of variance revealed significant differences among populations (29.2% average contribution to total variance for the four PCs) and among-trees within populations (39.2%), while 31.6% was due to intra-individual variation. For the first PC (related to leaf size), 52.1% resulted from among population differences. This variation was negatively correlated with latitude ( r = −0.86; P < 0.0001), indicating a steep clinal reduction in leaf size from south to north. Mean annual precipitation and an annual aridity index also significantly decreased and increased with latitude, respectively. It is suggested that the morphological cline is the result of plastic and/or adaptive responses to environmental conditions, and indicative of further ecophysiological latitudinal differences among Q. rugosa populations. Additionally, we estimated the magnitude of the least significant difference among populations for the first PC and translated it into a delineation of six latitudinal zones (each with a width of 2°30′), to be considered as preliminary zones for the movement of Q. rugosa seeds with management and conservation purposes, including management in response to global warming.

Effects of bean rust (Uromyces appendiculatus) epidemics on host dynamics of common bean (Phaseolus vulgaris)

Epidemics of bean rust (Uromyces appendiculatus) and their effects on host dynamics of common bean (Phaseolus vulgaris) were studied in three controlled greenhouse experiments, with and without fungicide sprays, on two susceptible bean cultivars, Dufrix and Duplika. Bean plants were artificially inoculated with a suspension of 105 U. appendiculatus urediniospores mL−1 water and temporal disease progress, as well as host growth dynamics (leaf size and defoliation), were monitored on a leaflet basis in comparison with non‐inoculated plants, which were sprayed with deionized water. Progress curves of bean rust, expressed as the proportion of leaf area occupied by pustules (uredinia), or as the proportion occupied by total lesion area (= halo areas + pustule area), were well described by logistic functions with maximum disease levels clearly lower than 1. Bean rust epidemics substantially affected host growth by reducing the total leaf area formed by 17·4–35·6% and 35·3–46·2% compared with healthy plants for cvs Dufrix and Duplika, respectively. Fungicide sprays mitigated the negative effect of bean rust, leading to a gain in leaf area of 17–21% compared with unsprayed plants in both cultivars in two experiments, while in another experiment, disease control had no effect in Dufrix, but a clear effect in Duplika. In addition to the growth depression, bean rust also led to pronounced losses of leaf area as a result of reduced leaf size (leaf shrivelling) and accelerated defoliation.

Compensation in seasonal leaf area dynamics and leaf longevity after defoliation in Buddleja davidii

Summary Buddleja davidii is a major invasive weed and its success is attributed to its ability to recover rapidly following defoliation. To quantify the impacts of defoliation on seasonal leaf area dynamics, we measured rates of leaf area growth and loss on cohorts of leaves on control plants and on plants that were defoliated by 66% repeatedly at monthly intervals throughout two growing seasons. The rate of leaf area growth was closely related to cumulative air temperature, but the maximum rate for the defoliated plants in the first season was 2.2 times that of the control plants. This compensational leaf growth resulted in 52% greater total emergent leaf area, attributable to increased node production (34%) and leaf size (35%), compared with control plants. Leaf longevity during the first growing season in the defoliated plants was 12% greater than that in the control plants. During the second season, the degree of compensation was greatly reduced, attributable to reduced leaf size in defoliated plants compared with the control. Total emergent leaf area over both seasons in the defoliated treatment exceeded control values by only 15%. The impact of leaf loss on the vigour in this invasive shrub may, in the short term, be less significant than would be predicted. However, the impacts of continued defoliation are likely to increase markedly in subsequent years. The study provides the basis for modelling leaf area dynamics and plant growth in response to defoliation associated with biocontrol release programmes.

Plant drought stress: effects, mechanisms and management

Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

Photosynthetic responses to solar UV-A and UV-B radiation in low-and high-altitude populations of Hippophae rhamnoides

Two contrasting sea buckthorn (Hippophae rhamnoides L.) populations from the low (LA) and high (HA) altitudinal regions were employed to evaluate the plant physiological responses to solar UV-A radiation and near-ambient UV-B radiation (UV-B+A) under the sheltered frames with different solar ultraviolet radiation transmittance. LA-population was more responsive to solar UV-A. Some modification caused by UV-A only existed in LA-population, such as significant reduction of leaf size, relative water content, and chlorophyll (Chl) b content as well as &delta;13C elevation, coupled with larger increase of contents of total carotenoids (Cars). This higher responsiveness might be an effective pre-acclimation strategy adapting for concomitant solar UV-B stress. Near-ambient UV-B+A radiation caused significant reduction of leaf size and Chl content as well as slight down-regulation of photosystem 2 activity that paralleled with higher heat dissipation, while photosynthetic rate was modestly but significantly increased. The higher photosynthesis under near-ambient UV-B+A radiation could be related to pronounced increase of leaf thickness and effective physiological modification, like the increase of leaf protective pigments (Cars and UV-absorbing compound), constant high photochemical capacity, and improved water economy.

The Transcriptional Repressor ARR1-SRDX Suppresses Pleiotropic Cytokinin Activities in Arabidopsis

The signal transduction of the phytohormone cytokinin is mediated by a multistep histidine-to-aspartate phosphorelay system. One component of this system are B-type response regulators, transcription factors mediating at least part of the response to cytokinin. In planta functional analysis of this family is hampered by the high level of functional redundancy of its 11 members. We generated a dominant repressor version of the Arabidopsis (Arabidopsis thaliana) response regulator ARR1 (ARR1-SRDX) using chimeric repressor silencing technology in order to study the extent of the contribution of B-type response regulators to cytokinin activities. In a protoplast test system, ARR1-SRDX suppressed ARR6:beta-glucuronidase reporter gene activation by different B-type ARRs. 35S:ARR1-SRDX transgenic Arabidopsis plants showed phenotypic changes reminiscent of plants with a reduced cytokinin status, such as a strongly reduced leaf size, an enhanced root system, and larger seeds. Several bioassays showed that 35S:ARR1-SRDX plants have an increased resistance toward cytokinin. The rapid induction of a large part of the cytokinin response genes was dampened. The transcript levels of more than 500 genes were more than 2.5-fold reduced in 35S:ARR1-SRDX transgenic seedlings, suggesting a broad function of B-type ARRs. Collectively, the suppression of pleiotropic cytokinin activities by a dominant repressor version of a B-type ARR indicates that this protein family is involved in mediating most, if not all, of the cytokinin activities in Arabidopsis. In addition, a role for B-type ARRs in mediating cross talk with other pathways is supported by the resistance of 35S:ARR1-SRDX seeds to phytochrome B-mediated inhibition of germination by far-red light. This study demonstrates the usefulness of chimeric repressor silencing technology to overcome redundancy in transcription factor families for functional studies.

Silencing of the mitochondrial ascorbate synthesizing enzyme L-galactono-1,4-lactone dehydrogenase affects plant and fruit development in tomato.

L-Galactono-1,4-lactone dehydrogenase (EC 1.3.2.3) catalyzes the last step in the main pathway of vitamin C (L-ascorbic acid) biosynthesis in higher plants. In this study, we first characterized the spatial and temporal expression of SlGalLDH in several organs of tomato (Solanum lycopersicum) plants in parallel with the ascorbate content. P(35S):Slgalldh(RNAi) silenced transgenic tomato lines were then generated using an RNAi strategy to evaluate the effect of any resulting modification of the ascorbate pool on plant and fruit development. In all P(35S):Slgalldh(RNAi) plants with reduced SlGalLDH transcript and activity, plant growth rate was decreased. Plants displaying the most severe effects (dwarf plants with no fruit) were excluded from further analysis. The most affected lines studied exhibited up to an 80% reduction in SlGalLDH activity and showed a strong reduction in leaf and fruit size, mainly as a consequence of reduced cell expansion. This was accompanied by significant changes in mitochondrial function and altered ascorbate redox state despite the fact that the total ascorbate content remained unchanged. By using a combination of transcriptomic and metabolomic approaches, we further demonstrated that several primary, like the tricarboxylic acid cycle, as well as secondary metabolic pathways related to stress response were modified in leaves and fruit of P(35S):Slgalldh(RNAi) plants. When taken together, this work confirms the complexity of ascorbate regulation and its link with plant metabolism. Moreover, it strongly suggests that, in addition to ascorbate synthesis, GalLDH could play an important role in the regulation of cell growth-related processes in plants.

REVIEW: Endemic plants of serpentine soils

Plant adaptation to serpentine soils is a system ideal for studies in evolutionary ecology. Serpentine soils are characterized by low calcium to magnesium ratios with Ca at significantly lower concentrations relative to surrounding areas. Although some variation occurs between sites which identified three collective traits: poor plant productivity, high rates of endemism and vegetation type distinct from those of neighboring areas. The several morphological feature characteristic of serpentine-tolerant species is possess xeromorphic foliage, including reduced leaf size and sclerophylls, the stature is significantly reduced relative to counterparts on non serpentine soil and root systems of species growing on and off serpentine sites are often more developed on serpentine soils than on neighboring soils. Serpentine soils are ubiquitous, but patchily distributed and thus promote geographic isolation. Adaptation to edaphic conditions may also begetreproductive isolation. Adaptive mutation might be influenced frequently from related species inhabiting surrounding areas. For the future studies involving serpentine systems merge the fields of ecology, evolution, physiology, and genetics required for serpentine adaptation.© 2007 Jurusan Biologi FMIPA UNS SurakartaKey words: adaptive mutation, endemics, isolation, magnesium, serpentine

Identification of Tomato yellow leaf curl virus and Tomato mottle virus in Two Counties in Alabama.

During July 2005, approximately 23% of tomato plants (Solanum lycopersicum L. 'Sebring') in a commercial field in St. Clair County, Alabama showed symptoms of stunting, leaf deformation, mottling, and reduced leaf size, which resembled symptoms of Tomato yellow leaf curl virus (TYLCV). A high population of whiteflies (Bemisia tabaci) was observed in this field, and as the season progressed, 100% of the plants became symptomatic. During October 2006, similar symptoms in tomato were observed at low incidences (less than 10%) in a commercial greenhouse in Jefferson County. Two samples from St. Clair County and six from Jefferson County were collected and tested for the presence of a begomovirus by PCR using three pairs of primers, PAR1c496 and PAL1v1978, a degenerate primer pair designed to amplify regions of the begomovirus A component, PBL1v2040 and PCRc154, a degenerate primer pair that amplifies a hypervariable region of the begomovirus B component (3), and C473 and PTYC1v2406, which are specific to TYLCV (1,2). Primer pair PAR1c496 and PAL1v1978 produced two amplicons (1,360 and 1,159 bp) in all samples tested, which suggests the presence of a monopartite and bipartite begomovirus. Primer pair pBL1v2040 and PCRc154 produced a 678-bp amplicon that would be consistent with the presence of a bipartite begomovirus. Primer pair C473 and PTYC1v2406 produced an 850-bp amplicon that would be consistent with the presence of TYLCV. Sequence analysis revealed that the 1,360-bp amplicon had 98% sequence identity to isolates of TYLCV from Cuba (GenBank Accession No. AJ223505), the Dominican Republic (GenBank Accession No. (AF04715), Florida (GenBank Accession Nos. AF260331 and AY530931), Egypt (GenBank Accession No. AY594174), and Almeria (GenBank Accession No. AJ489258). The 1,159-bp amplicon had a 97 to 99% sequence identity to the A component of Tomato mottle virus (ToMoV) Florida (GenBank Accession Nos. L14460, EF028241, and M90495) and Puerto Rico (GenBank Accession No. AY965900). Each of the eight tomato samples were shown to be infected with TYLCV and ToMoV. Symptoms of plants infected with both viruses resembled those of TYLCV because the milder symptoms of ToMoV are masked in the field by the more severe symptoms of TYLCV. To our knowledge, this is the first report of ToMoV and TYLCV in the state of Alabama. Reference: (1) M. Ghanim et al. Virology 240:295, 1998. (2) M. K. Nakhla et al. Phytopathol. Mediterr. 32:163, 1993. (3) M. R. Rojas et al. Plant Dis. 77:340, 1993.

First Report of Tomato yellow leaf curl virus in Venezuela.

Tomato yellow leaf curl virus (TYLCV), a member of the family Geminiviridae, is a serious production constraint to tomato worldwide. In the new world, the virus had been identified as the causal agent of tomato yellow leaf curl disease in the Caribbean countries of the Dominican Republic, Cuba, and Guadeloupe and also in Florida and the Yucatan Peninsula (1). Molecular data from these TYLCV isolates identified the virus as the TYLCV prototype from Israel. During April 2004, tomato plants showing symptoms such as chlorotic leaf edges, upward leaf cupping, leaf mottling, and reduced leaf size indicative of TYLCV were observed in commercial fields in Zulia state, Venezuela. Whiteflies (Bemisia tabaci Gennadius) were present in the field and appeared to be associated with the disease. Leaf samples from nine symptomatic plants were collected and brought to the lab at Instituto Venezolano de Investigaciones Científicas (IVIC) for further analyses. Geminivirus infection of samples was confirmed by PCR amplification with the degenerate primer pair PAL1v1978 and PAR1c494 (2). TYLCV coat protein gene-specific primers KL04-06_TYLCV CP F and KL04-07_TYLCV CP R (3) were used to confirm the diagnosis. These primers amplified the expected 842-bp PCR product from the nine symptomatic samples. One of the resulting amplicons was cloned into the pCR-TOPO vector (Invitrogen, Carlsbad, CA) and sequenced (GenBank Accession No. DQ302033). Sequence comparison with those available in the NCBI database indicated that the sequenced portion of the genome shared 99% nucleotide identity with the TYLCV mild strain from Portugal (GenBank Accession No. AF105975) and 98% nucleotide identity with the TYLCV mild strain from Spain (GenBank Accession No. AF071228), TYLCV Israel isolate (GenBank Accession No. AM234066), and TYLCV Mexico isolate (GenBank Accession No. DQ631892). To our knowledge, this is the first report of TYLCV infecting tomato crops in South America. Further studies are needed to clarify how TYLCV has been introduced into Venezuela.

Zinc Stress–Induced Changes in Biochemical Parameters and Oil Content of Mustard

An experiment with mustard (Brassica campestris L.) cv T 59 was conducted in a glasshouse under controlled culture conditions to determine the effect of variable zinc (Zn) on biomass, reproductive yield, seed oil, and some aspects of metabolism. Plants were raised in refined sand, and Zn was administered at variable levels ranging from acute deficiency to excess (0.00065 to 65 mg Zn L−1). At 0.00065 mg Zn L−1, symptoms of Zn deficiency were depression in growth, short internodes, chlorosis and necrosis of mature leaves, along with development of purple pigmentation at leaf margins, which later covered the entire lamina. Biomass, economic yield, and seed oil content were highest at 0.065 mg Zn L−1. Zinc at less and more than 0.065 mg Zn L−1 reduced vegetative yield, pod and seed weight, concentration of chlorophyll (a and b), sugars, starch, and protein and elevated the concentration of phenols and activity of acid phosphatase and peroxidase in mustard leaves. The activity of carbonic anhydrase (except at 65 mg Zn L−1), aldolase, and leaf Zn increased with an increase in Zn from deficiency to excess whereas ribonuclease activity was related inversely to Zn supply. In addition to various changes in metabolism of mustard, Zn at higher levels (6.5 and 65 mg Zn L−1) induced symptoms of toxicity on old leaves as interveinal chlorosis, necrosis, and reduction in growth and leaf size. Phosphorus (P) content in leaves decreased concomitantly with an increase in Zn supply from 0.00065 to 65 mg Zn L−1. The values of deficiency, threshold of deficiency, threshold of toxicity, and toxicity in leaves were respectively 18, 25, 200, and 380 µg Zn g−1 dry matter. The activity of aldolase and carbonic anhydrase appear to be reliable biological indicators for evaluating the Zn status of mustard.

The BIG gene is involved in regulation of sulfur deficiency‐responsive genes in Arabidopsis thaliana

The Arabidopsis thaliana mutants altered sulfur response 1‐1 (asr1‐1) and asr1‐2 were isolated using the green fluorescent protein gene (GFP), as a marker, driven by a sulfur deficiency‐responsive promoter containing the βSR fragment, which is responsible for the induction of gene expression under sulfur deficiency. In the asr1 mutants, the expression of three sulfur deficiency‐responsive genes βSR‐driven GFP, sulfate transporter 2;2 (SULTR2;2) and adenosine‐5′‐phosphosulfate reductase 1 (APR1) were induced in medium containing a normal sulfate concentration. The ASR1 locus was mapped to a 53‐kb region on the upper arm of chromosome III; this is also the region of the BIG gene, which encodes a calossin‐like protein necessary for the polar transport of auxin. The morphology of the asr1 mutants, i.e. reduced leaf size and inflorescence elongation, resembled that of big mutants. Using nucleotide sequence analysis of the BIG gene, we identified independent nonsense mutations in asr1‐1 and asr1‐2. To confirm that ASR1 was BIG, we established lines of transgenic A. thaliana carrying a transfer DNA (T‐DNA) insertion in the BIG gene. In these T‐DNA insertion mutants, mRNA levels of βSR‐driven GFP and APR1 were upregulated in normal sulfate medium. The F1 plants from crosses between asr1‐1 and T‐DNA insertion lines exhibited reduced leaf size and inflorescence length, indicating that ASR1 was indeed BIG. Taken together, the present results established that BIG is involved in the regulation of βSR‐driven GFP and APR1 mRNA level gene expression. Indole‐3‐acetic acid also upregulated βSR‐driven GFP and APR1 together with SULTR2;2 mRNA level, suggesting that the big effect on βSR‐driven GFP and APR1 is a pleiotropic aspect of the BIG gene.

Cold nights impair leaf growth and cell cycle progression in maize through transcriptional changes of cell cycle genes.

Low temperature inhibits the growth of maize (Zea mays) seedlings and limits yield under field conditions. To study the mechanism of cold-induced growth retardation, we exposed maize B73 seedlings to low night temperature (25 degrees C /4 degrees C, day/night) from germination until the completion of leaf 4 expansion. This treatment resulted in a 20% reduction in final leaf size compared to control conditions (25 degrees C/18 degrees C, day/night). A kinematic analysis of leaf growth rates in control and cold-treated leaves during daytime showed that cold nights affected both cell cycle time (+65%) and cell production (-22%). In contrast, the size of mature epidermal cells was unaffected. To analyze the effect on cell cycle progression at the molecular level, we identified through a bioinformatics approach a set of 43 cell cycle genes and analyzed their expression in proliferating, expanding, and mature cells of leaves exposed to either control or cold nights. This analysis showed that: (1) the majority of cell cycle genes had a consistent proliferation-specific expression pattern; and (2) the increased cell cycle time in the basal meristem of leaves exposed to cold nights was associated with differential expression of cell cycle inhibitors and with the concomitant down-regulation of positive regulators of cell division.

Effects of simulated green turtle regrazing on seagrass abundance, growth and nutritional status in Moreton Bay, south-east Queensland, Australia

In some parts of their range, green turtles maintain grazing plots in seagrass beds by regular regrazing. The effects of simulated repeated grazing on subtropical seagrasses in Moreton Bay, Australia were investigated in a manipulative experiment over summer. Three seagrass species were subjected to two different clipping frequencies (simulating turtle cropping) and compared with unclipped controls over a 3.5-month summer period for the effects on seagrass biomass, leaf size and regrowth rates and water-soluble carbohydrate (WSC) and starch content. The order of the seagrass species’ relative tolerance to simulated grazing was Halophila ovalis &gt; Zostera capricorni &gt; Cymodocea serrulata. Frequent regrazing of the green turtle’s preferred seagrass, H. ovalis, resulted in an increase in leaf regrowth rate so that standing biomass of leaves and total plant material was maintained, suggesting an increase in productivity. Furthermore, whole-plant concentrations of WSC increased significantly in clipped H. ovalis plants relative to unclipped controls. In contrast, leaf biomass of the seagrass species less preferred by turtles, Z. capricorni and C. serrulata, decreased in response to repeated leaf removal relative to controls, despite maintenance of leaf regrowth rates. C. serrulata responded to repeated clipping with a reduction in leaf size and a decrease in rhizome WSC concentration. Z. capricorni also produced fewer and smaller new leaves. The ability of the preferred species, H. ovalis, to increase production of nutrient-rich standing crop in response to regrazing has major implications for green turtles and other seagrass grazers.

First record of aCandidatusphytoplasma associated with little leaf disease ofPortulaca grandiflora

This paper reports Portulaca grandiflora as a new host of a Candidatus phytoplasma and demonstrates its association with the little leaf disease on P. grandiflora. Plants exhibited typical visual symptoms of a phytoplasma, with reduced leaf size. Pleomorphic bodies, observed by transmission electron microscopy, were restricted to the phloem of symptomatic tissue. However, no bacteria, fungi or viruses were observed under transmission electron microscopy. Products of the expected size were amplified by PCR using nested, phytoplasma-specific primer pairs. In addition, plants temporarily showed amelioration of phytoplasma symptoms following treatment with the antibiotic tetracycline.

Distinct Begomoviruses Closely Related to Cassava Mosaic Viruses Cause Indian Jatropha Mosaic Disease

A begomovirus was recently shown to be causing Jatropha mosaic disease (JMD) on Jatropha for the first time in India. A typical begomovirus-like symptoms characterized by chlorotic specks on leaves, curling and malformation of leaves, severe reduction in leaf size, partial or complete sterility were seen on infected plants. In Karnataka state, South India, JMD caused significant yield losses by affecting the growth of the infected plant and by disease incidences of up to 47%. The putative Jatropha mosaic India virus (JMIV) was successfully transmitted through grafting, the dodder Cuscuta subinclusa and the whitefly, Bemisia tabaci. The JMIV was detected in infected plants and individual B. tabaci by polymerase chain reaction tests using two sets of begomovirus-specific degenerate primers. The core coat protein (CP) sequences of ~575 bases were obtained from two isolates collected at Bangalore and Dharwad, South India. Phylogenetic analysis of the core CP sequences with those of selected begomoviruses grouped JMIV in a separate cluster close to Indian cassava mosaic virus and Sri Lankan cassava mosaic virus and shared highest nucleotide identities (90-95%) with them. The two JMIV isolates were 94% similar to each other. The begomoviruses causing JMD in the Americas grouped separately from JMIV and shared only 72.8-75.2% core CP nucleotide identities thus they are distinct. These results further confirm that JMD in India was caused by begomoviruses and they were most closely related to cassava mosaic viruses from the Indian sub-continent.