Seasonal Senescence Research Articles

Image analysis to quantify herbicide efficacy for Cynodon dactylon control

Studies on the effectiveness of herbicides for control of Cynodon dactylon (L.) Pers. (cynodon) normally rely on visual recording methods, which are rapid and convenient, but lack objectiveness and reproducibility. To address this problem, a computer software package was customized for creeping grass studies and applied to determine the ’cover of green’ from digital images of cynodon to verify and interpret visual assessments of herbicide treatments applied to cynodon-infested plots, with or without disking. Whereas the digital analyses confirmed the main conclusions from the visual assessment, significant differences between the recording methods were found during early and late evaluations of the trial, thereby having implications for the interpretation of assessments during these times. These differences were attributed to the difficulty of visual assessments early in the trial when there was partial grass control and late in the trial where seasonal grass senescence was superimposed to the treatments. The advantages of digital analysis are ease of use, repeatability, accuracy and objectivity enabling more profound analyses; as well as reduced costs, although in this study, sample number and size was restricted due to labour-intensive maintenance of the target species.

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ18O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ18O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ18O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ18O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.

Volatile organic compounds from vegetation in southern Yunnan Province, China: Emission rates and some potential regional implications

Little information is currently available regarding emissions of biogenic volatile organic compounds (BVOCs) in southern Asia. To address the need for BVOC emission estimates in regional atmospheric chemistry simulations, 95 common plant species were screened for emissions of BVOC in and near the Xishuangbanna Tropical Biological Gardens in southern Yunnan Province, Peoples’ Republic of China in February 2003. In situ measurements with leaf cuvettes and branch bag enclosures were used in combination with portable gas chromatography, flame ionization, photoionization, and mass spectral detection to identify and quantify BVOC emissions. Forty-four of the species examined emitted isoprene at rates exceeding 20 μg C g −1 (leaf dry weight) h −1. An emphasis was placed on the genus Ficus, which is important in the region and occupies a wide range of ecological niches. Several species in the footprint of a nearby flux tower were also examined. Several palm species and an abundant fern ( Cyclosorus parasiticus) emitted substantial amounts of isoprene, and probably accounted for observed daytime mean isoprene fluxes from the understory of a Hevea brasiliensis plantation of 1.0 and 0.15 mg C m −2 h −1 during the wet and dry seasons, respectively. These measurements verify that both the forest floor and canopy in this region can be sources of isoprene. Monoterpene emissions exceeded 1.0 μg-C g −1 (leaf dry weight) h −1 from only 4 of 38 species surveyed, including some Ficus species and H. brasiliensis. However most of the trees of the latter species were sparsely foliated due to dry season senescence, and emission factors are approximately an order of magnitude lower than those reported during the wet season. BVOC emission rates and physiology of many species are impacted by reduced moisture availability, especially Mangifera indica. South Asia is a region undergoing rapid landuse change and forest plantation establishment, with large increases in area of high BVOC-emitting species in the genera Bambusa, Elaeis, Eucalyptus, Hevea, Pinus, and Populus (among others). This could result in profound changes in atmospheric chemistry in these regions, for instance, terpene emissions from H. brasiliensis could increase wet season biogenic organic aerosol burdens by approximately a factor of 2 in the Xishuangbanna region. Increases in plantation area established with high isoprene emitting species, (e.g. Bambusa spp. and Eucalyptus spp.) are also projected for China and other parts of Southeast Asia in the near future. Thus, landcover change in South Asian landscapes is usually associated with large increases in BVOC flux with the potential to alter the atmospheric chemical composition and air quality over this rapidly developing region.

Effects of nutrient enhancement on the fecundity of a coral reef macroalga

On coral reefs, there is concern that increased nutrient supply (e.g. due to eutrophication) causes increased algal growth and hence increased algal abundance, in turn causing colonisation and invasions of coral populations, resulting in reef degradation, or a coral–algal phase shift. For example, species of Sargassum, a highly seasonal, large, brown seaweed, are suggested to be colonising corals on inshore coral reefs of the Great Barrier Reef, as a result of anthropogenic increases in terrestrial runoff of sediments and nutrients. However, implicit in this argument is the assumption that nutrient-related increases in growth will lead to increased fecundity (and/or propagule success), since without such changes, increased abundance can only occur by vegetative means. Whilst plausible, there is no experimental evidence for this assumption in coral reef algae. We here present an initial study in which experimental increases in nutrient supply apparently did not lead to increased fecundity of Sargassum siliquosum; rather, density and biomass of receptacles were reduced in nutrient-enhanced algae. There was little effect of nutrient treatments on the proportional allocation of biomass to reproductive and vegetative structures: nutrient enhancement led to similar decreases in biomass of reproductive and vegetative tissue. Tissue nutrient levels indicated effective enhancement of nitrogen supply, although phosphorus levels were not different across nutrient treatments. The reduced fecundity with increased nutrients may reflect either a genuine inhibition by higher nutrient levels, as found in previous studies, or accelerated maturation, causing increased tissue losses due to more advanced seasonal senescence. Either way, this exploratory experiment provides initial evidence that nutrient effects on tropical coral reef macroalgae may be complex, and does not support the assumption that increased nutrient supply will result in a numeric increase in populations of Sargassum spp. Our results should not be taken as an unequivocal demonstration that nutrients inhibit fecundity overall, but illustrate the need to distinguish between effects on different life-history processes (e.g. growth and reproduction). For increased growth of pre-existing individuals to contribute to algal invasions and phase shifts, that growth must result in either increased fecundity, or increased propagule success.

Investigation of the biological activities of Siphonochilus aethiopicus and the effect of seasonal senescence

Siphonochilus aethiopicus (Schweinf.) B.L. Burtt (Zingiberaceae), commonly known as wild ginger, is one of the most important and threatened medicinal plants in South Africa. A study of the pharmacological properties of S. aethiopicus and the effect of seasonal senescence on antibacterial and anti-inflammatory properties was undertaken. Water, ethanol and ethyl acetate extracts were prepared from the leaves, rhizomes and roots of S. aethiopicus plants. The extracts were tested in a variety of pharmacological assays. Results for the general screening showed antibacterial and anti-inflammatory activity. Some cytotoxicity was observed with the aqueous extracts of the rhizome. However, no significant activity against the herpes simplex virus types 1 and 2, the influenza A virus, and in the anthelmintic, antischistosomal and biochemical induction assays were observed. In the microdilution antibacterial assay, no inhibitory activity against the test bacteria was detected with the aqueous extracts. The ethanol and ethyl acetate extracts tested showed greater antibacterial activity at minimal inhibitory concentrations ranging from 0.78 to 3.13 mg ml − 1 against the Gram-positive bacteria ( Bacillus subtilis , Staphylococcus aureus ) than the Gram-negative bacteria ( Escherichia coli , Klebsiella pneumoniae ). Little difference was observed between the ethanol and ethyl acetate extracts, or between the different plant parts. Ethanol extracts were prepared from the different plant parts before and after seasonal senescence, and tested for antibacterial and anti-inflammatory activity. There appeared to be a loss of antibacterial activity in the leaves with senescence, concomitant with an increase of activity in the alpha-roots. In the cyclooxygenase-1 assay, the aqueous extracts showed no significant prostaglandin synthesis inhibition. For the ethanol and ethyl acetate extracts, the leaves showed the highest levels of activity at a concentration of 250 ng ml −1 per test solution, in both the cyclooxygenase-1 and −2 assays. Some differences in the levels of anti-inflammatory activity in the roots following senescence were also observed. There was a slight loss of activity as a result of drying the rhizome material prior to extraction. This suggests that fresh rhizome material may be more effective for medicinal use, although it should be noted that the aqueous rhizome extracts displayed moderately high levels of cytotoxicity, and may require further investigation.

Investigation of the biological activities of Siphonochilus aethiopicus and the effect of seasonal senescence

Investigation of the biological activities of Siphonochilus aethiopicus and the effect of seasonal senescence

Competition in tree row agroforestry systems. 1. Distribution and dynamics of fine root length and biomass

Complementarity in the distribution of tree and crop root systems is important to minimise competition for resources whilst maximising resource use in agroforestry systems. A field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya to compare the distribution and dynamics of root length and biomass of a 3-year-old Grevillea robusta A. Cunn. ex R. Br. (grevillea) tree row and a 3-year-old Senna spectabilis DC. (senna) hedgerow grown with Zea mays L. (maize). Tree roots were sampled to a 300 cm depth and 525 cm distance from the tree rows, both before and after maize cropping. Maize roots were sampled at two distances from the tree rows (75–150 cm and 450–525 cm) to a maximum depth of 180 cm, at three developmental stages. The mean root length density (Lrv) of the trees in the upper 15 cm was 0.55 cm cm−3 for grevillea and 1.44 cm cm−3 for senna, at the start of the cropping season. The Lrv of senna decreased at every depth during the cropping season, whereas the Lrv of grevillea only decreased in the crop rooting zone. The fine root length of the trees decreased by about 35% for grevillea and 65% for senna, because of maize competition, manual weeding, seasonal senescence or pruning regime (senna). At anthesis, the Lrv of maize in the upper 15 cm was between 0.8 and 1.5 cm cm−3. Maize root length decreased with greater proximity to the tree rows, potentially reducing its ability to compete for soil resources. However, the specific root length (m g−1) of maize was about twice that of the trees, so may have had a competitive uptake advantage even when tree root length was greater. Differences in maize fine root length and biomass suggest that competition for soil resources and hence fine root length may have been more important for maize grown with senna than grevillea.

Changes in respiration and chemical content during autumnal senescence of Populus tremuloides and Quercus rubra leaves.

Changes in respiration rate, chemical content and chemical concentration were measured in leaves of field-grown Populus tremuloides Michx. and Quercus rubra L. trees throughout the growing season and autumnal senescence. Chlorophyll, soluble sugar, N, P, K and Mg contents and concentrations all declined during leaf senescence, whereas Ca content and concentration increased. Leaf dry mass per area declined 24 and 35% in P. tremuloides and Q. rubra, respectively, during senescence. In leaves of both species, respiration rates peaked during leaf expansion in the spring and then declined, as a result of reduced cytochrome-mediated respiration, to reach relatively constant rates by midsummer. In senescing P. tremuloides leaves, respiration rates remained relatively constant until mid-October and then declined rapidly. In senescing Q. rubra leaves, respiration rates increased in late September, as a result of the appearance of residual respiration that could not be reduced by respiratory inhibitors, and then declined quickly in early November. No changes in alternative pathway respiratory activity were observed in leaves of either species during senescence until late autumn when rates declined. Because respiration rates were correlated with both leaf sugar and nitrogen content during leaf senescence, we conclude that respiration rates were maintained or increased during leaf senescence to supply energy for degradation and mobilization of chemical constituents.

Fluoride effects on leaf cell ultrastructure of olive trees growing in the vicinity of the Aluminium Factory of Greece

Leaves of olive trees growing in the vicinity of the Aluminium Factory of Greece were ultrastructurally investigated in order to determine any malformations caused by environmental air pollutants, especially hydrogen fluoride, in comparison with control samples and normal seasonal senescence. Estimation of some elements accumulated by these leaves showed that they contained high amounts of F and Al attributable to the operation of the nearby factory. The most seriously effected cell components were found to be the mesophyll chloroplasts that show a dilation of the intrathylakoid space, increase of the number of plastoglobuli, discoloration of plastoglobuli, accumulation of large starch grains and an overall disorganized appearance of the organelle. The nuclear crystalloid inclusions have unusual shapes, while the vacuoles contain a fibrillar/granular material that increases their electron density. It is concluded that the ultrastructural malformations are caused by a combination of environmental stresses and air pollutants.

Seasonality of cooling tower drift in vegetation at ORGDP

Cooling tower operation at the Oak Ridge Gaseous Diffusion Plant (ORGDP) requires the addition of a chromated corrosion inhibitor to the recirculating cooling water (RCW) system. Fluctuations in the RCW flow rates of the ORGDP cooling towers correlates with operational levels of the enrichment plant. Drift fraction is a function of the RCW flow rate, with variations in chromium deposition reflected in vegetation contamination. Fescue grass and litter were collected in the plant environs along a distance gradient, during different seasons, and analyzed for chromium. Foliar chromium concentrations were maximum (1390 μg/g) during the winter months, decreasing in spring and summer months (190 and 173 μg/g, respectively) at 13 m as the demand for cooling decreased with lower plant operations. In contrast, concentrations in litter increased with time, ranging from 894 μg/g at 13 m during the winter to 1890 and 2140 μg/g during the summer and fall months. The accumulation of chromium in the litter component was likely related to more exchange sites associated with increased litter biomass due to the seasonal senescence of foliage, whereas decreased accumulations on foliage reflected high mobility of the drift residue due to leaching and a short lifespan of individual leaves. In addition to operational considerations, the data illustrate the importance of seasonal considerations in the use of biological materials in a monitoring program.

The life history and morphology of free-living Pilayella littoralis (L.) Kjellm. (Ectocarpaceae, Ectocarpales) in Nahant Bay, Massachusetts

Abstract A previously unreported free-living form of Pilayella is described. During all months of the year, drifting clouds of this plant may carpet up to half of the sandy bottom and beaches of Nahant Bay, a 15 km2 rectangular embayment on the north coast of Massachusetts. Four features distinguish the atypical free-living form from the typical attached form of this species: (1) a ball shape and a concomitant general lack of polarity in the development of the central axes; (2) a polystichous condition present throughout much of the central axes; (3) a dearth of reproductive organs; and (4) a peak summer-time abundance of hundreds of tonnes (live weight). Plant reproduction and growth appear to result from ball fragmentation and subsequent regeneration solely by vegetative cell division. Fragmentation results from seasonal senescence of the filamentous axes and from infection by the marine fungus Eurychasma dicksonii. The Pilayella drift community is characterized by its virtual monospecificity by weight,...

Effect of four crops on the reprodnction and development of Xiphinema diversicaudatum and on the pore space, moisture content and temperature of the soil

SUMMARYIn a two‐year experiment from 1977 to 1979 in microplots growing strawberry, perennial ryegrass, spring barley or sugar beet, Xiphinema diversicaudatum(Micol.) Thorne reproduced and developed best under strawberry and least well under sugar beet. Total numbers of X. diversicaudatum were similar under spring barley and perennial ryegrass but nematode development beyond first‐stage juveniles was less under barley, probably because of the early season senescence of this crop. Differences in the host status of strawberry, barley and sugar beet could not be attributed to changes, caused by cropping, in the pore space or moisture content of the soil. It was not possible to assess the effect of pore space or soil moisture under ryegrass. During the summer and autumn of 1977 and 1978, under strawberry, barley and sugar beet, soil pores with necks large enough to admit X. diversicaudatum and even those that would admit only the first‐stage juveniles were frequently drained. The small differences in the soil temperatures recorded under the four crops had little effect on nematode reproduction and development.