N添加对西南亚高山红桦林根系分泌物及其介导的养分转化过程的影响

目前缺乏对根系分泌物通量以及相关生态后果对不同氮(N)沉降水平响应方向和幅度的深入理解,该研究以西南亚高山典型的红桦(Betula albosinensis)林为研究对象,通过野外原位N添加试验模拟不同氮沉降水平(对照组,0 kg·hm^-2·a(-1);低氮处理,25 kg·hm^-2·a(-1);高氮处理,50 kg·hm^-2·a(-1)),分析了红桦林根系分泌物C输入通量及其介导的根际土壤养分循环过程对不同N添加水平的差异化响应,试图揭示不同N添加处理对红桦根系分泌物C输入通量及其介导的土壤养分转化过程的影响。结果表明：(1)N添加显著抑制了红桦林根系分泌物C输入速率(其中低氮(N25)条件下单位根生物量根系分泌速率均值降低约14.87%)和年C输入通量(低氮条件下降低了约45.01%)(P<0.05),其高氮处理的抑制效应更强。(2)N添加显著抑制了红桦林N矿化速率及其相关的微生物胞外酶活性(P<0.05),并显著降低了其根际效应;N沉降显著抑制了根系分泌物C输入通量及其介导的土壤养分转化过程,并且这种抑...     

The gum exudates from Chloroxylon swietenia,Sclerocarya caffra,Azadirachta indica and Moringa oleifera

Analytical data are presented for the polysaccharide and proteinaceous components of the gum exudates from Chloroxylon swietenia and Sclerocarya caffra, and for the amino acid compositions of the exudates from Azadirachta indica (two specimens) and Moringa oleifera. The gums from C. swietenia and S. caffra contain 4-O-methylglucuronic acid, glucuronic acid, galactose and arabinose; rhamnose is absent. Amino acid analysis shows that proteinaceous material is present in the gums from C. swietenia, S. caffra and M. oleifera despite their low nitrogen content. Hydroxyproline accounts for 28 % of the amino acid content of S. caffra gum. In contrast, A. indica gum has a high nitrogen content but contains very little hydroxyproline.     

Effects of soil compaction on the microbial populations of melon and maize rhizoplane

Effects of soil compaction on the microbial populations of melon and maize rhizoplane were investigated in quantity and quality. The numbers of culturable bacteria and fluorescent pseudomonads on the rhizoplane were higher when plants were grown in more compacted soil and the relative increase was larger in fluorescent pseudomonads. Total bacterial counts, however, did not appear to be affected by soil compaction, resulting in the increase in the culturable bacteria among total counts in more compacted soil. The determination of extracellular enzymatic properties (pectinase, -glucosidase, -glucosidase and -galactosidase) of each 100 isolates from bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in highly, compacted soil, were composed of high ratios of bacteria with abilities to utilize root exudates efficiently. The microbial community structure estimated from the colony forming curves of bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in compacted soil, were likely to be composed of more r-strategists which were defined as those who formed colonies within 2 days.     

The amino acid composition of gum exudates from Prosopis species

The amino acid compositions of the proteinaceous components of the gum exudates from Prosopis alba, P. chilensis, P. glandulosa, P. laevigata, P. torreyana and P. velutina, and for a sample of commercial gum mesquite, are presented. In agreement with data published previously for the polysaccharide components of their gums, only minor differences in composition are shown by these species. The amino acid compositions are characterized by very high proportions of hydroxyproline and by high proportions of proline and serine; these three amino acids account for 62.5% of those present in the gum from Prosopis velutina. The amino acid compositions of these Prosopis gums are remarkably similar to that established recently for the gum from Acacia senegal (gum arabic).     

Effects of intercropping of oat (Avena sativa L.) with white lupin (Lupinus albus L.) on the mobility of target elements for phytoremediation and phytomining in soil solution

This study aims to investigate how intercropping of oat (Avena sativa L.) with white lupin (Lupinus albus L.) affects the mobile fractions of trace metals (Fe, Mn, Pb, Cd, Th, U, Sc, La, Nd, Ge) in soil solution. Oat and white lupin were cultivated in monocultures and mixed cultures with differing oat/white lupin ratios (11% and 33% lupin, respectively). Temporal variation of soil solution chemistry was compared with the mobilization of elements in the rhizosphere of white lupin and concentrations in plant tissues. Relative to the monocrops, intercropping of oat with 11% white lupin significantly increased the concentrations of Fe, Pb, Th, La and Nd in soil solution as well as the concentrations of Fe, Pb, Th, Sc, La and Nd in tissues of oat. Enhanced mobility of the mentioned elements corresponded to a depletion of elements in the rhizosphere soil of white lupin. In mixed cultures with 33% lupin, concentrations in soil solution only slightly increased. We conclude that intercropping with 11% white lupin might be a promising tool for phytoremediation and phytomining research enhancing mobility of essential trace metals as well as elements with relevance for phytoremediation (Pb, Th) and phytomining (La, Nd, Sc) in soil.     

Chemicals on plant surfaces as a heretofore unrecognized,but ecologically informative,class for investigations into plant defence

Plants produce and utilize a great diversity of chemicals for a variety of physiological and ecological purposes. Many of these chemicals defend plants against herbivores, pathogens and competitors. The location of these chemicals varies within the plant, some are located entirely within plant tissues, others exist in the air‐ (or water‐) space around plants, and still others are secreted onto plant surfaces as exudates. I argue herein that the location of a given defensive chemical has profound implications for its ecological function; specifically, I focus on the characteristics of chemical defences secreted onto plant surfaces. Drawing from a broad literature encompassing ecology, evolution, taxonomy and physiology, I found that these external chemical defences (ECDs) are common and widespread in plants and algae; hundreds of examples have been detailed, yet they are not delineated as a separate class from internal chemical defences (ICDs). I propose a novel typology for ECDs and, using existing literature, explore the ecological consequences of the hypothesized unique characteristics of ECDs. The axis of total or proportional investment in ECDs versus ICDs should be considered as one axis of investment by a plant, in the same way as quantitative versus qualitative chemical defences or induced versus constitutive defences is considered. The ease of manipulating ECDs in many plant systems presents a powerful tool to help test plant defence theory (e.g. optimal defence). The framework outlined here integrates various disciplines of botany and ecology and suggests a need for further examinations of exudates in a variety of contexts, as well as recognition of the effects of within‐plant localization of defences.     

Nitrogen release from roots of alfalfa and soybean grown in sand culture

An enclosed root chamber containing sterile sand medium was used to study net nitrogen (N) release from actively growing root systems of ‘Saranac’ alfalfa (Medicago sativa L.) and ‘Fiskeby V’ soybean (Glycine max L. Merr.). Plants were inoculated with a rhizobial strain appropriate to each host, irrigated with N-free nutrient solution, and grown either to 85 or to 173 d after germination (alfalfa) or to physiological maturity (soybean). Alfalfa released 4.5% of symbiotically-fixed plant N into the root zone over its growth period; soybean released 10.4% of plant N. Root zone leachates were analyzed for total N and for amino acid and ammonium content. Significant ammonium-N release occurred from the alfalfa but not the soybean root system; little amino-N was released by root systems of either species. Shoot harvest and water deficit caused increased release of N from alfalfa roots. The results provide evidence that alfalfa and soybean released significant proportions of their N into the root zone, and indicate that while substantial ammonium-N was released from alfalfa roots, passive leakage of amino-N was not a primary mechanism for N release from root systems of either species. Cooperative investigation of USDA-ARS and the Minnesota Agric. Exp. Stn. (Scientific J. Series No. 16048). This research was supported in part by a Graduate School Fellowship to LSB from the Univ. of Minnesota.     

Study of soybean and lentil root exudates

Summary Some polyphenolic compounds have been identified in root exudates of two species of legumes (both cultivated in sterile conditions): for lentil, three desoxy-5 flavones (4′,7-dihydroxy, 3′,4′,7-trihydroxy and 4′,7-dihydroxy, 3′-methoxyflavones) and for soybean: two isoflavonoids, coumestrol and daidzein (or 4′,7-dihydroxyisoflavone). Identified compounds are polyphenolic aglycones whereas roots contain the corresponding glycosides, showing that a hydrolysis takes place at the time of exudation. Compounds are observed in plantlet exudates as soon as the first day after planting. Their daily quantitative variations parallel those of total phenolic compounds. Exudated coumestrol and 4′,7-dihydroxyflavone amounts are estimated at about 20% of root content in these molecules.     

Studies on groundnut hypocotyl exudates and the behaviour ofRhizoctonia solani in influencing the disease

Summary The behaviour ofRhizoctonia solani on the surface of groundnut seedling hypocotyls was studied at different stages of growth. Characteristic variation in the growth and production on infection cushions was observed. No infection cushions were observed as the seedling reaches the age of three weeks. Hypocotyl exudates were collected at various stages of growth and analysed for total phenols, carbohydrates and ninhydrin positive compounds. The exudates were also analysed for individual amino acids, sugars and organic acids. Gradual decline in the total quantities of various compounds was observed with age. Quantitative and qualitative change in the individual compounds was also evident and some of the compounds were not at all detected as age increases. The possible correlation between the exudation and behaviour of the fungus on host surface is discussed.