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David Eissenstat

Root turnover has been determined primarily in ecosystem studies with perennial vegetation since it is a key for understanding primary production and nutrient cycling. These topics have become of interest to agriculturalists as well. Apart from ecosystem-level questions, there has been limited study of the environmental factors that influence root death. Many techniques have been devised to estimate root turnover, each with its own set of limitations. In forest ecosystems, one of the most popular methods of estimating root production turnover is sequential biomass sampling. However, this method fails to account for the simultaneous production and decomposition of roots during active periods of net biomass increase. A second method is a mesh-bag technique, which estimates root production/turnover from the amount of new roots that grow into a mesh bag. A method that uses radiocarbon is one of the most accurate, since estimates of root turnover include losses by exudation, cortical cell sloughing, as well as root loss. A fourth method of estimating root turnover involves tracking the roots visible behind transparent glass or plastic. Ultimately, the choice of method depends to a large extent on the type of plants used in the investigation and resources available for study.

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Wei Qiang Yang, Amy K. Dunbar and Mary A. Topa

Loblolly pine (Pinus taeda L.) is the most widely planted tree species in the Atlantic Coastal Plain. To maximize its aboveground yield, it is vital to understand how root production, particularly fine root production, affects root carbon allocation to its root systems under various environmental conditions. Over a 2-year period (1998-99), we conducted a field study using minirhizotron technology to investigate fine root production and turn over in four families of a 6-year-old loblolly pine stand in Scotland County, N.C. A total of 144 minirhizotron tubes were installed to examine potential genetic differences in fertilizer effects on fine root turnover. Data analyses indicated an interaction between these families and fertilizer treatments for total fine root length and total fine root number. The effect of treatment on total root length was less clear in the faster-growing families. However, fertilization increased total root length in a slow-growing family but decreased total root length in a faster-growing family. Total root number was decreased by fertilizer treatment in the two fastest-growing families, but increased in the two slowest-growing families. Because ectomycorrhizae are significant carbon sinks in pine root systems and more than 90% of short roots in these loblolly pine families were colonized, ectomycorrhizal short roots (clusters) were classified into nine different morphotypes. No treatment and family interactions were found. Fertilizer treatment decreased the number of mycorrhizal clusters per unit root length. Dark and brown morphotypes were dominant mycorrhizal morphotypes among all the families. Our results suggest possible genetic differences and treatment effects on root system carbon demands of loblolly pine.

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S.S. Snapp and C. Shennan

Roots respond first to edaphic stresses, yet little is known about root response to stress in mature, soil-grown plants. We investigated the effects of salinity and phytophthora root rot on root growth and senescence in tomato (Lycopersicon esculentum Mill.). Using minirhizotron- and rhizotron-based methodologies, we quantified intraspecific differences in root-system response to salinity and inoculation. Genotype susceptibility to salt-induced disease was related to root vulnerability to salt. `UC82B' was vulnerable to infection by Phytophthora parasitica when subjected to salt stress and produced thinner roots and ≈50% higher root-senescence rates compared to the phytophthora root rot-resistant `CX8303'. Root growth at the peripheral regions of the `CX8303' root system was inhibited by salinity, but otherwise root dynamics were not affected by salinity or inoculation. Overall, roots from the central root system and roots from the periphery responded differently to salt stress. Monitoring the diameters of new initiated roots indicated the vulnerability of a stressed root system to disease and early senescence.

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Shengrui Yao, Ian A. Merwin and Michael G. Brown

Root observations in situ with a rhizotron camera enabled us to compare the performance of apple (Malus ×domestica Borkh.) trees on 3 rootstock clones planted in a New York orchard with a history of apple replant disease. Visual observations were conducted in situ at monthly intervals during 2 growing seasons through minirhizotron tubes for trees grafted onto 3 rootstocks: M.7 (M.7), Geneva 30 (G.30), and Cornell-Geneva 6210 (CG.6210). There were 3 preplant soil treatments (fumigation, compost amendment, and untreated checks) and 2 tree planting positions (within the old tree rows or in the old grass lanes of the previous orchard at this site). Preplant soil treatments and old-row versus grass-lane tree planting positions had no apparent influence on root systems, whereas rootstock clones substantially influenced root growth and demography. New root emergence was suppressed during the first fruit-bearing year (2004) on all 3 rootstock clones compared with the previous nonbearing year (2003). A root-growth peak in early July accounted for more than 50% of all new roots in 2003, but there was no midsummer root-growth peak in 2004. The median lifespan for roots of CG.6210 was twice that of G.30 and M.7 in 2004. Also, CG.6210 had more roots below 30 cm depth, whereas M.7 had more roots from 11 to 20 cm depth. Trees on CG.6210 were bigger, yielded more fruit, and had the highest yield efficiency in the third year after planting compared with trees on G.30 and M.7 rootstocks. Crop load appeared to inhibit new root development and changed root-growth dynamics during the first bearing year, with a resurgence in new root growth after fruit was harvested in October 2004. Rootstock genotype was the dominant influence on root lifespan and distribution in this study, whereas preplant soil fumigation, compost amendments, and replanting positions had little apparent impact on root characteristics despite their influence on above-ground tree growth and yield.

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David R. Bryla and Bernadine C. Strik

harvest. Additional nutrients may have been lost to root turnover ( Valenzuela-Estrada et al., 2008 ). An estimated 70% of the N in the plants at the end of the first year, and 25% of the N in the fruit the following year, was derived from fertilizer

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

aboveground plant characteristics. However, cold hardiness of Rhododendron roots also differs among cultivars, ranging from –20 to –7 °C ( Havis, 1976 ; Studer et al., 1978 ). The root turnover that occurred in December may be a result of less cold

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G. Psarras, I. Merwin, A. Lakso and R. Zobel

We are evaluating techniques for measuring intact apple rootstock (Malus domestica cv. M.9 and MM.111) responses to low, medium, and high soil-water potential, and low, medium, and high concentrations of N, K, and Ca, in sterile sand culture. Root respiration and functional surface area were estimated with an IRGA chamber and electric capacitance meter, respectively. Root length and surface area were determined by digital image analysis of extracted root systems. Low N supply reduced root respiration, while low K levels increased respiration relative to well-nourished controls. Calcium effects were inconsistent among the rootstocks. Total root length and respiration rates of MM.111 were higher than M.9, but M.9 had higher root:shoot ratios. Root capacitance was correlated with total root length (P < 0.001); and M.9 root systems had greater capacitance than MM.111. In a related field experiment, root growth and respiration of 4-year-old `Mutsu' apple trees on M.9 rootstock were measured in soil under low and moderate drought stress established by rain exclusion shelters, using capacitance and IRGA meters, and a minirhizotron video camera inserted into Plexiglas tubes transecting the rhizosphere. Root growth rates peaked in July (coinciding with maximal shoot growth), then declined gradually during late summer; but variability among trees was greater than among water stress treatments. Root/soil respiration maxima of 4.5 μmol CO2/m2 per s occurred in mid June, late July (when new root counts peaked), and the end of August (when root turnover was maximal).

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Shann Tanner, Christina Wells and Gregory Reighard

The effectiveness of soil solarization as an alternative to methyl bromide (MBr) fumigation in replanted peach orchards was investigated at the Musser Fruit Research Farm near Clemson, S.C. A split plot experimental design was used, with soil treatment as the whole-plot factor and rootstock as the sub-plot factor. In Spring 2002, preexisting trees were removed from the study site, and six orchard rows were cultivated and subsoiled. In June, two rows were covered with clear polyethylene sheeting and solarized for the remainder of the summer. In November, two additional rows were treated with MBr (474.3 kg·ha-1), while the two remaining control rows received no soil sterilization treatment. In Jan. 2003, 36 `Redglobe' peach trees budded on Guardian™ or Lovell rootstock were transplanted to the site, and one minirhizotron was installed beneath each tree. Minirhizotron observations were made every 14–21 days from Feb. through Oct. 2003, and stem caliper measurements were taken on four dates during this interval. Trees grew significantly larger in the MBr and solarized rows than in the control rows (P< 0.1; Tukey's hsd), but there were no differences in stem caliper growth between MBr and solarization-treated trees. Reduced aboveground growth in control trees may have been related to greater carbon expenditure belowground: in the absence of soil sterilization, fine root median life spans were reduced by 27–28 days (P< 0.0001; proportional hazards regression) and rates of root production and mortality were significantly higher (P< 0.1; repeated measures ANOVA). Solarization and MBr fumigation appeared to provide similar benefits in reducing root turnover and improving aboveground growth at this site.

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Georgios Psarras, Ian A. Merwin, Alan N. Lakso and John A. Ray

A 2-year field study of `Mutsu' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] on `Malling 9' (M.9) rootstock was conducted to observe root growth in situ, and compare patterns of root growth, root maturation and turnover rates, and soil-root respiration. Rhizosphere respiration was monitored with a portable chamber connected to an infrared gas analyzer; root emergence, browning, and turnover rates were measured by direct observation through minirhizotron tubes inserted in the root zone. Negligible root growth was observed before the onset of shoot growth in mid-May. In both years, a main peak of new root emergence in late June and early July coincided partially with major phases of shoot and fruit growth. A smaller peak of root emergence during August to September 1997 consisted primarily of new roots at 20 to 45 cm soil depths. Most roots remained <1 mm in diameter and developed in the upper 25 cm soil profile; no roots were observed at any time below 50 cm, due to a compacted soil layer at that depth. The cumulative survivorship of new roots was 38% in 1996 and 64% in 1997, and 50% of emergent white roots turned brown or senesced within 26 days in 1996 and 19 days in 1997. Root turnover rates were highest in mid-August both years. Rhizosphere respiration was correlated (r 2 = 0.36 and 0.59, P = 0.01 and 0.004) with soil temperatures in 1996 and 1997, with Q10 values of 2.3 in both years. The Q10 for root-dependent respiration (the difference between soil only and combined soil-root respiration) in 1997 was 3.1, indicating that roots were more sensitive than soil microflora to soil temperature. The temporal overlap of high rates of shoot, root and fruit growth from late May to mid-July suggests this is a critical period for resource allocations and competition in temperate zone apple trees.