Short-term and long-term root respiratory acclimation to elevated temperatures associated with root thermotolerance for two Agrostis grass species

This study was designed to investigate whether thermotolerant roots exhibit respiratory acclimation to elevated temperatures. Root respiratory acclimation traits in response to increasing temperatures were compared between two Agrostis species contrasting in heat tolerance: thermal A. scabra and heat-sensitive A. stolonifera. Roots of both species were exposed to 17, 27, or 37 degrees C. Root RGR declined with increasing temperatures from 17 degrees C to 37 degrees C in both species; however, root growth of A. scabra maintained a significantly higher RGR than A. stolonifera at 27 degrees C or 37 degrees C. A. scabra exhibited a significantly higher respiration acclimation potential to elevated temperatures, both in the short term (60 min) and in the long term (7-28 d) as compared with A. stolonifera, when temperatures increased from 17 degrees C to 27 degrees C or from 27 degrees C to 37 degrees C. Thermal A. scabra also maintained a significantly lower maintenance cost than A. stolonifera as temperatures increased to 27 degrees C or 37 degrees C. The results suggested that root thermotolerance of thermal A. scabra was associated with both short-term and long-term respiratory acclimation to changes in temperatures. The superior ability of adjusting the rate of root respiration to compensate for increases in carbon demand during short- or long-term temperature increases in the heat-tolerant A. scabra may result in the reduction in carbon expenditure or costs for maintenance, leading to extended root survivability in high temperature soils.     

Root respiratory characteristics associated with plant adaptation to high soil temperature for geothermal and turf-type Agrostis species

Respiration is a major avenue of carbohydrates loss. The objective of the present study was to examine root respiratory characteristics associated with root tolerance to high soil temperature for two Agrostis species: thermal Agrostis scabra, a species adapted to high-temperature soils in geothermal areas in Yellowstone National Park, and two cultivars ('L-93' and 'Penncross') of a cool-season turfgrass species, A. stolonifera (creeping bentgrass), that differ in their heat sensitivity. Roots of thermal A. scabra and both creeping bentgrass cultivars were exposed to high (37 degrees C) or low soil temperature (20 degrees C). Total root respiration rate and specific respiratory costs for maintenance and ion uptake increased with increasing soil temperatures in both species. The increases in root respiratory rate and costs for maintenance and ion uptake were less pronounced for A. scabra than for both creeping bentgrass cultivars (e.g. respiration rate increased by 50% for A. scabra upon exposure to high temperature for 28 d, as compared with 99% and 107% in 'L-93' and 'Penncross', respectively). Roots of A. scabra exhibited higher tolerance to high soil temperature than creeping bentgrass, as manifested by smaller decreases in relative growth rate, cell membrane stability, maximum root length, and nitrate uptake under high soil temperature. The results suggest that acclimation of respiratory carbon metabolism plays an important role in root survival of Agrostis species under high soil temperatures, particularly for the thermal grass adaptation to chronically high soil temperatures. The ability of roots to tolerate high soil temperatures could be related to the capacity to control respiratory rates and increase respiratory efficiency by lowering maintenance and ion uptake costs.     

Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface – a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant’s below‐ground respiratory costs, the effects of temperature and soil drying on citrus root respiration were quantified in controlled greenhouse experiments. Chambers designed for measuring the respiration of individual roots were used. Under moist soil conditions, root respiration in citrus increased exponentially with changes in soil temperature (Q₁₀ = 1·8–2·0), provided that the changes in temperature were short‐term. However, when temperatures were held constant, root respiration did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 °C, thereby reducing their metabolism in warmer soils. Under drying soil conditions, root respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was constructed from greenhouse data to predict diurnal patterns of fine root respiration based on temperature and soil water content. The model was then validated in the field using data obtained by CO₂ trapping on root systems of mature citrus trees. The trees were grown at a site where the soil temperature and water content were manipulated. Respiration predicted by the model was in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.     

Responses of tree fine roots to temperature

Soil temperature can influence the functioning of roots in many ways. If soil moisture and nutrient availability are adequate, rates of root length extension and root mortality increase with increasing soil temperature, at least up to an optimal temperature for root growth, which seems to vary among taxa. Root growth and root mortality are highly seasonal in perennial plants, with a flush of growth in spring and significant mortality in the fall. At present we do not understand whether root growth phenology responds to the same temperature cues that are known to control shoot growth. We also do not understand whether the flush of root growth in the spring depends on the utilization of stored nonstructural carbohydrates, or if it is fueled by current photosynthate. Root respiration increases exponentially with temperature, but Q ₁₀ values range widely from c . 1.5 to > 3.0. Significant questions yet to be resolved are: whether rates of root respiration acclimate to soil temperature, and what mechanisms control acclimation if it occurs. Limited data suggest that fine roots depend heavily on the import of new carbon (C) from the canopy during the growing season. We hypothesize that root growth and root respiration are tightly linked to whole-canopy assimilation through complex source–sink relationships within the plant. Our understanding of how the whole plant responds to dynamic changes in soil temperature, moisture and nutrient availability is poor, even though it is well known that multiple growth-limiting resources change simultaneously through time during a typical growing season. We review the interactions between soil temperature and other growth-limiting factors to illustrate how simple generalizations about temperature and root functioning can be misleading.     

Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.     

Adenylate control contributes to thermal acclimation of sugar maple fine‐root respiration in experimentally warmed soil

We investigated the occurrence of and mechanisms responsible for acclimation of fine‐root respiration of mature sugar maple (Acer saccharum) after 3+ years of experimental soil warming (+4 to 5 °C) in a factorial combination with soil moisture addition. Potential mechanisms for thermal respiratory acclimation included changes in enzymatic capacity, as indicated by root N concentration; substrate limitation, assessed by examining nonstructural carbohydrates and effects of exogenous sugar additions; and adenylate control, examined as responses of root respiration to a respiratory uncoupling agent. Partial acclimation of fine‐root respiration occurred in response to soil warming, causing specific root respiration to increase to a much lesser degree (14% to 26%) than would be expected for a 4 to 5 °C temperature increase (approximately 55%). Acclimation was greatest when ambient soil temperature was warmer or soil moisture availability was low. We found no evidence that enzyme or substrate limitation caused acclimation but did find evidence supporting adenylate control. The uncoupling agent caused a 1.4 times greater stimulation of respiration in roots from warmed soil. Sugar maple fine‐root respiration in warmed soil was at least partially constrained by adenylate use, helping constrain respiration to that needed to support work being performed by the roots.     

Impact of temperature on the relationship between respiration and nitrogen concentration in roots: an analysis of scaling relationships, Q10 values and thermal acclimation ratios

* The impact of nitrogen (N) supply on the temperature response of root respiratory O(2) uptake (R) was assessed in several herbaceous species grown in solution culture. Warm-grown (25 : 20 degrees C, day:night) plants differing in root N concentration were shifted to 13 : 8 degrees C for 7 d to cold-acclimate. * Log-log plots of root R vs root N concentration both showed that R increased with increasing tissue N concentration, irrespective of the growth temperature. Although the regression slopes of the log-log plots did not differ between the warm-grown and cold-acclimated plants, cold-acclimated plants did exhibit a higher y-axis intercept than their warm-grown counterparts. This suggests that cold acclimation of root R is not entirely dependent on cold-induced increases in tissue N concentration and that scaling relationships (i.e. regression equations fitted to the log-log plots) between root R and N concentration are not fixed. * No systematic differences were found in the short-term Q(10) (proportional change in R per 10 degrees C change in temperature), or degree of cold acclimation (as measured by the proportional difference between warm- and cold-acclimated roots) among roots differing in root N concentration. The temperature response of root R is therefore insensitive to tissue N concentration. * The insensitivity of Q(10) values and acclimation to tissue N concentration raises the possibility that root R and its temperature sensitivity can be predicted for a range of N supply scenarios.     

林木非同化器官与土壤呼吸的温度系数Q10值的特征分析

温度系数(Q₁₀,温度每变化10 ℃,呼吸速率的相对变化)不仅可以用来描述不同森林非同化器官(根系和树干)和土壤对温度升高的敏感性,并由此断定它们在全球变暖进程中的不同表现,而且是其呼吸总量定量估计中必不可少的参数。虽然目前已经进行了大量的研究,但不同研究者结论并不一致,影响我们对问题的整体把握。因此,有必要综合以往文献进行统计分析。该文综合大量文献,评述了林木非同化器官和土壤的Q₁₀值频率分布、不同研究方法对Q₁₀值的可能影响并探讨了它们对温度升高的敏感性。结果表明,不同非同化器官和土壤的Q₁₀值差异较大,但具有相对稳定的分布中心范围。其中,土壤呼吸Q₁₀值中,频率分布最集中的区域是2.0~2.5,占23%,其中超过80%的测定结果在1.0~4.0之间,中位数为2.74。 根系呼吸的Q₁₀值,频率分布最集中的区域2.5~3.0,占33%,而大部分(>80%)的研究结果在1.5~3.0之间,中位数为2.40。树干呼吸的Q₁₀值中,频率分布最集中的区域是1.5~2.0,占42%,而90%以上的测定结果在1.0~3.0之间,中位数为1.91。通过对比,发现不同非同化器官Q₁₀值不同(树干<根系<根系与土壤共同体<去除根系土壤)。其中树干和根系的Q₁₀值显著低于去除根系土壤的Q₁₀值(p<0.05),表明土壤微生物活动对于未来全球变暖的反应要比木质化器官更敏感。此外,常绿植物的根系和树干呼吸的Q₁₀值与落叶树木对应值差异不显著,说明同化器官叶片的着生时间长短对非同化器官Q₁₀的影响不大。不同的CO₂分析方法(碱吸收法,红外线测定技术和气相色谱方法)对土壤呼吸Q₁₀值测定结果的影响不显著(p>0.10),根系分离方法(断根测定和壕沟隔断测定)也对根系呼吸的Q₁₀值影响也不显著(p>0.10)。但是,与活体测定相比,离体测定树干呼吸显著提高了其Q₁₀值。总体来看,不同林分相同非同化器官以及不同非同化器官呼吸的Q₁₀值相对稳定但仍具有较大的差异性,研究方法也对结果产生一定影响,在进行呼吸总量的定量估计中应该注意这一点。今后研究的重点是进一步把影响森林非同化器官呼吸的外在因素和内在因素综合考虑于Q₁₀值相关模型中,以便准确定量估计其呼吸总量,而研究难点是深入研究Q₁₀值具有较大变异性的原因(如温度适应性)和内在机理以便更好的表征不同器官和生态系统组分对全球变暖的敏感性。     

Adjustment of Forest Ecosystem Root Respiration as Temperature Warms

Adjustment of ecosystem root respiration to warmer climatic conditions can alter the autotrophic portion of soil respiration and influence the amount of carbon available for biomass production. We examined 44 published values of annual forest root respiration and found an increase in ecosystem root respiration with increasing mean annual temperature (MAT),but the rate of this cross-ecosystem increase (Q10 = 1.6) is less than published values for short-term responses of root respiration to temperature within ecosystems (Q10 = 2-3). When specific root respiration rates and root biomass values were examined, there was a clear trend for decreasing root metabolic capacity (respiration rate at a standard temperature) with increasing MAT. There also were tradeoffs between root metabolic capacity and root system biomass, such that there were no instances of high growing season respiration rates and high root biomass occurring together. We also examined specific root respiration rates at three soil warming experiments at Harvard Forest, USA, and found decreases in metabolic capacity for roots from the heated plots. This decline could be due to either physiological acclimation or to the effects of co-occurring drier soils on the measurement date. Regardless of the cause, these findings clearly suggest that modeling efforts that allow root respiration to increase exponentially with temperature, with Qt0 values of 2 or more, may over-predict root contributions to ecosystem CO2 efflux for future climates and underestimate the amount of C available for other uses,including net primary productivity.     

Temperature-respiration relationships differ in mycorrhizal and non-mycorrhizal root systems of Picea abies (L.) Karst

Root respiration has been shown to increase with temperature, but less is known about how this relationship is affected by the fungal partner in mycorrhizal root systems. In order to test respiratory temperature dependence, in particular Q (10) of mycorrhizal and non-mycorrhizal root systems, seedlings of PICEA ABIES (L.) Karst. (Norway spruce) were inoculated with the ectomycorrhizal fungus PILODERMA CROCEUM (Eriksson and Hjortstam, SR430; synonym: PILODERMA FALLAX: [Libert] Stalpers) and planted in soil respiration cuvettes (mycocosms). Temperature dependence of hyphal respiration in sterile cultures was determined and compared with respiration of mycorrhizal roots. Respiration rates of mycorrhizal and non-mycorrhizal root systems as well as sterile cultures were sensitive to temperature. Q (10) of mycorrhizal root systems of 3.0 +/- 0.1 was significantly higher than that of non-mycorrhizal systems (2.5 +/- 0.2). Q (10) of P. CROCEUM in sterile cultures (older than 2 months) was similar to that of mycorrhizal root systems, suggesting that mycorrhizae may have a large influence on the temperature sensitivity of roots in spite of their small biomass. Our results stress the importance of considering mycorrhization when modeling the temperature sensitivity of spruce roots.     

The temperature responses of soil respiration in deserts: a seven desert synthesis

The temperature response of soil respiration in deserts is not well quantified. We evaluated the response of respiration to temperatures spanning 67°C from seven deserts across North America and Greenland. Deserts have similar respiration rates in dry soil at 20°C, and as expected, respiration rates are greater under wet conditions, rivaling rates observed for more mesic systems. However, deserts differ in their respiration rates under wet soil at 20°C and in the strength of the effect of current and antecedent soil moisture on the sensitivity and magnitude of respiration. Respiration increases with temperature below 30°C but declines for temperatures exceeding 35°C. Hot deserts have lower temperature sensitivity than cold deserts, and insensitive or negative temperature sensitivities were predicted under certain moisture conditions that differed among deserts. These results have implications for large-scale modeling efforts because we highlight the unique behavior of desert soil respiration relative to other systems. These behaviors include variable temperature responses and the importance of antecedent moisture conditions for soil respiration.     

Influence of soil temperature on root freezing tolerance of Scots pine (Pinus sylvestris L.) seedlings

The influence of soil temperature on the root freezing tolerance of one-year-old containerized Scots pine (Pinus sylvestris L.) seedlings was investigated. In addition, the TTC and electrolyte leakage methods were evaluated in terms of their suitability for use in detecting damage to roots caused by freezing. In mid-August, seedlings were placed in three thermostat-controlled soil beds in a greenhouse with an initial soil temperature of 14.3 °C. Soil temperature was lowered in two of the soil beds, resulting in temperatures of 10.7 and 5.3 °C respectively. Each soil temperature, i.e. 14.3, 10.7 and 5.3 °C was maintained for eight weeks. Starting in early September, damage to roots induced by artificial freezing was estimated biweekly by measuring electrolyte leakage, triphenyl tetrazolium chloride (TTC) reduction and potential root growth in a three-week cultivation test. In addition, the root freezing tolerance of seedlings placed outdoors was tested. Measurements showed that these seedlings were exposed to soil temperatures ranging from 13.0 °C in mid-August to 0.5 °C in November. Generally, the development of root freezing tolerance was more pronounced for seedlings exposed to lower (0.5 and 5.3 °C) soil temperatures compared with those exposed to higher (10.7 and 14.3 °C) ones. Root freezing tolerance was highest among the seedlings placed outdoors which were also exposed to the lowest soil temperatures registered in the study. To examine the effect of a temporary warm period, the soil temperature in one treatment was increased from 5.4 °C to 13.9 °C, maintained at the latter temperature for two weeks in October and then lowered to 5.7 °C. Root freezing tolerance was reduced by exposure to the warmer soil temperature. However, after four weeks at the colder soil temperature, the tolerance of the seedlings had returned to the level measured prior to exposure to the warm soil temperature. Methods based on the measurement of root electrolyte leakage and TTC reduction were both found to have limitations when used to detect root freezing damages in containerized seedlings. This revised version was published online in June 2006 with corrections to the Cover Date.     

东北地区落叶松人工林的根系呼吸

落叶松根系呼吸速率在6～9月期间逐渐升高,8月达到高峰,之后明显下降.幼林根系呼吸速率和根系呼吸占土壤总呼吸的比例均高于成熟林.根系呼吸速率与根生物量呈线性相关,与土温呈指数相关,与土壤含水量无明显相关关系,但温度较高时,土壤湿度的增加能促进根系呼吸.成熟林和幼林根系呼吸的Q10值分别为5.56和4.17.     

Respiration in postharvest sugarbeet roots is not limited by respiratory capacity or adenylates

Control of respiration has largely been studied with growing and/or photosynthetic tissues or organs, but has rarely been examined in harvested and stored plant products. As nongrowing, heterotrophic organs that are reliant on respiration to provide all of their metabolic needs, harvested plant products differ dramatically in their metabolism and respiratory needs from growing and photosynthetically active plant organs, and it cannot be assumed that the same mechanism controls respiration in both actively growing and harvested plant organs. To elucidate mechanisms of respiratory control for a harvested and stored plant product, sugarbeet (Beta vulgaris L.) root respiration was characterized with respect to respiratory capacity, adenylate levels and cellular energy status in roots whose respiration was altered by wounding or cold treatment (1 degrees C) and in response to potential effectors of respiration. Respiration rate was induced by wounding in roots stored at 10 degrees C and by cold temperature in roots stored at 1 degrees C for 11-13d. Alterations in respiration rate due to wounding or storage temperature were unrelated to changes in total respiratory capacity, the capacities of the cytochrome c oxidase (COX) or alternative oxidase (AOX) pathways, adenylate concentrations or cellular energy status, measured by the ATP:ADP ratio. In root tissue, respiration was induced by exogenous NADH indicating that respiratory capacity was capable of oxidizing additional electrons fed into the electron transport chain via an external NADH dehydrogenase. Respiration was not induced by addition of ADP or a respiratory uncoupler. These results suggest that respiration rate in stored sugarbeet roots is not limited by respiratory capacity, ADP availability or cellular energy status. Since respiration in plants can be regulated by substrate availability, respiratory capacity or energy status, it is likely that a substrate, other than ADP, limits respiration in stored sugarbeet roots.     

Photosynthesis,respiration, and carbon allocation of two cool-season perennial grasses in response to surface soil drying

The study was conducted to investigate carbon metabolic responses to surface soil drying for cool-season grasses. Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinaceae Schreb.) were grown in a greenhouse in split tubes consisting of two sections. Plants were subjected to three soil moisture regimes: (1) well-watered control; (2) drying of upper 20-cm soil (upper drying); and (3) drying of whole 40-cm soil profile (full drying). Upper drying for 30 d had no dramatic effects on leaf water potential (Ψ_leaf) and canopy photosynthetic rate (P_n) in either grass species compared to the well-watered control, but it reduced canopy respiration rate (R_canopy) and root respiration rate in the top 20 cm of soil (R_top). For both species in the lower 20 cm of wet soil, root respiration rates (R_bottom) were similar to the control levels, and carbon allocation to roots increased with the upper soil drying, particularly for tall fescue. The proportion of roots decreased in the 0-20 cm drying soil, but increased in the lower 20 cm wet soil for both grass species; the increase was greater for tall fescue. The Ψ_leaf, P_n, R_canopy, R_top, R_bottom, and carbon allocation to roots in both soil layers were all significantly higher for upper dried plants than for fully dried plants of both grass species. The reductions in R_canopy and R_top in surface drying soil and increases in root respiration and carbon allocation to roots in lower wet soil could help these grasses cope with surface-soil drought stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Oxidative Activity of Mitochondria Isolated from Plant Tissues Sensitive and Resistant to Chilling Injury

Arrhenius plots of the respiration rates of mitochondria isolated from chilling sensitive plant tissues (tomato and cucumber fruit, and sweet potato roots) showed a linear decrease from 25 C to about 9 to 12 C (with Q(10) values of 1.3 to 1.6), at which point there was a marked deviation with an increased slope as temperatures were reduced to 1.5 C (Q(10) of 2.2 to 6.3). The log of the respiration rate of mitochondria from chilling resistant tissues (cauliflower buds, potato tubers, and beet roots) showed a linear decrease over the entire temperature range from 25 to 1.5 C with Q(10) values of 1.7 to 1.8. Phosphorylative efficiency of mitochondria from all the tissues, as measured by ADP:O and respiratory control ratios, was not influenced by temperatures from 25 to 1.5 C. These results indicate that an immediate response of sensitive plant tissues to temperatures in the chilling range (0 to 10 C) is to depress mitochondrial respiration to an extent greater than that predicted from Q(10) values measured above 10 C. The results are also consistent with the hypothesis that a phase change occurs in the mitochondrial membrane as the result of a physical effect of temperature on some membrane component such as membrane lipids.     

Citrus root responses to localized drying soil: A new approach to studying mycorrhizal effects on the roots of mature trees

Because fine roots tend to be concentrated at the soil surface, exposure to dry surface soil can have a large influence on patterns of root growth, death and respiration. We studied the effects of arbuscular mycorrhizas (AM) formation on specific root length (SRL), respiration and mortality of fine roots of bearing red grapefruit (Citrus paradisi Macf.) trees on Volkamer lemon (C. volkameriana Tan. & Pasq.) rootstock exposed to drying soil. For each tree, the fine roots were removed from two woody lateral roots, the roots were surface sterilized and then each woody root was placed in a separate pair of vertically divided and independently irrigated soil compartments. The two split-pot systems were filled with sterilized soil and one was inoculated with arbuscular mycorrhizal fungi (Glomus etunicatum/G. intraradices). New fine lateral roots that emerged from the woody laterals were permitted to grow inside the pots over a 10-month period. Irrigation was then removed from the top compartment for a 15-week period. At the end of the study, roots inoculated with AM fungi exhibited about 20% incidence of AM formation, whereas the uninoculated roots were completely void of AM fungi. Arbuscular mycorrhizal roots exhibited lower SRL, lower root/soil respiration and about 10% lower fine root mortality than nonmycorrhizal roots after 15 weeks of exposure to dry surface soil. This study demonstrates the feasibility of examining mycorrhizal effects on the fine roots of adult trees in the field using simple inexpensive methods.     

Comparison of temperature effects on soil respiration and bacterial and fungal growth rates

Temperature is an important factor regulating microbial activity and shaping the soil microbial community. Little is known, however, on how temperature affects the most important groups of the soil microorganisms, the bacteria and the fungi, in situ. We have therefore measured the instantaneous total activity (respiration rate), bacterial activity (growth rate as thymidine incorporation rate) and fungal activity (growth rate as acetate-in-ergosterol incorporation rate) in soil at different temperatures (0-45 degrees C). Two soils were compared: one was an agricultural soil low in organic matter and with high pH, and the other was a forest humus soil with high organic matter content and low pH. Fungal and bacterial growth rates had optimum temperatures around 25-30 degrees C, while at higher temperatures lower values were found. This decrease was more drastic for fungi than for bacteria, resulting in an increase in the ratio of bacterial to fungal growth rate at higher temperatures. A tendency towards the opposite effect was observed at low temperatures, indicating that fungi were more adapted to low-temperature conditions than bacteria. The temperature dependence of all three activities was well modelled by the square root (Ratkowsky) model below the optimum temperature for fungal and bacterial growth. The respiration rate increased over almost the whole temperature range, showing the highest value at around 45 degrees C. Thus, at temperatures above 30 degrees C there was an uncoupling between the instantaneous respiration rate and bacterial and fungal activity. At these high temperatures, the respiration rate closely followed the Arrhenius temperature relationship.     

Root respiration in citrus acclimates to temperature and slows during drought

Citrus seedlings were grown in soil columns in which the root system was hydraulically separated into two equal layers; this enabled us to maintain roots in the upper layer without water for 110 d. The columns were placed into waterbaths modified so that soil temperatures in the top layer could be maintained at 25°C or at 35°C, while temperature in the bottom layer was maintained at 25°C. We hypothesized that, if citrus plants were grown in dry soil for an extended period, root mortality would increase if the cost of maintaining the roots was increased by elevating the soil temperature. However, during the drought period we did not observe any root mortality, even at the higher soil temperature. Moreover, we did not find that root respiration was increased by prolonged exposure to drought and higher soil temperature. We did find that root respiration rates slowed in dry soil. Furthermore, when the soil columns were switched from one temperature treatment to another, root respiration rates in wet soil rapidly increased when moved to a higher temperature or rapidly decreased when moved to a lower temperature. But after only 4 d, respiration rates returned to their original level; root respiration in dry soil was not affected by either short-or long-term shifts in soil temperature. Root respiration in citrus appears to acclimate rapidly to changes in soil temperature.     

Assimilation and allocation of carbon and nitrogen of thermal and nonthermal Agrostis species in response to high soil temperature

We studied whether changes in the assimilation and allocation of carbon and nitrogen are associated with plant tolerance to high soil temperatures. Two Agrostis species, thermal Agrostis scabra, a species adapted to high-temperature soils in geothermal areas in Yellowstone National Park (USA), and two cultivars of a cool-season species, Agrostis stolonifera, L-93 and Penncross, were exposed to soil temperatures of 37 or 20 degrees C, while shoots were exposed to 20 degrees C. Net photosynthesis rate, photochemical efficiency, NO(3) (-)-assimilation rate and root viability decreased with increasing soil temperatures in both species. However, the decreases were less pronounced for A. scabra than for both A. stolonifera cultivars. Carbon investment in growth of plants exposed to 37 degrees C decreased more dramatically in both A. stolonifera cultivars than in A. scabra. Nitrogen allocation to shoots was greater in A. scabra than in both creeping bentgrass cultivars at 37 degrees C soil temperature. Our results demonstrate that plant tolerance to high soil temperature is related to efficient expenditure and adjustment of C- and N-allocation patterns between growth and respiration.