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 tyro 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 degrees C or at 35 degrees C, while temperature in the bottom layer was maintained at 25 degrees 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 met 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. [KEYWORDS: Citrus volkameriana; biomass allocation; drought; root respiration; root turnover; soil temperature; Volkamer lemon

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 (Q(10) = 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 degreesC, 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. [KEYWORDS: root distribution, simulation, soil water content, temperature]

Effects of phosphorus availability and vesicular-arbuscular mycorrhizas on the carbon budget of common bean (Phaseolus vulgaris)

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean (Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 mu M) or moderate (10 mu M) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29-40 %) compared with plants grown with moderate P supply (18-25 %). Relatively greater belowground respiration in low P plants was mainly a result of their increased root:shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40 %. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low [KEYWORDS: carbon budget, Phaseolus vulgaris L. (common bean), phosphorus efficiency, root respiration, vesicular-arbuscular mycorrhiza]

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO2 concentration on root respiration rates during short-term CO2 exposure, and on plant growth during long-term CO2 exposure, have been reported, Here we examine the effects of both short-and long-term exposure to soil CO2 on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO2, Respiration rates of citrus roots were unaffected by the CO2 concentration used during the respiration measurements (200 and 2000 mu mol mol(-1)), regardless of the soil CO2 concentration during the previous month (600 and 20 000 mu mol mol(-1)). Bean plants were grown with their roots exposed to either a natural CO2 diffusion gradient, or to an artificially maintained CO2 concentration of 600 or 20 000 mu mol mol(-1), These treatments had no effect on shoot and root growth, Growth respiration and maintenance respiration of bean roots were also unaffected by CO2 pre-treatment and the CO2 concentration used during the respiration measurements (200-2000 mu mol mol(-1)). We conclude that soil CO2 concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean. [KEYWORDS: Citrus volkameriana L.; Phaseolus vulgaris L.; citrus; common bean; growth analysis; root respiration; soil CO2 concentration Carbon-dioxide; plants; o2]

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO2 concentration on root respiration rates during short-term CO2 exposure, and on plant growth during long-term CO2 exposure, have been reported, Here we examine the effects of both short-and long-term exposure to soil CO2 on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO2, Respiration rates of citrus roots were unaffected by the CO2 concentration used during the respiration measurements (200 and 2000 mu mol mol(-1)), regardless of the soil CO2 concentration during the previous month (600 and 20 000 mu mol mol(-1)). Bean plants were grown with their roots exposed to either a natural CO2 diffusion gradient, or to an artificially maintained CO2 concentration of 600 or 20 000 mu mol mol(-1), These treatments had no effect on shoot and root growth, Growth respiration and maintenance respiration of bean roots were also unaffected by CO2 pre-treatment and the CO2 concentration used during the respiration measurements (200-2000 mu mol mol(-1)). We conclude that soil CO2 concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean. [KEYWORDS: Citrus volkameriana L.; Phaseolus vulgaris L.; citrus; common bean; growth analysis; root respiration; soil CO2 concentration Carbon-dioxide; plants; o2

Estimating respiration of roots in soil: interactions with soil CO2, soil temperature and soil water content

Little information is available on the variability of the dynamics of the actual and observed root respiration rate in relation to abiotic factors. In this study, we describe I) interactions between soil CO2 concentration, temperature, soil water content and root respiration, and II) the effect of short-term fluctuations of these three environmental factors on the relation between actual and observed root respiration rates. We designed an automated, open gas-exchange system that allows continuous measurements on 12 chambers with intact roots in soil. By using three distinct chamber designs with each a different path for the air flow, we were able to measure root respiration over a 50-fold range of soil CO2 concentrations (400 to 25000 ppm) and to separate the effect of irrigation on observed vs. actual root respiration rate. All respiration measurements were made on one-year-old citrus seedlings in sterilized sandy soil with minimal organic material. Root respiration was strongly affected by diurnal fluctuations in temperature (Q(10) = 2), which agrees well with the literature. In contrast to earlier findings for Douglas-fir (Qi et al., 1994), root respiration rates of citrus were not affected by soil CO2 concentrations (400 to 25000 ppm CO2; pH around 6). Soil CO2 was strongly affected by soil water content but not by respiration measurements, unless the air flow for root respiration measurements was directed through the soil. The latter me [KEYWORDS: Citrus volkameriana Tan. & Pasq.; CO2-diffusion gradient; root respiration; soil CO2 concentration; Volkamer lemon]

Canopy and Environmental Control of Root Dynamics in a Long-Term Study of Concord Grape

• Below‐ground carbon allocation represents a substantial fraction of net photosynthesis in plants, yet timing of below‐ground allocation and endogenous and exogenous factors controlling it are poorly understood. • Minirhizotron techniques were used to examine root populations of Vitis labruscana Bailey cv. Concord under two levels of dormant‐season canopy removal and irrigation. Root production, pigmentation, death and disappearance to a depth of 110 cm were determined over two wet and two dry years (1997–2000). • There was continual root production and senescence, with peak root production rates occurring by midseason. Later in the season, when reproductive demands for carbon were highest and physical conditions limiting, few roots were produced, especially in dry years in nonirrigated vines. Root production under minimal canopy pruning was generally greater and occurred several weeks earlier than root production under heavy pruning, corresponding to earlier canopy development. Initial root production occurred in shallow soils, likely due to temperatures at shallow depths being warmer early in the season. • Our study showed intricate relationships between internal carbon demands and environmental conditions regulating root allocation