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 shortand 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 volkainenana 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 ^niol nioP'), regardless of the soil CO2 concentration dnring the previous month (600 and 20 000 pniol nioP'). 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 ^/niol niol"'. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected hy CO2 pre-treatment and the CO2 concentration used dnring the respiration measurements (200-2000 //mol mol"'). 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. Key-words: Citrtts volkaineriatui L.; Phaseolus vulgaris L.; citrus; cotntnon bean; gtowth analysis; root r'espitation; soil CO2 concentration. INTRODUCTION A large body of work describes the effects of elevated atmospheric CO2 on shoot photosynthesis, shoot r-espir-ation and shoot growth. The amount of research being carried out on the effects of elevated atmospheric COT on root growth and root respiration is increasing (e.g. review by Rogers, Runion & Krupa 1994). However, there is still little current research focusing on the effect of soil CO, concentrations on root processes, even though soil CO2 concentrations generally greatly exceed that of the Correspondence and presetit address: Tjeerd Bounia. Netherlands Institute of Ecology. Ceiilre for Estuarine and Coastal Ecology, PO Box 140. 4400 AC Yerseke. The Netherlattds. Fax. +31 113 573616: e-mail: tbouniad' cenio.nioo.knaw.nl © 1997 Blackwell Science Ltd • '• " . ; atmospher-e. Soil COj concentration is a function of CO2producing activity in the soil and soil diffusivity, resulting in concentr ations that var-y with depth (.lohnson et al. 1994; Duenas et al. 1995), soil water content (500 for dry ver-sus 50 000 iJmo\ mor ' for wet conditions; Bouma et al. 1997), soil type (4000-10 000 jUmol moP' CO2 at 50 cm depth; Duenas et al. 1995) and time of year (up to 14 000 /imol mol ' at 15 cm; Johnson el al. 1994). The high but var-iable soil CO2 concentrations may affect root physiology, as discussed in the next two par̂ agr̂ aphs. Reports on the short-term effects of soil CO2 on root respiration have been contradictory. Root respiration of seedlings of Douglas fir {P.setidotstiga ittenziesii (Mirt.) Franco] decreased by a factor of 4-5 when soil CO, concentrations were doubled (Qi, Marshall & Mattson 1994), The effect of CO2 on root respiration was most pr^ominent for concentrations between =200 and 2000 ^mol moP'. However, the same CO2 range had no effect on the root respir-ation r-ates of three desert species; their respiration r-ates decreased only for CO2 concentrations of 3000 P' y 2 3000 ^mol moP' and higher (Nobel & Palta 1989; Palta & Nobel 1989). These contradictory results clearly indieate that the CO2 concentrations used during r'espiration measureriients may be critical. Qi et al. (1994) hypothesized that root r-espiration rates of Douglas fir showed a str-onger CO-, response than those of desert succulents, because, in theirstudy, the total r-oot respiration rate of Douglas fir could be ascribed to maintenance, as the seedlings were kept at the light compensation point. We considered this hypothesis unlikely, because slow-growing citrus exhibited no CO, response over a range of 400-25 000 /jrnol mol ' (Bouma etal. 1997). However, none of the studies discussed above (Nobel & Palta 1989; Palta & Nobel 1989; Qi etal. 1994; Bouma et al. 1997) provides the quantitative data on root growth necessar-y for adequate testing of the hypothesis of Q\etal. (1994). Long-term r-espon.ses of root respiration to soil CO2 concentration may differ from the short-term responses discussed above. Respitatoty responses to soil CO-, may be adapted to growth conditions in such a way that the r-espiratory enzymes ar-e most sensitive to CO2 concentrations outside the concentr-ation range that is normally exper-ienced by those enzymes (Amthor 1991). Long-term CO2 effects on respiratory losses by the r-oot may be assessed by growth analysis of plants with roots exposed to different soil CO2 concentrations. Available reports are also contr-adictoty on the effect of high soil CO2 concentrations on