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- Materials and methods
Plant and ecosystem responses to increasing atmospheric CO2 are directly influenced by the size and activity of below-ground sinks (Curtis et al., 1990) and the magnitude of terrestrial ecosystem carbon storage may be determined by below-ground processes. Roots and their symbionts (mycorrhizas, nodulating bacteria) are responsible for most nutrient uptake but are also major sinks for plant photosynthates during vegetative growth (Paul & Kucey, 1981; Harris & Paul, 1987). Carbon enters the soil as a consequence of growth, activity and death of roots and root symbionts. The presence of nodulating bacteria and mycorrhiza can increase C allocation below-ground and use up to 10–20% of the fixed C (Harris et al., 1985; Jakobsen & Rosendahl, 1990; Peng et al., 1993).
Mycorrhizas usually promote plant growth by improving plant uptake of P and other immobile nutrients (Smith & Read, 1997). However, mycorrhizas may also alter the morphology, size and longevity of root systems (Eissenstat et al., 2000). Mycorrhizas increased root branching in the grass species Andropogon gerardii (Daniels Hetrick et al., 1988) and in Populus (Hooker et al., 1992), which had longer and more ramified roots than nonmycorrhizal controls, but decreased root branching and specific root length of cotton, Gossypium hirsutum (Price et al., 1989). Root systems of mycorrhizal leek, Allium porrum (Berta et al., 1990) and mycorrhizal grapevine, Vitis vinifera (Schellenbaum et al., 1991), formed more, but shorter and more profusely ramified roots than nonmycorrhizal plants. The size of root systems of mycorrhizal vs nonmycorrhizal plants is variable and this is due, at least in part, to differences in the effectiveness of the colonizing arbuscular mycorrhizal fungi and in soil fertilitity (Graham et al., 1996).
It is generally accepted that ectomycorrhizas increase root longevity (Vogt & Bloomfield, 1991) since increased longevity is consistent with an increased efficiency of nutrient acquisition (Eissenstat & Yanai, 1997). The effect of arbuscular mycorrhizas on root production and mortality, however, has only been studied in poplar trees (Hooker et al., 1995), where root longevity was actually decreased by mycorrhizal colonization. Root production and root mortality have been well studied, in particular since rhizotron and video-recording technology became available (Fitter et al., 1997). There are, nevertheless, no published reports on the effect of mycorrhiza on root production and loss of annuals or on how this might be affected by global climate change.
There are conflicting results on the effect of elevated CO2 on arbuscular mycorrhizas and some variation in magnitude and direction of the response of different arbuscular mycorrhizal fungi to consider (Klironomos et al., 1998; Fitter et al., 2000). However, studies in which changes in plant size are accounted for have not shown an effect of elevated CO2 on intra- and extra-radical mycorrhizal colonization (Staddon & Fitter, 1998; Staddon et al., 1999). Results on the effect of elevated CO2 on root production and mortality are also variable, but most of them indicate increased root production and mortality under elevated CO2 (Rogers et al., 1994; Pregitzer et al., 1995; Fitter et al., 1996).
We conducted an experiment with pea plants to study root responses to elevated CO2 and their possible interactions with the presence of mycorrhizal symbionts in the roots. We hypothesized that nonmycorrhizal controls would produce more roots of smaller diameter and shorter life compared with roots of mycorrhizal plants thereby improving soil exploration for nutrients. We expected the differences between nonmycorrhizal and mycorrhizal plants to be larger at elevated than at ambient CO2 because mycorrhizal plants would overcome an increased nutrient demand at elevated CO2 by enhancing nutrient uptake by mycorrhizas rather than by altering their root system.
We present here the effects of atmospheric CO2 and mycorrhizal inoculation on weekly root production and root loss followed with a video-minirhizotron system. Results from the same experiment concerning the evaluation of mycorrhizal colonization, plant biomass production and allocation, and nutrient uptake are published in a separate paper (Gavito et al., 2000).
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- Materials and methods
A limited number of the previous studies on CO2 effects on root production and root loss have dealt with annuals, and the present work is the first to our knowledge to include mycorrhizas as a treatment factor in such a study. Our prediction of higher root production and faster root loss in nonmycorrhizal than in mycorrhizal plants was in general supported by the results. Contrary to our expectations, these differences were significant and clear only at ambient CO2. Our prediction of a CO2–mycorrhiza interaction was based, however, on the premise of increased nutrient demand at elevated CO2 presumably leading to nutrient limitation.
It is important to consider that our experiment was carried out with a nodulated legume that was given additional N and P and grown under favourable conditions to minimize nutritional or other effects on plant and mycorrhiza development. Mycorrhizal colonization was similar at both CO2 levels (Gavito et al., 2000) and there were no differences in root mass, root length density, intraradical or extra-radical mycorrhizal colonization of ambient and elevated CO2 plants that may have indicated confounding changes in total mycorrhizal colonization or fungal biomass. Our observations of biomass production and nutrient uptake in this experiment (Gavito et al., 2000) suggested that there was little persistent nutrient limitation. Assuming that there was no nutrient limitation, it is difficult to explain why mycorrhizal inoculation affected root production and root loss only at ambient CO2, a result that has been confirmed in later experiments (M. E. Gavito & I. Jakobsen, unpublished).
Our only possible explanation is based on the use of plant C by mycorrhizas. Mycorrhizal fungi are obligate symbionts and depend on plant C but would not respond to an increase in C supply unless this resource was limiting. Photosynthate use by mycorrhizal symbionts does not seem to be controlled by the plants (Fitter et al., 2000) and can be high enough to become detrimental as illustrated from reports of growth depressions caused by inoculation with mycorrhizal fungi (Eissenstat et al., 1993; Peng et al., 1993). We believe that mycorrhizal fungi are not C limited and, with adequate nutrients and at elevated atmospheric CO2, they might at some point respond to an increased availability of root space to colonize, not to an increased C supply to the roots. If so, mycorrhizal fungi would use the same amount of C at both atmospheric CO2 levels, at least during the vegetative stage where fungal growth is usually not ‘root space limited’. Without nutrient limitation but limited C supply for the plant (ambient CO2), shoot C in mycorrhizal plants would be partitioned to mycorrhizal fungi at the expense of root development whereas in nonmycorrhizal plants it would be used entirely for root development. At elevated CO2, if a larger amount of photosynthate was sent below-ground, the additional C could be used for root growth in mycorrhizal plants thereby resulting in a more similar root production in nonmycorrhizal and mycorrhizal plants. This is, at present, still highly speculative because most work on the regulation of the mycorrhizal symbiosis has been centred on the plant side and plant-limiting resources such as P (Fitter et al., 2000).
We found that vegetative growth was accompanied by increasing root production, and the onset of flowering marked a drastic reduction in root production and a period of high root loss. Such a clear period of root shedding implies that large amounts of C and nutrients enter the soil in the form of dead roots at flowering, when between 40 and 50% of the root system dies. At ambient CO2, root length just before flowering was approx. 30% lower in mycorrhizal than in nonmycorrhizal plants. Two wk after flowering root length was reduced by 24% in mycorrhizal plants and by 43% in nonmycorrhizal plants compared to root length measured before flowering. Jakobsen (1986) and Jakobsen & Nielsen (1983) used a soil-core method to measure changes in root length of ambient CO2 field-grown peas in similar soil and temperature conditions as our study. They found that nonmycorrhizal plants produced more roots of smaller diameter and shorter life than mycorrhizal plants and that there were two clear periods of root production and loss. One period occurred before flowering and one occurred after flowering, indicating some replacement or turnover of roots at pod filling. In their study, root length before flowering was 30% lower in untreated P-fertilized soil than in fumigated soil (to eliminate mycorrhizas) and after flowering root length was reduced by 23% in untreated soil and by 33% in fumigated soil (Jakobsen, 1986). This indicates higher loss of roots without mycorrhiza. Our results at ambient CO2 are, therefore, in general agreement with previous observations of field-grown peas but differ from those of Hooker et al. (1995) and Hooker & Atkinson (1996) where arbuscular mycorrhizas decreased root longevity in the woody perennial Populus generosa inter americana. Cohort survivorship showed that between 40 and 50% of the root segments born had disappeared by week 8, but with no obvious treatment differences. Roots of annual crops seem to live for 1 or 2 months and to have a shorter life than perennials, based on few studies conducted to date (Eissenstat & Yanai, 1997). Since our criterion for root death was disappearance, and not change of colour or appearance, as in most root mortality studies, it is possible that we underestimated root loss (Comas et al., 2000).
The root systems of mycorrhizal and nonmycorrhizal plants seem to be similar under nonlimiting nutrient supply and to differ progressively as nutrient availability decreases or increases from the optimum (Daniels Hetrick et al., 1988; Amijee et al., 1989). If we assume that there was no nutrient limitation in our experiment, the differences in root length should be attributed to size or morphology changes in the root system induced by the presence of mycorrhiza independently from its effects on nutrient uptake. Hooker et al., 1992) reported such mycorrhiza-induced changes unrelated to plant nutrition in root morphology and architecture of Populus. We found no mycorrhiza effects on mean root diameter suggesting changes in root morphology or architecture. We are aware of no other published reports of mycorrhiza effects on root diameter, production and loss in annual plants with which to compare our findings. Altogether, our results indicated that under controlled growth conditions of nonlimiting nutrient supply, root production and root loss of pea plants was affected only slightly by the presence of mycorrhiza but mycorrhizal inoculation effects on root architecture could not be ruled out.
Elevated CO2 had almost no effect on standing root length or root production in our experiment, which is contrary to our expectations but in agreement with the results of van Vuuren et al. (1997) from a study with spring wheat and other results discussed by Arnone et al. (2000). The lack of response to atmospheric CO2 is consistent with the results discussed above if plants were not nutrient-limited. Root loss was also unaffected by elevated CO2, in accordance with other studies showing that elevated CO2 does not affect the relationship between root production and root loss (Pregitzer et al., 1995; Berntson & Bazzaz, 1996). That is, root production, root loss and root longevity (turnover) are affected equally by elevated CO2, although CO2 effects are likely to be different in annual and perennial plants (Pritchard & Rogers, 2000).
Overall, our results indicated modest effects of atmospheric CO2 and mycorrhizal inoculation on root production or root loss in pea. However, it is clearly premature to draw general conclusions from a single study. Further experimentation will be needed if we are to make accurate predictions regarding the effects of atmospheric and climatic change on root production and root loss, two crucially important processes affecting the amount of C entering the soil.