Discussion

Plant growth in response to CO 2 enrichment with two different Rhizobium inoculation treatments

2.4 Discussion

2.3.5 Root:shoot ratios

Rootshoot ratios were greatest in plants grown without rhizobial inoculation in both CO₂ treatments, and were, on average,higher in ambient CO₂treatments than elevated CO₂ treatments for both species (Table 2.3).

al. 1996). It is possible that CO₂ enrichment will alter the quality of organic matter and thereby alter the rates at which carbon and nitrogen are cycled in plant systems (BAZZAZ 1990,HENNING et al. 1996).

REEKIE and BAZZAZ (1989),ARNONE (1996) and WAND et al. (1996) concluded that the success of a species was positively related to its mean canopy height and that leaf and canopy morphological changes could alter patterns of resource availability. Plant height, and mass of the plants in this study increased in both species when they were exposed to elevated CO₂ and rhizobia I inoculation,this was a trend-like increase, and was only highly significant inA. ni/otica, and in inoculated plants. The increase in leaf area caused by elevated CO₂ was not significant in A. sieberana (Table 2.2) and decreased in inoculated plants. Rhizobial inoculation contributed significantly to growth and subsequent increases in mass and height. VOLlN and REICH (1996) found that photosynthetic rates were significantly greater for plants grown with high nitrogen compared to those grown in low nitrogen. MITCHELL et al. (1995) found that increased nitrogen and CO₂ supplies and the interaction of these two factors markedly increased respiration rate. However, nitrogen supply had a greater effect on both respiration and growth than did atmospheric CO₂ concentration (GRIFFENet a/.1993,MITCHELLet al.1995) .

Differential shoot morphological response to increased CO₂ (and nitrogen if inoculated) such as changes in leaf area, branching and tillering may lead to changes in canopy architecture. This may alter the species competitive balance by modifying foliar light interception patterns (BAZZAZ and MCCONNAUGHY 1992;Luoand MOONEY 1995) which may be particularly important during seedling establishment (WAND et al. 1996). Rootshoot ratio (RSR) is commonly used to assess compensatory changes in root growth characteristics in response to CO2enrichment. RSRs are often (DAY et al. 1996) but not always increased under elevated CO_{2 ,} This ratio is an important index of compensatory changes in carbon allocation (and a useful overall measure of treatment response) and a poor index of plant potential for nutrient acquisition (POORTER 1993; BASSIRIRAD et al.

1996). In this study RSRs were greatest in plants grown without rhizobial inoculation in both CO2 treatments (Table 2.3). RSRs were also higher under ambient CO

2 when

compared to elevated CO2 treatments in both species. Allocation of photoassimilates between shoots and roots is partly determined by genetics, but also changes adaptively, with greater allocation to roots under nutrient and water stress (NORBYetal. 1986; KORNER

and ARNONE 1992; HUNT et al. 1996). The absence of rhizobial inoculation and CO2

enrichment seemed to have a similar 'Iow nutrient' effect on the growth of both species.

This effect was comparatively equally pronounced in both species, suggesting that both species will fare equally well under conditions of nutrient or water stress,regardless of the CO₂ concentration, if their root systems can grow extensively, and if plants are inoculated with root nodule bacteria. If temperature and water status are unchanged, an increase in CO₂ might increase RSR by way of the plant's adaptive response to decreasing carbon limitation relative to nutrient limitation. However, elevated CO₂ may also improve plant water status thereby affecting RSR (HUNT et al. 1996). Rhizobial inoculation will become especially important in determining a plant's success under nutrient stress if the RSR is affected significantly.

In A. nilotica elevated CO₂ and inoculation increased branching. Enhanced numbers of specific parts (stems,branches, tillers and flowers) have often been reported in response to elevated CO₂ concentration (ROGERS and DAHLMAN 1993). The small differences between A.sieberana treatments with respect to branchingconfound the results.Although there were more branches on plants grown without rhizobial nodules, the reason for this .is unclear. Other studies have found that some plants grown under elevated CO₂ conditions are not able to convert additional photosynthate into increased growth. For these plants no changes in total biomass,accumulations of non-structural carbohydrates in leaves, leaf discolouration, and increased below ground carbon occurred (DIAZ et al.

1993,cited in REYNOLDS 1996). These studies were however,usually conducted under low fertilityconditions.Species with nitrogen-fixing abilityor mycorrhizae may be able to avoid competition for soil nutrients,and demonstrate potential growth responses to elevated CO₂ (REYNOLDS 1996). In the present study this was demonstrated where inoculated plants showed greater increases in mass,and height than uninoculated plants.

REEKIE and BAZZAZ (1989) found highly significant shifts in the contributions of individual tree species to community above ground biomass with increasing CO₂ concentration. These shifts occurred even though the CO₂level had no effect on above ground biomass or on leaf area index.Thus,it may be assumed that even greater changes in the allocation of carbon to various plant parts will occur in plants such as Acacias which show obvious changes under experimental conditions. These increases in carbon allocation will inevitably lead to changes in light interception, nutrient acquisition, species composition

and plant distribution.Since CO2affects a wide range of plant functions both directly and indirectly, predicting the direction, much less the magnitude,of changes in plant function and community structure is difficult. This is because there is a lack of understanding of the mechanisms that control overall plant response to CO₂ concentration (MITCHELL et al.

1995).

Shifts may also occur in species composition in favour of nitrogen-fixing species. This suggests that when species are differentially infected, nitrogen-fixing ability will be important in understanding shifts in species composition in response to elevated CO_2, even when soil fertility is too low to support a community level response (REYNOLDS 1996).

As a demonstration of this phenomenon, POORTER (1993) reported for data compiled from 106 species, an averageCO2induced mass increase of 41%, which was increased to 50%

in the case of nitrogen-fixing species. Evidently nitrogen fixing species will be favoured in aCO2 enriched world,and the results obtained in this study confirm this supposition.