AI/^ = ZEE
Chapter 3: Resource Balance in TILLERTREE
1.1 Concepts
1.1.1 Maintenance respiration and growth respiration
Non-structural carbohydrate provides the energy used for synthesis of new structures and renewal of existing structures. Therefore total respiration can be divided into two components: growth respiration is that respiration associated with the synthesis of new phytomass and maintenance respiration supplies energy to keep existing phytomass in a
healthy state (Amthor 1989). In general it is assumed that maintenance respiration takes priority over growth when soil water conditions are unfavourable for carbohydrate assimilation and cell expansion. However at a sub-cellular level, the two respiratory components cannot be separated since ATP energy for both components is derived from the same process: oxidation of glucose by the mitochondria. McCree (1982) measured maintenance coefficients of white clover at different growth rates. The coefficients varied from 2.4 mg C02 (g DM)"1 h"1 down to 1.4 mg C02 (g DM)"1 h"l at zero growth, but the estimated value of growth conversion remained constant at 0.67 (720 mg CO2 (g DM)"1 grown). In other words maintenance respiration did decline as growth declined. This probably reflects the rate of protein turnover that is associated with cellular division and growth (Amthor 1989). Moser et al. (1982) found that tall fescue meristems can continue to grow for up to 16 days of darkness, providing additional evidence which suggests that maintenance respiration does not take full priority over growth. It would appear this is some form of sub-cellular control that regulates priority to maintenance processes over production of new structures, since maintenance is less affected by adverse environmental conditions such as water stress than growth (Wilson et al. 1980).
1.1.2 Reserve non-structural carbohydrates
Non-structural carbohydrate is divided into two components: ethanol-soluble material, which includes glucose, fructose and sucrose, and ethanol-non-soluble material, fructan and starch (Farrar & Williams 1991). Farrar & Williams (1991), working on non-structural carbohydrate concentration in roots of young barley plants, observed that total non- structural carbohydrate was about 70 - 100 mg (g DM)"1, of which the ethanol solubles are half. They further noted that the soluble sugars were unevenly distributed in cellular tissues and that about 60 % of these sugars were compartmented into vacuoles. Farrar & Williams (1991) observed fluxes of sucrose into the root of 14 mg (g DM)"1 h"1, and that the vacuolar loading and unloading rate of soluble carbohydrate in response to shoot defoliation was fairly rapid at 5 mg (g DM)"1 h" . By contrast there was little or only a slow change in the production or breakdown of starch polymers at 1.4 - 2.0 mg (g DM)"1 h . Douce et al.
(1991) examined the autophagy in sycamore (Acer pseudoplatanus) cells subjected to sucrose deprivation. Notably starch mobilization only occurred after 10 hours of sucrose starvation, during which time the cells consumed most of the available soluble
carbohydrates. Therefore it is apparent that carbohydrates can be divided into two components, namely stored carbohydrates (starch and fructan), which are less available, and soluble carbohydrates (glucose, sucrose and fructose), which are more freely available for use in both growth and maintenance respiration.
The traditional role of stored carbohydrates in plant recovery following defoliation has been questioned (e.g. Wolfson 1999). This is because numerous researchers found a lack of clear correlation between non-structural carbohydrate concentrations (NSC, the traditionally perceived form of carbohydrate reserves in grass plants) and plant regrowth rates following defoliation. Richards & Caldwell (1985) suggested three reasons for this:
1. The contribution of photosynthesis from residual leaf material to regrowth is large relative to that of reserve substrates.
2. Non-structural carbohydrate reserves, as measured by traditional methods, do not adequately represent the available substrates for regrowth.
3. Meristematic restrictions resulting from the defoliation may limit regrowth more substantially than energy reserves.
Penning de Vries et al. (1989) note that much of the protein-rich cytosolic components are re-absorbed as plant tissues die, but none of the carbohydrate structures such as the cell walls are broken down. They estimated that about half of the cellular biomass of leaves may be re-absorbed, although they give no empirical backing for this estimate. The present model includes the ability to re-allocate carbohydrate as plant tissues senesce to account for this carbohydrate gain. It has been shown that grass species are differentially susceptible to defoliation because of their ontogenetic growth characteristics (Bridgens 1968; Caldwell et al. 1981;Coughenoure?a/. 1985).
The major storage region in grasses is the stem base, including underground stems in rhizomatous grass species (Wolfson & Tainton 1999), but stems and sheaths also contain major reserves of carbohydrates in some grass species (Richards 1984, Danckwerts &
Gordon 1990). Roots have also been linked to carbohydrate storage in grasses (Nursery 1971).
The overall impression is that stored carbohydrate reserves are small and are used to initiate regrowth at any point in the season (Danckwerts 1984; Farrar & Williams 1991). As soon as active photosynthetic surface has been created, utilisation of stored carbohydrate
reserves begins to decline rapidly and soon switches to carbohydrate storage as supply rate exceeds demand rate (Farrar, Pollock & Gallagher 2000).
1.1.3 Seasonal patterns of reserve carbohydrate storage
Grasses require carbohydrate storage for two purposes: to provide energy for regrowth after defoliation events and to tide them over when environmental conditions are unfavourable for photosynthesis and growth. Tufted grasses of the Southern Tall Grassveld (Acocks 1953) apparently have a growing season from spring though to autumn, and a non-growing season during the unfavourable environmental conditions that prevail over winter every year. During the latter period, the grasses are unable to grow or photosynthesise appreciably because the temperatures are too cool and the soil is too dry for growth. Maintenance respiration continues at a much-reduced rate.
Reserve carbohydrate storage studies on native South African grasses in humid grasslands (Weinmann 1947; Weinmann 1948) and semi-arid grasslands (Danckwerts 1984) recorded maximum carbohydrate storage in mid-winter, strong depletion in spring and early summer and accumulation in autumn. Carbohydrate storage is generally at most about 10 % by plant tissue mass (Bartholomew 1968; Steinke & Booysen 1968; Danckwerts 1984;
Farrar & Williams 1991).
The seasonal patterns of carbohydrate storage may reflect some seasonal strategy, but this explanation seems doubtful. An alternative explanation can be provided: there is a strong negative relationship between protein concentration and NSC concentration (e.g.
Van Herwaarden, Richards & Angus 2003). Protein content reflects nitrogen (N) content, so it is probable that a plant's ability to use available carbohydrates is based on the availability of other resources. Consequently when nitrogen content is low, NSC is high and vice versa.
In a subsequent section, nitrogen will be discussed more fully. However at this stage, it is pertinent to point out that nitrogen availability from the soil is highly seasonal and has rapid turnover periods (Blair et al. 1998), and is dependent on wetting and drying cycles (Birch 1958). Soil available nitrogen is high during early spring because it follows the longest period of low growth activity in a season.