A Co-ordinated Demand-and-Supply-Led Root-to-Shoot Partitioning Model
3.1 Carbohydrate restriction, no nitrogen restriction
3.1.2 Comparison of the two co-ordinated partitioning models with the fixed partitioning model
Second season growth data, as presented in Table 6.1 below, indicates that plant growth during the second spring was improved by the two co-ordination models over the fixed model, with the dual demand-and-supply-led partitioning model accumulating greater mass than the demand-led partitioning model.
The improvement of the performance of the two co-ordination models over the performance of the fixed model was achieved because a greater portion of the growth resources were allocated to shoot growth than to root growth during early spring, which improved the growth of photosynthetic structures. This increased the photosynthetic machinery of the single-tiller ramets of the two co-ordination models, which in turn increased the photosynthetic production later in the second season, when roots received a greater portion of the available carbohydrate thereby increasing overall plant growth (Figure 6.4).
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101 201 301 401 Iteration (days)
501
Figure 6.1 The effect of carbohydrate storage restriction on the daily available non- structural carbohydrate (AVC) of a single-tiller ramet subjected to a late winter mow of height 8 cm on day 360. Resource partitioning to roots and shoots is fixed. Nitrogen is non-limiting.
(Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
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201 301 401 Iteration (days)
Figure 6.2 The effect of carbohydrate storage restriction on the daily carbohydrate demanded ( ) and supplied ( ) for growth of a single-tiller ramet subjected to a late winter mow of height 8 cm on day 360. Resource partitioning to roots and shoots is fixed.
Nitrogen is non-limiting. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
101 201 301 401 Iteration (days)
501
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mass 9Aj
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0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
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301 401 Iteration (days)
Figure 6.3 Comparison of summed live mass of shoot organs of a single-tiller ramet without carbohydrate restriction (a) and with restriction on carbohydrate storage (b). The ramet has been subjected to a late winter mow of height 8 cm on day 360. Resource partitioning to roots and shoots is fixed. Nitrogen is non-limiting. Summed live mass of organ types ( blade mass;
sheath mass; internode mass; flower mass) of a ramet with a single tiller subjected to a cutting treatment. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
101 201 301 401 Iteration (days)
5 0 1
a
5
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101 201 301 401
Iteration (days)
5 0 1
3
2 a
1 0 1 201 301 401
Iteration (days)
5 0 1
Figure 6.4 Live root mass ( ) and live shoot mass ( ) of a single-tiller ramet with restricted AVC storage subjected to a late winter mow, for the three partitioning models, a.
Fixed partitioning; b. Demand-led partitioning c. Demand-and-resource-led partitioning.
(Defoliation: height 8 cm on day 360.) Nitrogen is non-limiting. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4.
Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
Table 6.1 The maximum live mass of organs and shoots and roots generated during the reproductive phase by each of the partitioning models
Component
Shoot (g) Root (g) Blade (g) Sheath (g) Intenode (g) Inflorescence (g) Culm length (mm)
Unrestricted growth*
0.690 0.786 0.115 0.062 0.421 0.092 794.58
Fixed
0.383 0.438 0.049 0.016 0.277 0.033 345.68
Partitioning model Demand-led
0.500 0.570 0.070 0.031 0.341 0.049 520.85
Demand-and-supply- led
0.647 0.664 0.083 0.040 0.441 0.083 737.77
* potential growth of single-tiller ramet using a fixed partitioning function, without resource limitation
The imbalance functions and the ideal and actual root-to-shoot ratios generated by each partitioning model are shown (Figures 6.5 and 6.6). The imbalance function of the fixed partitioning model of course does not affect partitioning, but it does help illustrate the effect of the two co-ordination models. The imbalance function of the fixed partitioning model is continuously positively skewed during the growth period in the second season indicating that carbohydrate is continuously limited during this time in this model. By contrast the demand-led partitioning model has many days where carbohydrate is not limiting to growth, indicating that the feedback reduction on root growth demand has a beneficial effect by reducing the integral of daily imbalance in spring. The dual demand-and-supply-led partitioning model has a longer period of continuous resource imbalance which results from the daily allocation of proportionately more resources to shoots on the basis of the resource imbalance. This reduces the actual root-to-shoot ratio further below the ideal root-to-shoot ratio than occurs with the demand-led partitioning model. However this strategy increases the blade growth in early spring over the demand-led partitioning model which ensures more photosynthesis for greater shoot and root growth later in the spring.
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a
101 201 301 401 501
Iteration (days)
101 2 0 1 301 401 501
Iteration (days)
101 201 301 401 5 0 1
Iteration (days)
Figure 6.5 Response of the imbalance function to restriction on AVC storage of a single- tiller ramet subjected to a late winter mow, for the three partitioning models, a. Fixed partitioning;
b. Demand-led partitioning c. Demand-and-resource-led partitioning. (Defoliation: height 8 cm on day 360.) Nitrogen is non-limiting. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
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Iteration ( d a y s )
5 0 1
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1 0 1 2 0 1 301 4 0 1
Iteration ( d a y s )
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0
1 0 1 2 0 1 3 0 1 4 0 1
Iteration ( d a y s )
5 0 1
Figure 6.6 Response of the actual R:S ratio (- alrlshy5t) and ideal R:S ratio (•
QalrlshY5)t) to restriction on AVC storage of a single-tiller ramet subjected to a late winter mow, for the three partitioning models, a. Fixed partitioning; b. Demand-led partitioning c. Demand- and-resource-led partitioning. (Defoliation: height 8 cm on day 360.) Nitrogen is non-limiting.
(Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year.)
All three models are able to achieve non-restricted growth before the end of the reproductive growth phase. The fixed partitioning model achieves this first, followed by the demand-led partitioning model and then the dual demand-and-supply-led partitioning model respectively.
Immediately post the start of spring regrowth, both roots and shoots grow for a few days under all three partitioning strategies (Figure 6.4). This indicates that the release of carbohydrate from stored sources ensures that there is no imbalance in the resources at this time. This is because the carbohydrate storage function releases up to 67 % of its resources into AVC in a single day. Therefore the available carbohydrate for growth at the start of spring exceeds daily demands. This increased concentration is probably not realistic, and the net effect is that some carbohydrate is allocated to roots by the imbalance function that could potentially be better utilised in shoots if the amount of carbohydrate released from storage was slower.