Senescence

AI/^ = ZEE

Chapter 3: Resource Balance in TILLERTREE

2.2 The nitrogen balance model

2.2.3 Senescence

Senescence of leaf material (sheaths and blades), stem material and root material provides recycled nitrogen, which is returned to the AVN. The inflorescence is a sink for nitrogen that is entirely lost to the plant. The average estimate for nitrogen re-allocation (RNdiebacki_j0) from senescent leaves in graminoids is 0.585 g g"¹ N (Aerts 1996).

Ndiebackyt - ^T J ] RNdiebackio * aNgot * dwloany t + RNdiebackj r * aNgrt * dwlry t . P a

3.22

3 ALGORITHM FOR RESOURCE ALLOCATION

This section describes the procedural algorithm for resource movements within all objects on a bunchgrass clone during a single iteration.

1. Firstly carbohydrate and nitrogen additions from photosynthesis, soil nitrogen absorption, storage and senescence are added to the AVC and AVN pools of each ramet respectively. Secondly the demands for carbohydrate and nitrogen for growth, maintenance and storage in individual organs are passed to each ramet.

2. Following this all carbon and nitrogen in individual ramet AVCs and AVNs and total demands (CDem_y,5_(t) are passed to their parent ramet group. The ramet group supplies resources back to the ramets in proportion to their demands (CSupDem6,t). If there are excess resources in the ramet group after the demands have been supplied (CExcess5_>t) then the remaining carbohydrate in the ramet group is allocated back to the ramets in proportion with their shoot masses. Therefore the net change in carbohydrate in the AVC of a given ramet on a given day (CRpool_T)5_it) is equal to the difference between the amount of carbohydrate that has been supplied back to the ramet by the ramet group and the amount in the AVC before it was passed up to the ramet group.

CDem_y § _t wly_{y s t}

CSupDemgj — \- CExcessg_t — AVC_y§_t.

Yj

^CDem

r,S,t Z

^{w /}

? V ^ r r

3.23 The procedure is identical for nitrogen allocation to idnvidual ramets in the ramet group.

3. After resources have been reassigned to individual ramets, allocation to organs proceeds in the following manner: First carbohydrate maintenance demands are supplied, then carbohydrate storage demands, and then growth demands are supplied. The separation and prioritization of maintenance over growth allocation is a reasonable assumption for models and is generally used (Coughenour 1984; Sequeira et al. 1991). Insertion of allocation to storage before growth ensures more conservative behaviour. This model assumption may be reviewed in future research because there is evidence that reserve carbohydrate storage is inversely related to the nitrogen status of a plant (Oparka et al.

1986), and therefore of lower priority than growth.

CRpool_r5j

4. If there is insufficient carbohydrate in an iteration to satisfy the maintenance demand, the ramet is forced to dieback leaf and root material to satisfy the respective deficits of shoots and roots. Leaf material is senesced from the distal end, in identical fashion to programmed senescence. This dieback proceeds at the maximum dieback rate, and begins on the oldest live leaves first. Root dieback also proceeds at the maximum rate.

5. Allocation to growth within ramets is limited by the more restricted resource of nitrogen and carbohydrate. This is the point of interaction between the two models, and reflects the stoichiometric relationship between the growth requirements for the two resources.

CDemGrow_yt if [(A VN_Yjt > NDemGrow_yt) n {A VC_yJ > CDemGrow_yt)],

Cgrow_/t =

CDemGro\Vy_t AVN_Yt

AVC r,s,t

NDemGroWy _t

if [(A VNyj < NDemGrowyt) n (A VCy5t > CDemGrowyt)], if [(A VNyj > NDemGrowyt) n (A VC_y5t < CDemGrow_yt)],

CDemGroWyf min AVN_rt AVCy_t NDemGroWy, CDemGrow_v ,

ft¹ if¹

otherwise.

3.24 6. Once total resource-limited growth carbohydrate has been calculated for each ramet, this

is divided between the root and shoot on that ramet. Subsequently, the resource assigned to each ramet shoot is divided among the connected tillers, and then divided among the phytomers on each tiller. Finally the growth resource assigned to each phytomer is allocated to its component organs.

7. Secondary tiller recruitment is attempted on the basis of resource availability (described subsequently) after primary growth allocation.

8. Finally at the end of the iteration, nitrogen is allocated to storage, and if carbohydrate in AVC exceeds the maximum allowable carbohydrate concentration, the excess is converted to secondary metabolite and removed from the system.

4 TILLER MORTALITY

Tiller mortality can be classed into two types. A grass tiller may die after completing its life-cycle (flowering and seeding), as often occurs in annual species. This is termed programmed senescence. Alternatively a tiller may be killed prematurely by unfavourable environmental conditions. This is termed premature termination. During favourable environmental conditions, an individual tiller of a perennial grass species does not necessarily die on completion of seed set. If it is linking other live tillers to a root system, then the stem portion of that tiller will continue to live as long as its dependent tillers are still alive. It is probable that there is some ageing process which limits the lifespan of individual tillers (Everson, Everson & Tainton 1985), perhaps based on some reduced efficiency of the conduits in the stem and roots. However this process is not understood and it is not considered.

In the model tiller death is induced by resource carbohydrate starvation combined with an inability to overcome this resource deficit in the near future. This is characterised by both a lack of sufficient stored reserves and a photosynthetic rate that is insufficient to supply maintenance demands and is therefore unable to supply any carbohydrate for growth.

Since maintenance demands are determined by the size of the plant, induced dieback of the plant could provide a twofold benefit to overcome resource restriction, by firstly reducing the maintenance demand and secondly providing some additional non-structural carbohydrate from cellular deconstruction. Therefore a size constraint must be placed over plant death that ensures that tillers have died back substantially before they are terminated.

These rules are sufficient to cover both premature termination and programmed senescence.

Tillers and even whole tufts are often killed during extended drought periods (Danckwerts & Stuart-Hill 1988; O'Connor 1994). However grass tillers appear to survive dry conditions during winter in the humid grasslands of Natal (Tainton & Booysen 1965).

The reason for this dichotomy may be explained in the following manner. Most C4 grasses lack true dormancy. Rather tiller growth rate and photosynthesis are restricted by environmental limitation due to both ambient temperatures and soil water pressure.

Importantly chemical reaction rate is mainly determined by temperature. This means that maintenance demands during dry periods may be relatively large if temperatures are high even if there is no potential for growth and no potential to photosynthesise, thereby depleting stored reserves rapidly. In contrast low temperatures in winter slow chemical reaction rates almost to zero, thus slowing resource depletion substantially. Therefore water

stress induces carbohydrate exhaustion of tillers when ambient temperatures are high.

Consequently water stress is not included directly in the rules for mortality.