Ageing & Senescence Module in WIMOVAC

The ageing and senescence of plant structures is typically associated with a slowing down of metabolic activity, decreasing growth, and often with remobilization of carbon and nitrogen resources from the affected component. Limitation of the functional life of metabolically active parts of a plant by means of genetically programmed ageing is an economy measure ensuring timely transition to a period of dormancy in regions or periods where the growing potential is limited {Larcher, 1995 #2042}. The patterns of ageing in all components is highly variable and species specific. The ageing of leaves, for example, follows different patterns: in some species the leaves turn yellow in succession, in the order that they opened, in others all of the leaves of one period of leaf formation become senescent at the same time. Continuously growing plants including many herbs typify the former type and geophytes the latter. Woody species include examples of every degree of transition between the successive and the simultaneous patterns of leaf senescence. At a mechanistic level the regulation of senescence is thought to be coordinated by phytohormones, chiefly abscisic and jasmonic acids and ethylene but is often also initiated by external stress factors such as short day length or low temperatures. Ageing cells typically show disproportionality in protein metabolism in which the rate of protein breakdown outweighs its synthesis leading to an accumulation of soluble amino compounds, which can be diverted to other sinks in the plant. Up to 60% of the protein of the leaf can be withdrawn for re-use and valuable bio-elements like nitrogen, phosphorus and sulphur are recovered. The stepwise breakdown of leaf proteins to the advantage of the remaining plant parts prior to the completion of the growth cycle is an important component of the material balance of the plant {Larcher, 1995 #2042}.

Organ senescence & death

Although the biochemical processes involved in ageing and decomposition are broadly understood {Matile, 1989 #2044} few detailed mechanistic models of the processes involved currently exist. As a consequence of this wimovac adopts a relatively simple approach in which a table of growth for each plant structural type is maintained (Figure 18).

Ageing and senescence table with an indication of typical thermal times after 1 day and 200 days. Note that stem fine and structural root elapsed thermal times since formation have independent rows in the program array.

The ageing table takes the form of a two dimensional array consisting of 8 rows and 200 columns. Each table entry contains either the dry matter gain for a given plant structure on a given simulation day or the associated elapsed thermal time since the structure was formed. Once daily the central scheduler takes the calculated dry matter gains for the leaf, stem, structural and fine root pools and calculates the leaf area, stem length and root length formed. The ageing and senescence module is then called and table entries moved one column to the right using a variant ‘bubble sort’ algorithym. The information is then placed in the leftmost column of the table. To minimize table manipulation only net positive growth figures are added to the table, zero values are discarded. The rows containing elapsed thermal time since formation information for each of the plant structures are updated each day by adding the cumulative thermal time for the given plant structure for the simulated day. The underlying assumption to using thermal time rather than simple chronological time is that ageing and senescence are temperature sensitive. The rate of ageing being in part determined by the rate of turnover of essential metabolic components such as proteins which at higher temperatures turnover at faster rates.

Once daily the ageing and senescence module checks each entry in the elapsed thermal time rows starting at the rightmost entry. If the tabulated entry exceeds the average thermal time to senescence value set for each plant structure in the model parameter database, the amount of dry matter in the form of leaf area or stem or root length corresponding to that age is ‘retired’ from the table. If the rightmost entry is not sufficiently old to merit retirement no further check of cell entries in the row is required. Retired material is moved out of the living plant structure pools into one of the dead pools which are maintained as state variables in the decomposition sub module of the Century soil model (section 2.327). Retired leaf dry matter becomes surface litter or is reabsorbed for use by other plant structures. Stem dry matter contributes to the standing dead and surface litter pools in a proportion determined by model parameter database settings and retired fine and structural roots become below ground dead material. Storage root and seed material is not included in this analysis.

If any plant material has not reached its required thermal time to senescence, within the 200 calendar days since formation wimovac retains the entry in column 200 and adds to it any subsequent days growth that are moved into column 200 until the elapsed thermal time for the column reaches the required value. At this point all of the material accumulated in the table cell is retired even though some will not be sufficiently old to merit this. The cell entry is then cleared and the thermal time counter for the column reset ready for the following days data. This method slightly overestimates retirement rates for material which is not retired within 200 calendar days and may result in an irregular rate of retirement for this material. It is possible to extend the size of the senescence table to more than 200 columns and so to consequently more than 200 calendar days but this would have calculation/performance implications which make such changes unattractive for the current usage. Further most vegetative tissues senescence is considerably less than 200 calendar days and so the error associated with this approach is minimal.

This simple approach is useful because it is very easy to parameterise the ageing and senescence rates for different plant tissues using experimentally measurable properties. However on its own it would miss a number of important ageing responses observed naturally. Three of the most significant of these are i). The accelerated leaf ageing effects associated with low plant water potentials and discussed in section 2.3262. ii). A gradual decline in metabolic efficiency of leaves with senescence has been noted and may also occur in other plant tissues (section 2.3263). iii). The senescence pattern predicted by the model is not uniform with respect to time, as it is a complex function of formation rates determined by photosynthesis and partitioning, the average age to senescence of plant structures and the thermal conditions for the model. However it is essentially sequential in nature and material dies in the order that it was formed and a sequential algorithym is unlikely to provide an adequate description of the coordinated senescence patterns of some species, particularly geophytes {Larcher, 1995 #2042}.

Water stress ageing accelerator

In many plant systems the onset of low soil water potential conditions in which very little water is available to the plant leads to an accelerated ageing and senescence of leaf material. Water stress induced senescence is usually associated with changes in abscisic acid concentration and may result in remobilization of proteins and other structural components away from the leaves and into the formation of root, stem or reproductive structures {Addicott, 1968 #2045}. Wimovac uses an empirical formulation to simulate this effect in which a multiplier of the elapsed thermal time used in the leaf ageing table is related to the plant water potential.

Thermal time multiplier response to plant water potential (y _plant).

The ageing accelerator function has properties of threshold soil water potential above which there is no effect on ageing and a gradient term which determines the rate of response to decreasing soil water potential (Figure 19). With the default parameterisation supplied by the model parameter database, and shown in Figure 19, leaf ageing occurs at a rate determined simply by the cumulative thermal time when y _plant is in the range 0 to -700kPa and is accelerated in a linear fashion with y _plant values below -700kPa such that at a value of -2000kPa ageing occurs at eight times the rate determined by cumulative thermal time alone. Currently only the leaves are aged in this fashion and the model assumes that the ageing of other plant structures are unaffected by water stress.

Ageing effects on leaf metabolism

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Last modified: August 19, 1997