Ecophysiology and Plant Functioning

Plant growth results from biomass production (photosynthesis) and its partitioning among organs. Both environmental factors and organ functioning are involved. Growth may occur independent from development, that is, organs may still expand when no further organogenesis occurs as is the case for sunflower or chrysanthemum after the terminal inflorescence has been initiated. Studies based on explanatory models linking environmental factors to crop production started in agronomy with C. T. de Wit in the 1960s.

Role of environmental factors

The main driving factors for plant growth are light, temperature, water, and CO2; secondary ones are nitrogen, potassium, and other essential elements. A challenge is to compute the climate effect on growth potential and to find, by inverse problem, the parameters of efficient empirical functions that can assess biomass production in a variable environment. In order to simplify, we consider here the cumulated effect of the environmental factors on plant growth that is relevant for biomass production, rather than the instantaneous one, where things are much more complicated.

Temperature controls the speed of shoot development and the duration of organ expansion (Figure 4a). Within a certain temperature range (i.e., when development rate is linearly related to temperature), there is a linear relationship between the number of phytomers developed on a shoot and the sum of daily effective temperatures received by the plant. This corresponds to the so-called 'temperature sum' factor, and allows the definition of a 'thermal time' that is linearly related to development. It is different from the 'calendar time', with the use of which the observed speed of development (the phyllochron) may be variable depending on temperature.

• Light produces photosynthates via green leaf functioning. Empirically, the effect of incident light is well known. According to light intensity, one can observe a linear effect coming progressively to saturation. Light has also a strong influence on plant plasticity. It can modify plant development by affecting meristems' rules of production. It can also change sink values and organ allometries. In shadow conditions for instance, internodes will have greater biomass and length, and consequently other organs will be reduced.

• Water is taken up by roots from the root environment and evaporates by transpiration at the leaf level. As both transpiration and photosynthesis are strongly influenced by light intensity, often a close relation between crop transpiration and biomass production is observed. Plant transpiration depends primarily on radiation and leaf area. It can be limited by water shortage in the root environment (stomata will close). Cumulated effect of water transpiration at long term is often linearly related with plant biomass production and allows the definition of the so-called 'water-use efficiency' (Figure 4b). It should be noticed that this relationship is not functional but statistical, and it should be used with caution. Under normal conditions, more than 90% of the water withdrawn from the soil will be evaporated out of the leaf surface and only 10% results in fresh mass increase. The water efficiency depends on the plant species. For example, the production of 10 kg of fresh potato requires 6001 of water whereas that of 10 kg of fresh maize cob needs 2501 of water. So the proportion is quite variable according to the plant species, but fortunately it is quite stable for a given cultivated plant in field conditions from year to year. Water stress mainly reduces the growth and has usually little impact on biomass partitioning between shoots and roots.

Plant functioning (respiration)

Plant functioning corresponds to growth and maintenance of the living structure. Taking into account environmental factors, the biomass production at the level of square meters per time unit is summarized by the following equation:

Thermal time (sum of temperatures °C days) Transpiration (kg m 2)

Figure 4 (a) Speed of plant development depending on daily average temperatures (°C days for each phytomer; base temperature is 15 °C). (b) Biomass production depending on plant transpiration. Data for cotton from Guo Yan, Chinese Agricultural University, 2002.

Here, Pg is the gross photosynthesis, Rm is the cost of the structure maintenance, Yg the growth conversion efficiency, and d W/dt the biomass production in the crop per square meter per unit time.

In a stress condition (e.g., soil salinity), the main part of sugars is used to fight against the external salt concentration and the growth process can be very much reduced. To simplify, we consider that plant growth is proportional to the amount of sugars produced.

A common pool of biomass

The branching system ofa plant may be quite complex. Each leaf has its own functioning, in its local environment, and its biomass production has to spread to each organ according to its sink and through the complex network of branches.

This induces us to consider both topological and geometrical structures to ensure the connection between organs and the matter transport. Fortunately, ecophysiol-ogists have proved for many crops that the final balance of the source and sink relationships for a long term is similar to the action of a common pool of biomass that enables us to skip the details of the transport resistance system of the biomass. One can consider that each organ is connected directly and independently from others to a virtual reserve from which it withdraws biomass as a sink or provides biomass as a source (Figures 5 and 6). Direct or indirect proof of this comes from skilled experiments on crops, but in the case of a big structure (trees), this assumption could fail as it has been demonstrated that growth in thickness at a particular point is proportional to the leaf surface seen above this point. However, in most cases, the assumption of a common assimilate pool is valid. In the case of big trees, the actual sink of an organ becomes proportional at the same time to its own strength multiplied by its possibility to access to the sources that is proportional to the leaf surface it 'sees' above its position.

Dry matter partitioning

Keeping the notion of a common pool of biomass, we can skip the study of a complex transport path resistance of the sugars within a complex plant topological and geometrical structure. In this case, we can define the sink strength as the 'potential demand of an organ for biomass accumulation'. Although this demand, So, is absolute and follows usually a bell-shaped curve as a function of organ developmental stage, one may consider that it is relative,

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Figure 5 Cumulative total (closed symbols) and fruit (open symbols) dry weight per plant as a function of day of the year (day 1 = 1 January). Plants were decapitated above cotyledons and had two equal stems originating from the axillary buds of the cotyledons. Tomatoes located on one stem (100-0; triangles) or two stems (50-50; squares). Details are given in Heuvelink E (1995) Dry matter partitioning in a tomato plant: One common assimilate pool? Journal of Experimental Botany 46: 1025-1033.

Figure 6 Growth of a nonphotosynthetic shoot of Hedera helix (ivy) using the common pool of biomass. Here leaves are no more source but only sink organs.

Leaf area index

Figure 7 Light interception of young tomato plants arranged at different plant densities in order to vary LAI. Measurements -three symbols for three different dates/plant sizes. Line represents regression equation y = 1 - e-a83x. Details are given in Heuvelink E (1996) Tomato Growth and Yield: Quantitative Analysis and Synthesis. PhD Thesis, Wageningen University.

Leaf area index

Figure 7 Light interception of young tomato plants arranged at different plant densities in order to vary LAI. Measurements -three symbols for three different dates/plant sizes. Line represents regression equation y = 1 - e-a83x. Details are given in Heuvelink E (1996) Tomato Growth and Yield: Quantitative Analysis and Synthesis. PhD Thesis, Wageningen University.

because it has to be balanced by the sum of the plant sinks S that is the total plant demand. Eventually, the relative sink strength, fo, can be written:

where fo represents the fraction of assimilates partitioned to an organ with sink strength So.

Roles of organs in the plant functioning

Each organ contributes to the plant processes, during its functioning period, in different ways. Leaves ensure light interception and biomass production; stems with their constitutive internodes build the hydraulic plant architecture for the water transport from the root to the leaves and for transport of assimilates from sources to sinks. All organs (even those that can also be sources like leaves) are sinks during their expansion and are thus involved in biomass partitioning.

Role of leaves

Crop light interception is no more proportional to leaf area when the canopy is dense. In homogeneous conditions (that is to say most cases), a crop can be considered as a turbid environment and light interception is well described by Lambert-Beer law (Figure 7). At the stand level, the number of leaves overlapping is controlled by a simple parameter, the leaf area index (LAI = leaf area per square meter), and the fraction of intercepted light (FIL) is deduced from the relationship

The light extinction coefficient k depends on leaf orientation and reflectance and transparency, that can be assessed by measurements inside a crop. Biomass production can be calculated as the product of FIL, incident light (PAR, photo-synthetically active radiation), and light-use efficiency (LUE; gMJ-1 PAR). LUE is a robust parameter, and its value is species dependent and also prone to environmental influences (e.g., CO2 concentration), but often a value between 2 and 3 g MJ- PAR is reported. For low planting densities, the biomass production per unit of ground area is proportional to the LAI (and the number of plants). At highest densities, the production per square meter of ground area becomes independent of the plant density (Figure 8).

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Figure 8 Effect of density on plant weight and biomass production per square meter for Chrysanthemum. The individual weight is lower at higher planting density, but the biomass production per square meter per day is independent of density, once the LAI reaches a high value. From Lee J-H, Heuvelink E, and Ortega L (unpublished) (Wageningen University).

The photosynthetic active functioning duration of a leafis limited. For a plant, the highest resistance to water transpiration is located at the leafsurface level, where the water changes from liquid to vapor phase. For a 10-year-old poplar tree, the leaf area represents 95% of the resistance to transpiration monitored by the hydric potential between soil and atmosphere.

The ratio between the leaf weight and the leaf surface is called specific leaf weight (SLW). A low value means thin leaves and this favors in a young crop the fast buildup of light-intercepting capacity and hence growth.

Role of stems and their constitutive internodes

Beyond the mechanical role that ensures plant stability, stems play a functional role for transport of water and assimilates and form a hydraulic network. Resistance to water flow of the hydraulic architecture for the above-mentioned 10-year-old poplar is low and assessed to be only 3% of the total resistance. For most crops it may thus be considered as quite negligible. For mature trees it can increase and consequently reduce water transpiration and photosynthesis. Stems in woody plants have secondary growth that increases their diameters. Eventually, stems contained a pith and a stack ofrings. The pith has a variable sink linked to the phytomer; meanwhile, the rings belong to the whole plant architecture and can be considered as a single big sink for biomass that is always in expansion.

Role of fruits

Fruits (reproductive organs) are sinks during their whole lifetime. They are often the strongest sinks on a plant and can reduce dramatically the expansion ofthe other organs such as in sunflower (the inflorescence) and maize (the cob). They have no significant influence on the biomass production that depends on the LAI. However, an indirect influence occurs when leaf area development is drastically reduced because of the strong sink capacity of the reproductive organs.

The place where fruits can occur in the plant structure depends on the plant species; for example, in a sweet pepper plant typically there is a flower in every leaf axil, whereas a tomato plant produces a truss after every three leaves and internodes. Whether a flower turns into a fruit or not depends on the assimilate status of the plant (source/sink ratio) but besides assimilate availability, hormonal regulation also plays a separate role.

Role of roots

The root system is seldom accessible to measurements. The weight ratio shoot/root is supposed to be constant by default. In the case of loose soil where the roots can find their way, the root system can be considered as a single sink. For the above-mentioned 10-year-old poplar tree, its resistance to water transport is considered negligible and assessed at about 2%.

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