Theoretical production ecology tries to quantatively study the growth of crops. Main parameters kept into consideration are temperature, sunlight, standing crop biomass, plant production distribution, nutrient and water supply.
Modelling
Modelling is essential in theoretical production ecology. Unit of modelling usually is the crop, the assembly of plants per standard surface unit. Analysis results for an individual plant are generalised to the standard surface, e.g. the Leaf Area Index is the generalised surface of all crop leafs per surface unit.
Plant model
Basic production model used in SUCROS
The usual system of describing plant production divides the plant production process into four separate processes, which are influenced by several external parameters. Two cycles of biochemical reactions form the basis of plant production, the light reaction and the dark reaction.
- In the light reaction, sunlight photons are trapped in chloroplasts which split water into an electron, proton and oxygen radical which is recombined with another radical and released as molecular oxygen. The recombination of the electron with the proton yields the energy carriers NADH and ATP. The rate of this reaction depends on sunlight intensity, leaf area index, leaf configuration and amount of chloroplasts per leaf surface unit.
- The dark reaction or Calvin cycle ties atmospherical carbon dioxide and uses NADH and ATP to convert it into glucose. The available NADH and ATP, as well as temperature and carbon dioxide levels determine the rate of this reaction.
In order to achieve optimum plant production, this cycles of plant production must complement each other. Together those two reactions are termed photosynthesis. The rate of this reaction depends on temperature and availability of NADH, ATP and carbon dioxide.
- The produced glucose is transported to other plant parts, such as storage organs and converted into secondary products, such as amino acids, lipids, cellulose and other chemicals needed by the plant or used for respiration. Lipids, sugars, cellulose and starch can be produced without extra elements. The conversion of glucose into amino acids and nucleic acids requires nitrogen, phosphorus and sulphur. Chlorophyll production requires magnesium, while several enzymes and coenzymes require trace elements. This means, nutrient supply influences this part of the production chain. Water supply is essential for transport, hence limits this too.
- The production centers, i.e. the leafs, are sources, the storage organs, growth tips or other destinations for the photosynthetic production are sinks. The lack of sinks can be a limiting factor for production too, as happens e.g. in apple orchards where insects or night frost have destroyed the blossoms and the produced assimilates cannot be converted into apples. Biannual and perennial plants employ the stored starch and fats in their storage organs to produce new leafs and shoots the next year.
- The amount of crop biomass and the relative distribution of biomass over leafs, stems, roots and storage organs determines the respiration rate. The amount of biomass in leafs determines the leaf area index, which is important in calculating the gross photosynthetic production.
- extensions to this basic model can include insect and pest damage, intercropping, climatical changes etcetera.
Important parameters in theoretical production models thus are:
Temperature
The temperature determines the speed of respiration and the dark reaction. A high temperature combined with a low intensity of sunlight means a high loss by respiration. A low temperature combined with a high intensity of sunlight means that NADH and ATP heap up but cannot be converted into glucose because the dark reaction cannot process them swiftly enough.
Sunlight
Sunlight is the energy source for plant growth. Sunlight powers the light reaction, which converts carbon dioxide and water into glucose and molecular oxygen. When temperature, carbon dioxide and nutrient levels are optimal, light intensity determines maximum production level.
Carbon dioxide levels
Atmospherical carbon dioxide is the carbon source for plants. About half of all proteins in green leaves have the sole purpose of capturing carbon dioxide. Although CO2 levels are constant under natural circumstances, CO2 fertilization is common in greenhouses and is known to increase yields by on average 24% [1]. C4 plants like maize and sorghum can achieve a higher yield at high solar radiation intensities, because they prevent the leaking of captured carbon dioxide because of the spatial separation of carbon dioxide capture and carbon dioxide use in the dark reaction. This means that their photorespiration is almost zero. This advantage is often offset by a higher maintenance respiration level.
Standing crop biomass
Unlimited growth is an exponential process, which means that the amount of biomass determines the production. Because an increased biomass implies higher respiration per surface unit and a limited increase in intercepted light, crop growth is a sigmoid function of crop biomass.
Plant production distribution
Usually only a fraction of the total plant biomass consists of useful products, e.g. the seeds in pulses and cereals, the tubers in potato and cassava, the leafs in sisal and spinach etc. The yield of usable plant portions will increase when the plant allocates more nutrients to this parts, e.g. the high yielding varieties of wheat and rice allocate 40% of their biomass into wheat and rice grains, while the traditional varieties achieve only 20%, thus doubling the effective yield. Different plant organs have a different respiration rate, e.g. a young leaf has a much higher metabolism rate than roots, storage tissues or stems.
Water supply
Because plants use passive transport to transfer water and nutrients from their roots to the leafs, water supply is essential to growth, even so that water effiency rates are known for different crops, e.g. 5000 for sugar cane, meaning that each kilogram of produced sugar requires up to 5000 liter of water.
Nutrient supply
Nutrient supply has a twofold effect on plant growth. A limitation in nutrient supply will limit biomass production as per Liebig's Law of the Minimum. With some crops, several nutrients influence the distribution of plant products in the plants. A nitrogen gift is known to stimulate leaf growth and therefore can work adversely on the yield of crops which are accumulating photosynthesis products in storage organs, such as ripening cereals or fruit-bearing fruit trees.
Plant production models
Plant production models exist in varying levels of scope (cell, physiological, individual plant, crop, geographical region, global) and of generality: the model can be crop-specific or be more generally applicable. In this section the emphasis will be on crop-level based models as the crop is the main area of interest from an agronomical point of view.
As of 2005, several crop production models are in use. The crop growth model SUCROS has been developed during more than 20 years and is based on earlier models. Its latest revision known dates from 1997. The IRRI and Wageningen University more recently developed the rice growth model ORYZA2000. This model is used for modeling rice growth. Both crop growth models are open source. Other more crop-specific plant growth models exist as well.
SUCROS
SUCROS is programmed in the Fortran computer programming language. The model can and has been applied to a variety of weather regimes and crops. Because the source code of Sucros is open source, the model is open to modifications of users with FORTRAN programming experience. The official maintained version of SUCROS comes into two flavours: SUCROS I, which has non-inhibited unlimited crop growth (which means that only solar radiation and temperature determine growth) and SUCROS II, in which crop growth is limited only by water shortage.
ORYZA2000
The ORYZA2000 rice growth model has been developed at the IRRI in cooperation with Wageningen University. This model, too, is programmed in FORTRAN. The scope of this model is limited to rice, which is the main food crop for Asia.
Other models
The United States Department of Agriculture has sponsored a number of applicable crop growth models for various major US crops, such as cotton, soy bean, wheat and rice. [2] Other widely-used models are the precursor of SUCROS (SWATR), CERES, several incarnations of PLANTGRO, SUBSTOR, the FAO-sponsored CROPWAT, AGWATER and the erosion-specific model EPIC. [3]
External resources
- Wageningen University, Netherlands, Department of Theoretical production Ecology
- Leuven University, Belgium, Department of Theoretical Production Ecology
- SUCROS manual
- SUCROS download page
- ORYZA2000 project page
- ORYZA2000 download page
- Summary page with US government sponsored crop growth models