The crop's environment can be broken down as follows:
I will use resources to refer to the things plants consume in their growth and reproduction, and environmental conditions to refer to the things, both abiotic and biotic, that influence the rates and efficiencies at which plants capture (or lose) supplies of these resources.
It is axiomatic that crop plants must consume resources to grow and produce a harvestable yield. In most agroecosystems, crop productivity is limited by the availability of one or more required resources, most often nutrients, water, and light. The amount of yield achieved by a crop is a function of both the level of limiting resources available to the crop, and the efficiency with which it uses these resources. This gives rise to the following rather simplistic, mechanistic conceptual model:
Y = f (kiRi)C[(cl, s, m, o, 1)](t)
In words, this model states that the yield of a crop (Y) is determined by the capture of resources (Ri, i = 1 to n) converted to biomass and/or harvestable yield with a characteristic efficiency (ki) by a particular crop or combination of crops (C) growing in an agroecosystem characterized by a particular climate (cl), soil (s), management (m), other organisms in the agroecosystem (o), and characteristics of the landscape (l) in which the agroecosystem is embedded. Both the resources Ri and the factors c, cl, s, m, o, and l interact in complex ways which may result in changes in yield over time (t). Efficiencies is the timing and placement of input resources are part of the management (m) factor.
The mechanistic premise of the model is that productivity (Y) of the agroecosystem depends on the consumption of resources; that no combination of crops and management system can be expected to produce more yield than an alternative combination of crops and management system unless it either captures more resources or uses these resources more efficiently
Low crop yields may be due to (1) low levels of resources, (2) low resource-use efficiencies, or (3) both.
Resource-use efficiencies (RUE) (ki) describe the rates at which crops use resources to produce biomass; RUEs are not constants; rather they are influenced by environmental conditons. Generally, RUEs are the slopes of the lines relating yield to resource levels.
RUEs relate yield to resource supply (Si):
Y = kiSi
However, it is useful to breakdown the RUE (ki) into two components that describe the supply of the resource in the environment (Si) and the uptake of the resource by the crop (Ui):
ki = Y/Si = (Y/Ui)(Ui/Si)
The term (Ui/Si) refers to the efficiency of capturing or taking up resources from the environment and can be called the ecological RUE. Ui/Si is determined by the crop, cropping system, and management system. The other term (Y/Ui) refers to the efficiency of the crop in converting captured resources into biomass or yield; that is, it is the physiological RUE. Y/Ui is determined mostly by the traits of the crop.
The majority of plant scientists believe that the tools of molecular biology and genetic engineering can be applied in various ways to increase the physiological RUE of crops. See Huang et al. (Nature 418: 678, 2002), for example. Increases in harvest index, which involve changes in the crop plant's allocation of energy, represent an improvement in physiological RUE. Plant scientists, however, have not been able to identify potential alterations in other traits that might result in improvements in physiological RUE.
RUEs are scale-dependent; we can define RUEs at organ (for example, leaf), whole plant, or population (crop) levels. Examples to be discussed in class: radiation-use efficiency, nitrogen-use efficiency.
Yield is maximized in the absence of stress; stess may be brought on by:
Here I expand the conceptual model, considering just three resources (perhaps light, water, nitrogen) to include the interaction terms:
Much is known about crop responses to single resources under controlled conditions (first line of expanded model); this is the subject of much formal agronomic research. However, crops seldom if ever respond to just single resources or conditions, and it is the interactions of these environmental factors (second and third lines of expanded model) that accounts for the complexity of ecological systems.
The actual functions can be defined either (1) empirically, that is, using data from experiments and observations, or (2) theoretically, based on the physiological mechanisms involved in resource uptake and use (see, for example, the discussion of water use efficiency in Sinclair et al., 1984). Functions are likely to be nonlinear. Nonlinearities may be due to thresholds-points "beyond which a system's behavior suddenly changes (Meadows et al. 1992). For example, soil depth may have a threshold at crop root depth.
The purpose of this conceptual model is not to suggest that we can write equations to predict crop yield (although crop growth models attempt to do essentially that). Crop plants require many different resources and grow in many different environments, where the resources and environmental factors can interact in very complex ways. The purpose of the model is to acknowledge this complexity, while focusing attention on the resources needed to produce a yield and on the fact that these resources interact in determining yield.
This model is a modification of Jenny and Major's conceptual model for soil and vegetation properties:
S, V = f(cl, o, p, r, t) where p = parent materials, and r = relief (similar to l, landscape factor, above)
We will first look at some of the environmental conditions that influence the way plants capture and use resources, then we will look at the each of the major resources required by plants.
URL: http://ag.arizona.edu/~spmcl/lecturenotes/conceptualyield.html
10 January 2003