46
Computer Simulation of Plant and Crop Allocation
Processes
Donna M. Dubay and Monica A. Madore
University of California, Riverside, California
909I. INTRODUCTION
Plant growth is balanced between growth above ground (shoot growth) and below ground (root growth).
This balance provides adequate surface area to intercept light for photosynthesis as well as sufficient area
to acquire needed water and nutrients in the soil. In this way, the requirements for carbon (C) and mineral
nutrients such as nitrogen (N) within the plant are satisfied to maximize plant growth or productivity.
Each plant species appears to have a preferred balance, which is apparently genetically controlled but
which can be altered by environmental conditions. For example, if a needed resource such as water or a
nutrient is limiting, a plant often tends to grow a proportionally greater amount of root [1,2]. This gives
the plant more surface area within the soil to absorb the limited supply of resources and also to search out
pockets of resources deeper within the soil. In a similar sense, if the shoot is exposed to a limit in a re-
source (most commonly light, which is necessary to acquire C through photosynthesis), growth will shift
to favor shoot growth over root growth. This supplies the plant with greater surface area to intercept light
and absorb carbon dioxide and brings balance back to the plant.
Although these growth responses have been repeatedly demonstrated experimentally, no physiolog-
ical mechanism has yet been found that regulates these processes. In fact, how the balance of root and
shoot growth in a plant is attained even without a limited resource is still a mystery [3,4].
II. MODELING PLANT GROWTH
Allometrically, the relationship between the mass of the shoot and root can be explained by the formula
[5]:
ybxk (or: log ylogbklogx) (1)where yroot dry weight
xshoot dry weight
bandkare constants
This allometric depiction is purely empirical and does not explain to any degree the plasticity of growth
responses observed in nature in response to limitations of substrate during the growth period.