524 H. Zhang et al.
SELECT THE ROOT TIP WITH
THE LARGEST FITNESS VALUE
AS THE FIRST ONE TO BRANCH
SELECT THE ROOT TIP WITH
THE LARGEST FITNESS VALUE
IN THE REMAINING ONES
STOP CRITERIA
CALCULATE
DISTANCE BETWEEN
THE ROOT TIPS
DETERMINE THE ROOT TIP
TO BRANCH
NO
THE SELECTION
PROCESS ENDS
YES
GREATER THAN
THE THRESHOLD
LESS THAN THE
THRESHOLD
Fig. 1 The selection process of the root tips
from the old root tip in memory, the proposed model uses the
following expression:
pg
lj
=
x
ij
+ (2 ×δ
ij
− 1) j = k
x
ij
j = k
(2)
where k ∈{1, 2,...,D} are randomly chosen indexes and
j ∈{1, 2,...,D}. pg
l
(i = 1, 2,...,S) are S new growing
points. δ
ij
is a random number between [−1, 1].
3.4 Root hair growth
After new growing points are produced, root hairs begin to
grow from these growing points. Root hair growth depends
on its growth angle and growth length. The growth angle is
a vector for measuring the growth direction of root hair. The
growth angle of each root hair ϕ
i
(i = 1, 2,...,n) which is
produced randomly can be expressed as:
(φ
1
,φ
2
,...,φ
D
) = rand(D) (3)
ϕ
i
=
(φ
1
,φ
2
,...,φ
D
)
φ
2
1
+ φ
2
2
+···+φ
2
D
(4)
The growth length of each root hair is defined as δ
i
(i =
1, 2,...,n) which is an important parameter in the root
growth model. Some strategies of tuning the parameter
can produce multiple versions of the root growth model.
After growing, a new root tip is produced by the following
expression:
x
i
= x
i
+ δ
i
ϕ
i
(5)
In order to simulate the trophotropism of root system, some
rules are defined as follows:
(1) If morphactin concentration (fitness) of a new root tip is
better than old one in the same cycle t, the root tip will
continue to grow. A new root tip in the inner loop can be
expressed as:
x
t
i
= x
t
i
+ δ
i
ϕ
i
(6)
But the number of iterations in the inner loop is a prede-
fined value. While the number of iterations in the inner
loop equals a predefined value, the inner loop stops.
(2) If morphactin concentration of a new root tip is worse
than old one in the same cycle t, the root tip will stop
growing and t = t +1. A new root tip can be expressed
as:
x
t+1
i
= x
t+1
i
+ δ
i
ϕ
i
(7)
3.5 Root growth algorithm
The root growth model proposed is instantiated as RGA for
simulation of root system of plant and higher-dimensional
numerical function optimization. The threshold of the dis-
tance between root tips and the growth length of each root
hair are important parameters for RGA. The flowchart of the
RGA is presented in Fig. 2. The pseudocode for the RGA is
listed in Table 2.
4 Simulation of root system of plant
4.1 Simulating root–soil interaction
As the first stage of a comprehensive root growth model, the
objective of the study was to simulate the interactive rela-
tionships between changing soil environment and the root
growth. In the simulation, Ackley function is used as soil
environment in computer. Ackley function is presented in
Sect. 4.1 and the setting of the parameters is the same as one
in Sect. 4.2. Figures 3 and 4 shows the simulation result of
root–soil interaction. The color changes in the simulation of
soil indicate the uneven distribution of nutrients in the soil.
The root system is represented by points and a point rep-
resents a root tip. It is simulated successfully that the roots
of plant grow towards the directions with the higher con-
centration of nutrients. It is also illustrated that RGA can
obtain optimal solution for numerical function optimization
in Figs. 3 and 4.
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