Emerging Technologies Inspired by Plants - Bioinspired Approaches for Human-Centric Technologies - page 112

These are only some of the examples of artificial solutions that were inspired by
plants. This chapter gives a brief overview of some attempts to translate various
plant features into artificial solutions, including plant root-like robotic devices and
actuators.
4.2.1 Plant Root Sensory Systems and Tropisms
For efficient uptake, plant roots must move toward mineral ions and water in soil.
For this reason, plant roots must sense and respond to a variety of environmental
(both biotic and abiotic) stimuli as they move through soil. A plant's root system
adapts itself morphologically to explore and penetrate the soil, which results in the
capillary exploration of the entire volume of soil. Plant roots can perform these
complex actions by using a large number of tips (apices) that contain many different
types of sensors (e.g., for touch, humidity, gravity, and ions) so that various parts of
the plant can communicate information and implement complex, adaptive behav-
iors. Sensory capabilities allow roots to develop specific growth responses (tro-
pisms) to react to changes in their environment. In general, a tropic movement is the
directional movement of a plant or part of a plant that results in the curvature of
plant organs toward or away from certain stimuli. Tropisms can be positive (the
plant will bend toward a stimulus) or negative (the plant will bend away from a
stimulus). To form a curvature toward or away from a directional stimulus, plant
roots use a differential growth response in which cells in one region actively
elongate at a faster rate than those located in the opposite region. Signal transduc-
tion pathways of tropic sensing result in a differential redistribution of auxin in the
responding plant organ (Esmon et al. 2005 ). Current models of tropic responses are
primarily based on the Cholodny-Went theory, which states that tropic stimuli
induce lateral redistribution of auxin, which results in unequal accumulation
between opposing sides of a responding organ and promotes differential growth
(Esmon et al. 2006 ). A wide range of tropisms exists in plants, including phototro-
pism (light; Hohm et al. 2013 ), gravitropism (gravity; Blancaflor and Masson
2003 ), thigmotropism (touch; Hart 1990 ), thermotropism (temperature; Ding and
Pickard 1993 ), chemotropism (chemicals; Estabrook and Yoder 1998 ; Van Norman
et al. 2004 ; Loreto et al. 2006 ), and hydrotropism (water or humidity gradient;
Takahashi et al. 2009 ; Esmon et al. 2005 ; Eapen et al. 2005 ). In a root apparatus, a
single root must move through a substrate by orienting along the gravity vector,
negotiating obstacles, and locating resources.
4.2.2 Strategies of Plant Roots for Soil Penetration
Plant roots are excellent natural diggers, and their characteristics such as adaptive
growth, energy movements, and the capability of penetrating soil at any angle are