The Plant - Agroecology: The Ecology of Sustainable Food Systems - page 35

3 The Plant

The design and management of sustainable agroecosystems
has important foundations in our understanding of how indi-
vidual plants grow, develop, and eventually become the plant
matter we use, consume, or feed to our animals. This chapter
reviews some of the more important plant physiological pro-
cesses that allow a plant to live, convert sunlight into chemical
energy, and store that energy in parts of the plant and in forms
we can use. The chapter also reviews some of the principal
nutritional needs of plants. Finally, by way of introduction to
the rest of the chapters in Section II, the chapter reviews some
of the most important concepts and terms used to describe
the ways individual plants respond and adapt to the range of
environmental factors we will be examining.

Photosynthesis is actually made up of two distinct pro-
cesses, each with multiple steps. These two processes, or
stages, are called the light reactions and the dark reactions .
The light reactions function to convert light energy into
chemical energy in the form of adenosine triphosphate
(ATP) and a compound called oxidized from of nicotina-
mide adenine dinucleotide phosphate (NADPH). These
reactions use water and give off oxygen. The dark reactions
(which take place independently of light) take carbon atoms
from carbon dioxide in the atmosphere and use them to form
organic compounds; this process is called carbon fixation
and is driven by the ATP and NADPH produced by the light
reactions. The direct end product of photosynthesis, often
called photosynthate , is made up mainly of the simple sugar
glucose. Glucose serves as an energy source for growth and
metabolism in both plants and animals, because it is readily
converted back to chemical energy (ATP) and carbon diox-
ide by the process of respiration. Glucose is also the building
block for many other organic compounds in plants. These
compounds include cellulose, the plant's main structural
material, and starch, a storage form of glucose (Figure 3.1).
From an agroecological perspective, it is important to
understand how photosynthesis can be limited. Temperature
and water availability are two important factors. If temper-
atures are too high or moisture stress too great during the
day, the openings in the leaf surface through which carbon
dioxide passes begin to close. As a result of the closing of
these openings — called stomata — carbon dioxide
becomes limiting, slowing down the photosynthetic pro-
cess. When the internal concentration of CO 2 in the leaf
goes below a critical limiting concentration, the plant
reaches the so-called CO 2 compensation point , where pho-
tosynthesis equals respiration, yielding no net energy gain
by the plant. To make matters worse, the closing of the
stomates under water or heat stress also eliminates the leaf's
evaporative cooling process and increases leaf O 2 concen-
tration. These conditions stimulate the energetically waste-
ful process of photorespiration , in which O 2 is substituted
for CO 2 in the dark reactions of photosynthesis, producing
useless products that require further energy to metabolize.
Some kinds of plants have evolved different ways of
fixing carbon that reduce photorespiration. Their alternate
forms of carbon fixation constitute distinct photosynthetic
pathways. Altogether, three types of photosynthesis are
known to exist. Each has advantages under certain condi-
tions and disadvantages in others.

PLANT NUTRITION

Plants are autotrophic (self-nourishing) organisms by vir-
tue of their ability to synthesize carbohydrates using only
water, carbon dioxide, and energy from the sun. Photo-
synthesis, the process by which this energy capture takes
place, is thus the foundation of plant nutrition. Yet manu-
facturing carbohydrates is just part of plant growth and
development. An array of essential nutrients, along with
water, are needed to form the complex carbohydrates,
amino acids, and proteins that make up plant tissue and
serve important functions in plants' life processes.

P HOTOSYNTHESIS

Through the process of photosynthesis, plants convert
solar energy into chemical energy stored in the chemical
bonds of sugar molecules. Since this energy-trapping pro-
cess is so important for plant growth and survival, and is
what makes plants useful to humans as crops, it is impor-
tant to understand how photosynthesis works.
The descriptions of the processes of photosynthesis
that follow are very simplified. For our purposes, it is more
important to understand the agroecological consequences
of the different types of photosynthesis than to know their
actual chemical pathways. However, if a more detailed
explanation is desired, the reader is advised to consult a
plant physiology text.
As a whole, the process of photosynthesis is the solar-
energy-driven production of glucose from water and car-
bon dioxide, as summarized in this simple equation:

6CO 2 + 12H 2 O + light energy —> C 6 H 12 O 6 + 6O 2 + 6H 2 O

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