Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 48

(600-700 nm) wavelengths of the solar spectrum. Plants
also can use light in the red and violet parts of the spectrum
for photosynthesis, however, which are less available than
green light. Thus, plants usually have more light-
sequestering structures than just chlorophyll. This is
indicated by colors other than green in the fall foliage of
many deciduous trees that result from the lack of chlorophyll
a and b production in the chloroplasts and the appearance of
accessory pigments in the chromoplasts, such as the
carotenes and xanthophylls. Moreover, some leaves remain
red year round because anthocyanins are present in the sap
rather than in the chloroplasts, and they absorb the more
plentiful green light.
The rate and magnitude of photosynthesis is dependent on
many variables. As may be expected, the rate is related to the
amount of light and air temperature of a particular area.
Photosynthesis is directly proportional to light intensities
greater than 25% of maximum sunlight, in terms of incom-
ing radiation in kilocalories per square meter per minute
([kcal/m 2 ]/min). Light intensities less than 25% of maxi-
mum result in a decrease in the rate of photosynthesis for
most plants. The rate of photosynthesis also is directly
related to the amount of chlorophyll present. The conversion
of light energy into sugar is relatively inefficient, usually
between 1% and 2% for most plants. This is similar to the
very small amount of energy actually converted to light,
rather than lost as heat, in a typical incandescent light bulb.
In reality, because the incident light on the surface of the
earth is not a constant but varies with location and time and
the summation of nights equate to about 6 months per year,
the photosynthetic efficiency globally is about 0.1%.
Rates of photosynthesis increase as air temperature
increases to a maximum and then decreases with further
increasing temperature. The rate of increase is about 2-5
times per 10 C increase in temperature under conditions of
normal light intensity such that it is not limiting. All plants
respond differently to changes in temperature, and optimum
rates of photosynthesis can occur when temperature ranges
from 16 Cto40 C. As such, photosynthesis occurs in most
plants in the morning before noon when temperatures are
lower and evapotranspiration demands also are lower, and
then later in the afternoon when light is still available but air
temperatures decrease from peak levels. As will be discussed
later in this chapter, this temperature effect is initiated by
stomatal closing and opening—stomata tend to close as
temperatures increase.
Because temperature and light affect rates of photosyn-
thesis the rate of photosynthesis also is directly related to the
rate of transpiration. If the air is dry, transpiration may be
initially high until stomata close, which decreases both pho-
tosynthesis and transpiration rates. On the other hand, if
the air is humid, photosynthesis can outpace transpiration.
The transpiration efficiency, TE , of plants is expressed as the

Fig. 3.3 The structure of chlorophyll a , showing the magnesium ion in
the porphyrin ring, the center of light capture.

molecules of chlorophyll a are attached to each thylakoid.
The thylakoid is the location where photons of light are
processed. Chlorophyll is protected from being oxidized by
too much solar radiation by another pigment, the carotenoid
zeaxanthin, which acts as a heat sink (Fleming and Niyogi
2005).
The green color of most plants begs the question of why
green, with all the visible colors of the rainbow? The incom-
ing light that strikes the leaf surface appears invisible to the
naked eye, but it contains all of the different wavelengths, or
white light, that viewed individually are seen as different
colors. Plants are green because they reflect light in the green
spectrum. In general, during photosynthesis, the green-
pigmented chloroplasts absorb incoming light energy. This
source of light can be the sun or artificial lighting. Pigments
have distinctive light absorption patterns, and chlorophyll
absorbs the blue (400-500 nm [nanometers]) or red