Light - Agroecology: The Ecology of Sustainable Food Systems - page 47

goes below the compensation point, the energy balance
for the plant is negative. If the negative balance is not
offset by a time period of active photosynthesis and energy
gain, the plant may die.

L ATITUDE

The closer to either of the poles, the greater the seasonal
variation in daylength. Above the arctic circle, 24-h
periods of daylight in the summer are balanced by 24-h
periods of night in the winter. Near the equator, the
constancy of 12-h d throughout the year makes for a
light environment that promotes year-round high net
primary productivity and permits an agriculture that is
characterized by either multiple plantings during the
annual crop calendar or an abundance of perennial crops
that provide a mixture or succession of harvests through-
out the year.

D URATION

The length of time that leaf surfaces are exposed to sun-
light each day can impact photosynthetic rates as well as
longer-term plant growth and development. Duration of
light exposure is also an important variable in how light
intensity or quality can affect a plant. Exposure to exces-
sive levels of light for a short time, for example, can be
tolerated, whereas a longer period of exposure can be
damaging. Or a short period of intensive light, allowing
the plant to produce an excess of photosynthate, can then
allow for tolerance of a longer period below the light
compensation point.
The total number of hours of daylight — the photoperiod
— is also an important aspect of the duration of light expo-
sure. A variety of plant responses, as will be discussed in
detail below, have specific chemical triggers or control mech-
anisms that can be activated or deactivated depending on the
number of hours of daylight, or in some cases, the number
of dark hours without sunlight.

A LTITUDE

As elevation increases, light intensity also increases
because the thinner atmosphere absorbs and disperses
less light. Plants growing at higher elevations, therefore,
are more subject to conditions of light saturation and
face greater danger of chlorophyll degradation than
plants at sea level. Many high-elevation plants have
evolved reflective coloration or protective hairs or scales
on leaf cuticles to reduce the amount of light penetrating
the leaves.

T OPOGRAPHY

DETERMINANTS OF VARIATIONS IN THE
LIGHT ENVIRONMENT

The slope and direction of the soil surface can create
localized variations in the intensity and duration of expo-
sure to sunlight. Although the temperature effects of this
variation may be of greater significance, steep slopes
facing the poles can receive significantly lower, direct
insolation than other sites. Slope orientation usually
becomes more important during the winter months, when
a hillside or other topographic feature can cast a shadow
over the vegetation. In farming systems, minor topographic
variation can create subtle differences in microclimate that
affect plant development, especially when plants are still
very small (Figure 4.4).

The quality and quantity of light received by a plant in
a specific location and the duration of its exposure to
light are a function of several important factors includ-
ing (1) seasonality, (2) latitude, (3) altitude, (4) topog-
raphy, (5) air quality, and (6) the structure of the
vegetation canopy.

S EASONALITY

Except at the equator, daylight hours are longest during
the summer and shortest in the winter, reaching their
extremes at the corresponding solstice. Since the angle
of the sun in relation to the surface is much lower toward
the poles during the winter, the sunlight that is available
has to pass through more atmospheres before it reaches
the plant, making that sunlight much less intense. There-
fore, both intensity and duration of light are affected by
seasonality. Many plants have adapted to the seasonal
variations in day length and light intensity through the
selection of adaptations that either prepare the plant for
the upcoming winter or get it ready to take advantage
of more optimal conditions for growth and development
as spring progresses into summer. The timing of
many agricultural activities — planting and pruning —
correspond to the changing hours of daylight at specific
times of the year.

A IR Q UALITY

Suspended materials in the atmosphere can have a signifi-
cant screening effect. Smoke, dust, and other pollutants,
either natural or human-produced, can greatly interfere
with photosynthetic activity, either by reducing the
amount of light energy that reaches the leaf or by coating
the leaf and cutting down the amount of light that pen-
etrates the cuticle. Such air quality problems are usually
most common in and around urban or industrial regions,
but poor air quality associated with agricultural activi-
ties such as burning and soil disturbance can also
occur. Greenhouse horticulture is particularly affected
by deposition of particulates from dirty air; even when
glass is clean it reduces light passage by about 13%
(Figure 4.5).