Download C4_3 Notes

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Cool season crops
Hardiness
Growing degree day
(GDD)
Metabolism
Plant heat-zone map
Plant hardiness zone
map
Stratification
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Thermoperiod
Thermoperiodic
Vernalization
Warm season crops
All of the chemical reactions in a plant, including
photosynthesis and respiration, fall under a
term, metabolism. The speed at which
metabolism occurs is affected by temperature.
This is because the enzymes that drive the
reactions are sensitive to temperature.
Cool or cold temperatures slow metabolic
processes, while warmer temperatures speed
the processes.
Most plants show optimum growth when night
temperatures are 10 to 15 degrees cooler
than day temperatures. Under ideal conditions
photosynthesis occurs at a high rate during
the day. The cooler temperatures at night slow
respiration. For growth to occur, the
rate of photosynthesis must exceed that of
respiration.
High temperatures can speed the rate of
respiration beyond that of photosynthesis.
As a result, the products of photosynthesis are
used more rapidly than they are produced.
In many plants, photosynthesis shuts down at
temperatures above 86°F.
So, during hot summer days plants draw upon
stored energy reserves.
Different species of plants respond differently to
temperature. Temperatures also play a role
in different plants functions.
The change of daily temperatures is called
thermoperiod. When changes in daily
temperatures influence plant responses, such as
flowering, the plants are said to be
thermoperiodic. Poinsettias and
chrysanthemums are thermoperiodic plants.
They initiate flowers when temperatures
become cooler and days grow shorter.
Wheat, oats, barley, rye, spinach, and lettuce are
cool season crops meaning they prefer
cooler growing temperatures and are tolerant of
frost. When temperatures rise in combination
with longer days, spinach and lettuce initiate
flower production
Warm season crops require warmer temperatures
for best growth. Cotton, corn, soybeans, and
sorghum are an example of a warm season
crop. Another warm season crop, tomatoes,
will not flower if temperatures are cool.
Some plants require a cold treatment for
physiological processes to occur. This is known
as vernalization. Tulips and narcissus require
vernalization to flower.
Some cereal grains, including winter wheat, also
require vernalization. Apples require 1,000 to
1,200 hours of temperatures between 32°F and
45°F to break their rest period.
Seeds of some plants have a dormancy
mechanism that is broken by a cold period. The
seeds do not germinate until the seed has
undergone a cold period.
This cold requirement for seeds is known as
stratification.
Plants can be classified as hardy or non-hardy
depending on their ability to withstand cold
temperatures. Hardiness is a plant’s ability to
tolerate cold temperatures.
A. The USDA has established a plant hardiness
zone map for the United States, that reflects
the average minimum winter temperatures for
given areas. The map shows eleven
zones of temperatures. It is valuable in selecting
plants adapted for growing in those areas.
Heat also plays a role in the performance of plant
species. Some plants are more sensitive
to heat than others. The plant heat-zone map has
been developed by the American
Horticultural Society to help identify areas in
which landscape plants can flourish. The
map shows 12 zones. Each zone reflects a rating
of summer heat based on the average
number of days above 86°F.
Eighty-six degrees is a temperature at which
plants are unable to process water fast enough
to maintain normal functions. Plants also
experience damage to cellular proteins.
Growing degree days (GDD) is a measure of
temperature requirements for plants and can
be used to estimate growth and development.
A. The basic concept of growing degree days is
that plant development will occur when
temperatures exceed a base temperature. For
corn, the maximum temperature plus the
minimum temperature in a day divided by 2
minus 50.
Fifty is selected as the constant because corn
grows very little at temperatures of 50°F or
below.
Growth is also checked when temperatures rise
above 86°F, so all temperatures recorded above
86°F are counted as 86 in the formula.
Most corn hybrids have fairly specific GDD
specifications.
Example : If a low temperature was 60°F and the
high was 90°F, the GDD would be 60 +
86 = 146 divided by 2 = 73 – 50 = 23 GDD. The
GDD are added during the growing
season. If a variety of corn required 2,450 GDD, it
would require 107 days to maturity
using the figure calculated in the example.