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 speedthe 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 uponstored energy reserves.

Different species of plants respond differently to temperature. Temperatures also play a rolein 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 whentemperatures exceed a base temperature. For

corn, the maximum temperature plus theminimum 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.