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<br />I <br />I <br /> <br />1868 <br /> <br />fluctuations and generally recover from short term temperature <br /> <br />stresses. <br /> <br />t' <br />jl <br />,. <br />II <br />l' <br />I. <br />I <br />-a <br />I <br />t <br />I <br />., <br />I' <br />ti <br />t <br />I <br />J' <br /> <br />In addition to direct plant growth effects, temperature also impacts <br /> <br />seed germination, dormancy, and flowering. In many plants a cold <br /> <br />period is necessary to overcome natural substances which act to <br /> <br />prevent or delay germination. Many perennial plants will enter a <br /> <br />period of dormancy after the onset of cold temperature. Different <br /> <br />varieties have different dormancy requirements. For example, <br /> <br />alfalfa dormancy and tolerance to cold are variety dependent. <br /> <br />Alfalfa varieties are classed non-dormant, semi-dormant, or <br />dormant. Dormant alfalfa varieties, which have the highest <br /> <br />tolerance to winter cold, are probably best suited to reservation <br /> <br />conditions. <br /> <br />Deciduous trees have specific cold weather requirements which must <br /> <br />be satisfied to promote normal flowering, growth, and fruit <br />production. This cold weather requirement is measured by <br /> <br />determining the number of chilling hours that occur over the winter <br /> <br />period. Chilling hours refer to the accumulation of time during <br />which the temperature is less than 45 degrees F. Temperatures below <br /> <br />32 degrees F or above 45 degrees F are not considered effective and <br /> <br />warm temperatures during the dormant period are thought to have an <br />offsetting effect. Plant species and varieties vary in their <br />chilling requirement. <br /> <br />3- 6 <br />