Freezing tolerance of citrus, spinach, and petunia leaf tissue : osmotic adjustment and sensitivity to freeze induced cellular dehydration.
Seasonal variations in freezing tolerance, water content, water and osmotic potential, and levels of soluble sugars of leaves of field-grown Valencia orange (Citrus sinensis) trees were studied to determine the ability of citrus trees to cold acclimate under natural conditions. Controlled environmental studies of young potted citrus trees, spinach (Spinacia pleracea), and petunia (Petunia hybrids) were carried out to study the water relations during cold acclimation under less variable conditions. During the coolest weeks of the winter, leaf water content and osmotic potential of field-grown trees decreased about 20 to 25%, while soluble sugars increased by 100%. At the same time, freezing tolerance increased from lethal temperature for 50% (LT(50)) of -2.8 to -3.8 degrees C. In contrast, citrus leaves cold acclimated at a constant 10 degrees C in growth chambers were freezing tolerant to about -6 degrees C. The calculated freezing induced cellular dehydration at the LT(50) remained relatively constant for field-grown leaves throughout the year, but increased for leaves of plants cold acclimated at 10 degrees C in a controlled environment. Spinach leaves cold acclimated at 5 degrees C tolerated increased cellular dehydration compared to nonacclimated leaves. Cold acclimated petunia leaves increased in freezing tolerance by decreasing osmotic potential, but had no capacity to change cellular dehydration sensitivity. The result suggest that two cold acclimation mechanisms are involved in both citrus and spinach leaves and only one in petunia leaves. The common mechanism in all three species tested was a minor increase in tolerance (about -1 degrees C) resulting from low temperature induced osmotic adjustment, and the second in citrus and spinach was a noncolligative mechanism that increased the cellular resistance to freeze hydration.