Heat Injury in Leaves of Alpine, Temperate and Tropical Plants

Temperatures producing heat injury in leaves of 30 species were measured by following heat-induced changes in fluorescence of the leaf chlorophyll. Except for six alpine plants and four cereals, mainly leafy vegetables and leaves of young fruit trees were used. Dark-adapted leaves were heated at a rate of one Celsius degree per minute. Chlorophyll fluoroscence began to increase between 30 and 40°C and reached a peak around 50°C. The peak of fluorescence was reached at 47.1°C in an alpine plant Plantago glacialis, at 51.2°C in lettuce (Lactuca sativa) and at 57.6°C in pawpaw (Carica papaya). Similarly, in the cereals, the peak of fluorescence was reached at lower temperatures in the temperate ones (49.5°C in barley and 48.9°C in oats) than in the two from warm climates (55.2°C in maize and 53.5°C in sorghum). This difference in heat sensitivity between plants from the three groups did not hold for all plants surveyed and there was some overlapping between individual plants in different groups. However, the mean temperature, for the inital rise in fluorescence as well as for the peak of fluorescence, occurred at a lower temperature in the alpine group of plants and at a higher temperature in the tropical group of plants compared with the temperate group. The results demonstrate a relationship between heat resistance and temperature of the environment in which the plants evolved, and illustrate the potential use of the chlorophyll fluorescence method for in vivo measurements of heat sensitivity in green plants.