This study was undertaken to see if Macadamia trees vary in their resistance to heat and desiccation. Plant tissue was subjected to high temperatures and then tested for viability by the tryphenyltetrazolium (TTC) method (Parker. 1953; Roberts. 1951) Fifty trees were compared : nineteen M.integrifolia seedlings, nine M.tetraphylla seedlings, two hybrids and twenty named varieties. The named varieties include trees of 333. Burdick UCLA, 246, Beaumont, Jordan, 764, and Santa Ana. Tests on the 50 trees were carried out during November and December 1967 the heat killing temperature of each of the 50 trees was determined. Thirteen of the 50 trees were selected for retesting during May 1968.

PROCEDURE AND PRELIMINARY TESTS

In order to attempt to establish a reliable procedure to compare the tissues from different trees, a number of preliminary tests were made. Leaf tips, petiole sections, pieces from the center of the leaf sections of stem, with bark, sections of stem with bark peeled off and sections of both old and young roots were taken from each of three trees. Each piece was put into a test tube on a small amount of cotton. One test tube with cotton was reserved for a thermometer. The test tubes were placed in a rack in a water bath at room temperature. It took approximately an hour for the water to heat to 60 C as soon a desired temperature was attained, the tubes were immediately removed and cooled with tap water. Then the tissue pieces were placed immediately in a 1 per cent solution of TTC. The color of the tissue was checked 24 hours later. The results of the preliminary tests showed that all samples from a given plant died at the temperature that did not vary more than one degree centigrade. This was important because it mean that leaves could be used instead of stems.

Then a check was made on the influence of various methods of handling the leaves between the time they were picked to the time of testing. These included 1) a comparison of the effects of leaving the leaves in plastic bags overnight at room temperature or under refrigeration. 2) Picking leaves at different times of the day, and 3) waiting for various periods of time before the tests were started. The results were that as long as the leaves were treated with reasonable care and not allowed to dry out that the methods of handling the leaves had no effect on the results of the tests.

Following these preliminary tests, the 50 trees were selected and tests were started during the first week in November. Leaf samples were used. The samples were cut from the central portion of mature leaves and were approximately .5 by 1 cm. Samples from all 50 trees were tested at the same time and the tests were repeated 14 times during the two months of November and December. Thirteen of the .50 trees were retest in May 1968.

RESULTS AND DISCUSSION

Results of the November-December tests were as follows: at 58 C all tissue samples were alive at the end of the test period. Samples from nine trees were killed by a temperature of 59 C. Samples from 34 more trees were killed at 60 C, five more at 61 C and the last two at 62 C. The three degrees difference is not great but it is of significance because of the consistency of the results that were obtained with each test. Tests were run during November and December and the temperature at which samples from any given tree died seldom varied during these two months. It is interesting to note that the trees with the greatest resistance to heat as indicated by the samples tested are Burdick, 764, UCLA, a seedling M.integrifolia, and a seedling M. tetraphylla. The nine trees showing the least resistance were all M. integrifolia and included the varieties 333 and 246.

Table I summarizes the results of the retesting of thirteen of the fifty trees during May 1968. Many of the leaf samples were able to withstand a slightly higher temperature during May than during the winter months. This was not a surprise since Mainer 1967) reported heat resistance to temperatures of 65 C in samples she tested during July. Levitt (1956) discusses seasonal changes in heat and dehydration hardiness in various plants. It is planned to run further tests in Macadamias during the time of the hot Santa Ana winds in September and October. An explanation of the increase in heat resistance during the summer months is of course complicated by the fact that some of the trees show no increase.

We plan to continue the problem of heat resistance in Macadamias by investigation of differences at the cellular level and also by studying the effect of heat on photosynthesis and nitrogen uptake.

LITERATURE CITED

Mainer, Karen, 1967. Studies on the anatomy and heat resistance of Macadamia. Unpublished thesis.