Differential thermal analyses, 2,3,5-triphenyltetrazolium chloride stain, electrical conductance, and whole tree freezing test measurements were performed on ‘Golden Delicious’ apple (Malus domestica Borkh.) to monitor annual variations in cold resistance and compare test responses. A distinct pattern of exotherms was observed by thermal analyses throughout the year. The first exotherm at −6 to −9 °C represented the extracellular water. The second and third exotherms varied with the time of year and were at −10 to −39 °C, representing pith cells and xylem tissue, respectively. The lowest temperature exotherm occurred at −35 to −40 °C. Tetrazolium tests, performed at temperatures corresponding to each exotherm, showed that the cell damage occurred in the shoot interior and spread radially outward, with the secondary xylem being the most cold resistant. Comparison of electrical conductance with the xylem exotherm showed a nonlinear relationship. Stress tests on trees under controlled conditions showed that the temperature at the xylem exotherm was critical to the survival of the tree. Both the tetrazolium test and tree stress tests indicated that the lowest temperature exotherm could not be used as a measure of cold resistance. The possibility that thermal analysis is an improved measure of cold resistance is discussed.

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Additional index words. Malus domestica, yield, flowering, fruit set, fruit size, L/D ratio, biennial bearing index Abstract. A cooperative experiment begun in 1980 was designed to evaluate the productivity and other characteristics of 16 spur and 12 standard strains of 'Delicious' apple on two rootstocks (M.7 EMLA and MM.111 EMLA). All trees were propagated at one nursery, and plantings were established in Michigan, Washington, Oregon, Idaho, Indiana, and British Columbia. In Michigan, trees of standard strains were consistently larger than trees of spur strains 8 years after planting. Early yields were greater on M.7 EMLA than on MM.111 EMLA, but trees were more subject to leaning under heavy crops on M.7 EMLA. In general, spur-type strains yielded more fruit per unit trunk cross-sectional area than standard strains. Yield was more closely associated with fruit-setting ability than with flower density, and fruit size declined as fruit set increased. Length/diameter ratios and biennial bearing indices also differed significantly among strains.

Phospholipid (PL) and fatty acid composition of chloroplasts of pine needles (Pinus sylvestris L.) and apple bark tissue (Malus sylvestris Mill. cv. Golden Delicious) was determined in a series of experiments in which growth temperature and daylength were changed. Trees were exposed to 0 and 20°C and to daylength conditions of 9 and 14 h. All 16 possible combinations were investigated by transfer of the trees from the original condition to each of the other conditions. There was no direct relation between cold hardiness and PL composition in apple bark and pine chloroplasts, when temperature and/or daylength were changed. PL composition seemed to be strongly determined by the sequence of the imposed sets of daylength and temperature. The effect of these environmental factors on PL composition strongly differed from that for cold hardiness. The correlation between the levels of PL (and phosphatidylcholine) and cold hardiness, as reported in the literature, was also evident in this experiment, when treatments, presenting the normal seasonal order, were compared. It seems that the yearly cycle of temperature and daylength is important in determining the PL composition of apple bark and pine chloroplasts.