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  • Author or Editor: Ronald L. Perry x
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`Imperial Gala' apple trees (Malus ×domestica Borkh.) on M.9 EMLA, MM.111, and Mark rootstocks were subjected to two drought-stress and recovery periods in a rainshelter. Water relations, gas-exchange parameters per unit leaf area and per tree, chlorophyll fluorescence, and leaf abscisic acid content were determined during each stress and recovery period. Whole-plant calculated gas exchange best indicated plant response to drought stress, with consistent reductions in CO2 assimilation, transpiration, and leaf conductance. Variable and maximal chlorophyll fluorescence and fluorescence quenching were not as sensitive to stress. Other fluorescence parameters showed little difference. The most consistent decreases due to stress for gas exchange per square meter were in transpiration and leaf conductance, with few differences in CO2 assimilation and fewer for mesophyll conductance, internal CO2 concentration, and water-use efficiency. Leaf water potential was consistently lower during drought stress and returned to control values upon irrigation. Leaf abscisic acid content was higher for drought-stressed trees on M.9 EMLA than control trees during the stress periods but inconsistently different for the other rootstock treatments. Trees on M.9 EMLA were least affected by drought stress, MM.111 was intermediate, and Mark was the most sensitive; these results are consistent with the growth data.

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`Imperial Gala' apple (Malus domestica Borkh.) trees, trained to two shoots, on M.9 EMLA, MM.111, and Mark rootstocks were subjected to two drought-stress and recovery periods in a rainshelter. Leaf growth rate, leaf area, leaf emergence, shoot length, and trunk cross-sectional area were measured during each stress and recovery period. Leaf growth rate was reduced during both stress periods but most consistently during the second drought stress. Length of the less-vigorous shoot was reduced most consistently due to drought stress but did not recover upon irrigation. Leaf emergence and trunk cross-sectional area increment were inconsistent in response to stress. Tree growth was reduced by drought stress to the greatest extent for trees on Mark, with MM.111 intermediate and M.9 EMLA least affected. At termination, the plants were separated into roots, current-season shoot growth, previous-season shoot growth, and rootstock, and dry weights were measured. Dry weights confirmed the growth measurements taken during the experiment with a 16%, 27%, and 34% reduction in total plant dry weight for drought-stressed trees on M.9 EMLA, MM.111, and Mark, respectively, compared to corresponding controls. It was concluded that Mark was the most sensitive of the three rootstocks followed by MM.111; M.9 EMLA was the most drought resistant.

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Root distribution of `Starkspur Supreme Delicious' on nine apple (Malus domestics Borkh.) rootstock grown in two different soil types in the 1980 NC-140 Uniform Apple Regional Rootstock Trial (Michigan and Ohio sites) was determined using the trench profile method. Based on the number of roots counted per tree, rootstock could be separated into five groups for the Marlette soil from most to least: MAC.24 > OAR1 > M.26EMLA = M.9EMLA > M.7EMLA = 0.3 = M.9 = MAC.9 > M.27EMLA. For the Canfield soil, rootstock were ranked for number of roots counted from most to least as follows: MAC.24 > OAR 1. MAC.9 = M.7EMLA > M.26EMLA = O.3 = M.9 EMLA = M.9. Root distribution pattern by depth was affected by soil type with roots fairly well distributed throughout the Marlette soil but restricted primarily above the fragipan in the Canfield soil. Two rootstock performed differently from others in adapting to soil conditions at the different sites. MAC.9 had the second lowest number of total roots/dm2 in the Marlette soil yet the second most in the Canfield soil, while the opposite was found for M.9EMLA. Regression analysis demonstrated positive correlations between number of roots counted and vigor and yield of the scion.

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The influence of rootstock on average fruit weight was evaluated for a subset of data from a multilocation NC-140 apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] rootstock trial. Data for eight dwarf rootstocks were collected at four locations for 2 years. Analysis of covariance was used to evaluate the effect of rootstock on average fruit weight when crop density or number of fruit per tree was included in the linear model as a covariate. When number of fruit harvested per tree was used as a covariate, average fruit weight was not affected by rootstock in either year in Ontario. In Michigan and Virginia, rootstock and number of fruit per tree, but not the rootstock × number of fruit interaction, were significant, so common slopes models were used to estimate least squares means for average fruit weight. In general, trees on M.27 and P.1 produced the smallest fruit, and trees on B.9, M.9 EMLA, and Mac.39 produced the largest fruit. In New York the interaction of rootstock × number of fruit was significant, so least squares means were estimated at three levels of number of fruit per tree. Both years, at all levels of number of fruit, trees on M.26 EMLA produced the smallest fruit and trees on M.27 EMLA produced the largest fruit. Average fruit weight was most affected by number of fruit per tree when Mark was the rootstock. In general, results were similar when crop density was used as the covariate, except that trees on M.27 EMLA did not produce small fruit in Michigan and Ontario.

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One-year-old `Imperial Gala' on Mark, M.9 EMLA, and MM.1ll; and `Indian Summer' on MM.lll and MM.106 rootstocks were planted in a rain exclusion shelter in May 1991. All trees were irrigated. Half the trees were drought stressed and received no water for two, 30-day drought cycles. Four trees from each scion ×rootstock×irrigation combination were excavated in mid-October. Nonstructural carbohydrate reserves of stems and roots were determined. Cold hardiness, determined by visual examination of tissue after controlled freezing, was influenced by rootstock, drought, and stem age. Concentrations of several carbohydrates were correlated with cold hardiness. Regression models of carbohydrate concentrations on cold hardiness were significant. Removal of root tissue, which was cold sensitive and had high carbohydrate levels, altered the regression equations. Rootstock significantly influenced root concentrations of sorbitol, sucrose, and starch. Root sorbitol increased following drought stress. Mark and MM.106 roots had the largest increases in sorbitol. Irrigated Mark roots had very low levels of sorbitol.

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