COMPARATIVE GROWTH AND PHYSIOLOGY OF SELECTED ONE-YEAR-OLD RED-FRUITED AND GREEN-FRUITED EUROPEAN PEAR CULTIVARS

Red-fruited pear trees are reported to be more difficult to grow, less vigorous and less productive than green-fruited pear trees, This stud y was undertaken to examine some hypotheses that have been proposed to explain the observed differences in red-vs. green-fruited pear trees. Several growth and gas exchange parameters plus ribulose 1,5-bisphosp hate carboxylase/oxygenase (Rubisco) activity, relative total soluble protein, and total chlorophyll were measured on potted, I-yr-old green house-grown trees of the green-fruited pear cultivars, 'Bartlett', 'd' Anjou', and 'Clapp's Favorite', and their respective red-fruited sport s, 'Red Bartlett', 'Red d'Anjou' and 'Kalle'. Green-fruited 'd'Anjou' trees had double the mean total leaf area of 'Red d'Anjou' (P less tha n or equal to 0.05). Specific leaf area of 'Clapp's Favorite' was 77% that of 'Kalle'. The asymptotic maximum net photosynthetic rates (P-ma x) for red-fruited sports were 69% of those of their green-fruited par ents. Stomatal conductance to water vapor of the sports was 78% of tha t of the parents under light-saturated conditions. Dark respiration of green-fruited 'd'Anjou' leaves was 77% of that of 'Red d'Anjou' leave s. Intercellular CO2 concentration was not significantly different bet ween green-fruited parents and red-fruited sports, Although the relati ve total soluble leaf protein content and total chlorophyll content di d not differ significantly between green-fruited parents and red-fruit ed sports, both characteristics tended to he higher in the parental se ries, which was consistent with higher net photosynthesis (P-n) in the green-fruited parents than in the red-fruited sports. The ratio of ch lorophyll n to chlorophyll b in 'Red Bartlett' and 'Red d'Anjou' was 6 2% and 54% of that in 'Bartlett' and 'd'Anjou', respectively, Rubisco activity in the red-fruited sports was 60% of that in the green-fruite d parents. On the basis of these observations, we cannot reflect (he h ypothesis that lower chlorophyll content, particularly chlorophyll a, and lower P-n contribute to the reduced vigor ill trees of red-fruited sports compared to their green-fruited parents. (C) 1997 Elsevier Sci ence B.V.