ABSTRACT
Blueberries are perennial flowering plants with indigo-colored berries within the genus Vaccinium. Recently cultivated area and consumption have been increased rapidly. The objective of pruning blueberries is to control crop load for improving the fruit quality and the production of new bearing shoots for the following year.
These studies were carried out to estimate the effect of 1) vigor of a bearing shoot, 2) pruning severity and 3) flower bud load of a bearing mother branch on plant growth and fruit quality.

CHAPTER 1. Characteristics of Shoots and Fruits Depending on Vigor of a Bearing Shoot in ‘Bluecrop’ Blueberry
The vigor of a bearing shoot was classified with BS (bearing shoot) and MB (bearing mother branch). The vigor of a bearing shoot was divided into A) thin MB and short BS, B) thin MB and long BS, C) thick MB and short BS and D) thick MB and long BS. Shoots from D were longer (6.5 cm) and thicker (1.7 mm) than those from the others. Shoots of D had more leaves (5.8) than those of the others. Leaf area of D was larger (13.5 cm2) than those of the others. Stomatal length was not affected by the vigor of a bearing shoot. Number of stomata per cm2 was affected by BS. Content of chlorophyll a and b was lower at A (0.63 and 0.21 mg·g-1 FW respectively) than the others. MB × BS interaction did not show significant difference in content of chlorophyll. Leaf photosynthetic rate of A was increased rapidly from 0 to 300 μmol·m-2·s-1 PPF and was saturated at about 500 μmol·m-2·s-1 PPF. ETR (electron transport rate) and NPQ (non-photochemical quenching) of PSⅡ (photosystem Ⅱ) did not show significant difference depending on the vigor of a bearing shoot. Cluster weight on B was heavier (5.83 g) than those on the others. Ratio of middle berry (14 - 16 mm) from the long BSs was higher (43.9%) than that from the short BSs. Soluble solid content of middle fruits on the long BSs was higher (15.9oBx) than those on the short BSs. The long BSs would be proper than the short BSs for bearing bigger fruits.

CHAPTER 2. Characteristics of Plant Growth and Fruits Depending on Pruning Severity in ‘Bluecrop’ Blueberry
Pruning severity was classified into light, intermediate and heavy pruning. Light pruning was thinning out succulent shoots, intermediate pruning was thinning out succulent and short (< 10 cm) shoots, and heavy pruning was thinning out succulent and short shoots and heading cut to 1/3 flower buds.
Heavy pruned plants had longer shoots (7.8 cm), thicker (2.0 mm) and more number of leaves per shoot (6.6 leaves) on bearing shoots than the others. There was no different shoot growth on bearing shoots between light and intermediate pruned plants. Pruning severity did not affect new shoot growth over 30 cm of plant height but growth of short shoot under 10 cm was higher from light and intermediate pruning and growth of long shoot over 20 cm was higher from heavy pruning. Number of succulent shoots (≥ 50 cm) under 30 cm of plant height from heavy pruned plants was higher than those from the others. There was no significant pruning severity effect on leaf photosynthetic rate and ETR and NPQ of PSⅡ. Leaf photosynthetic rate was saturated at about 1,000 μmol·m-2·s-1 PPF. Small fruits (< 14 mm)were produced more from light and intermediate pruning, and big fruits (≥ 16 mm) were produced more from heavy pruning. Ratio of big fruit on heavy pruning was higher as 17.6% than those of the others. Soluble solid content, free sugars and organic acidity were not affected by pruning severity. Heavy pruning would be proper pruning severity for sustaining plant vigor and producing bigger fruits.

CHAPTER 3. Characteristics of Plant Growth and Fruits Depending on Flower Bud Load of a Bearing Mother Branch in ‘Bluecrop’ Blueberry
Three flower bud loads of a bearing mother branch were 20, 40, and 60 MCA (flower number per bearing mother branch cross-sectional area, cm-2). Shoots from 20 MCA were longer (7.6 cm) and thicker (1.8 mm) and had more number of leaves per shoot (4.5 leaves) on bearing shoots than those from the others. Flower bud loads of a bearing mother branch did not affect leaf area. Content of chlorophyll a and b, and SPAD value did not show significant difference (0.64, 0.23 mg·g-1 FW and 56.4 respectively). Leaf photosynthetic rate was saturated at about 1,000 μmol·m-2·s-1 PPF. Yield of middle and big fruit were not affected by flower bud loads of a bearing mother branch. But yield of small fruit from 60 MCA was higher (129 g) than those from 40 and 20 MCA. Soluble solid content, free sugars and organic acidity did not affected by flower bud loads of a bearing mother branch. 20 MCA would be proper flower bud load of a bearing mother branch for increasing big fruit ratio.