Winter bud content according to position in 3-year-old branching systems of 'Granny Smith' apple

Abstract

An investigation was made of the number of preformed organs in winter buds of 3-year-old reiterated complexes of the 'Granny Smith' cultivar. Winter bud content was studied with respect to bud position: terminal buds were compared on both long shoots and spurs according to branching order and shoot age, while axillary buds were compared between three zones (distal, median and proximal) along 1-year-old annual shoots in order 1. The percentage of winter buds that differentiated into inflorescences was determined and the flowers in each bud were counted for each bud category. The other organ categories considered were scales and leaf primordia. The results confirmed that a certain number of organs must be initiated before floral differentiation occurred. The minimum limit was estimated at about 15 organs on average, including scales. Total number of lateral organs formed was shown to vary with both bud position and meristem age, increasing from newly formed meristems to 1- and 2-year-old meristems on different shoot types. These differences in bud organogenesis depending on bud position, were consistent with the morphogenetic gradients observed in apple tree architecture. Axillary buds did not contain more than 15 organs on average and this low organogenetic activity of the meristems was related to a low number of flowers per bud. In contrast, the other bud categories contained more than 15 differentiated organs on average and a trade-off was observed between leaf and flower primordia. The ratio between the number of leaf and flower primordia per bud varied with shoot type. When the terminal buds on long shoots and spurs were compared, those on long shoots showed more flowers and a higher ratio of leaf to flower primordia.

Fig. 1. Positions of the buds sampled in the branching system. 1, Apical buds of 3‐year‐old long shoots in order 1; 2, 3 and 4: axillary buds, respectively, in distal, median and proximal positions; 5 and 6, apical buds on 1‐ and 2‐year‐old long shoots in order 2; 7 and 8, apical buds on 1‐ and 2‐year‐old spurs in order 2; 9, apical buds on long shoots in order 3.

Fig. 2. Mean number of total preformed organs (▴), leaf primordia (▪) and scales (♦) according to the number of flowers per bud and at different bud positions in 3‐year‐old branching system of ‘Granny Smith’.