Frequency of occurrence of the median and distal zones per shoot depending on the genotype and year of shoot development.

The year effect was tested by Pearson χ2 test. Comparisons among genotypes (cvs and VP clones) and between years were performed with a Pearson χ2 test. Different letters in a same variable indicate significant difference (p < 0.05). The two-year proportion of zone occurrence among cultivars and VP clones was compared separately.