Vegetative phase change in Metrosideros: Shoot and root restriction

Abstract

Plants of Metrosideros excelsa Sol. ex Gaertn. ‘Scarlet Pimpernel’, which had undergone reversal of ontogenetic ageing (rejuvenation) following micropropagation, were subjected to shoot and root restriction treatments over 35 weeks to accelerate vegetative phase change. Shoot restriction was imposed by removal of axillary branches, while control plants were allowed to branch. Root restriction, imposed by growing plants in a range of container sizes, was applied in factorial combination with shoot restriction. Image analysis techniques were used to measure changes in leaf dimensional (roundness, area, length, width, length/width ratio and perimeter) and optical (hue, saturation and lightness) properties which change gradually between juvenile and mature forms of Metrosideros excelsa. Leaves of single-stemmed plants became progressively mature with increasing node position, and developed the downy tomentum on the abaxial surface characteristic of mature leaves. In general, leaves on the branched plants did not become progressively mature with increasing node position. The acceleration in vegetative phase change in single-stemmed plants was not due to a greater number of nodes produced along the main axis, nor because of a greater distance from root to shoot apex. Root restriction reduced root growth in branched plants, and increased shoot/root dry weight ratio in both sets of plants. However, it did not affect shoot growth, nor did it accelerate vegetative phase change.