Apple root study

Root growth of deciduous fruit occurs in two main growth flushes per season according to literature – typically during spring and autumn. The timing and duration of these flushes can be influenced by various factors e.g. soil environment, irrigation, rootstock, tree age and climate. No root studies to quantify the timing and duration of root flushes of apple trees under local, commercial conditions are available. In this study we quantified white root dynamics for mature, bearing trees (‘Golden Delicious’, Cripps Pink’), young, bearing trees (‘Fuji’) on M793, and a young, non-bearing ‘Corder Gala’ orchard on M7 in the EGVV area. The ‘Cripps Pink’ and ‘Corder Gala’ orchards were on lighter, sandy soils and the other orchards on heavier, clay soils.

The second objective was to determine the effect of cover crops on white root growth in the young, non-bearing ‘Corder Gala’ orchard. The third objective comprised the quantification of a young, non-bearing (‘Golden Delicious’) and mature, bearing orchard (‘Golden Delicious’) in the Ceres Koue Bokkeveld with special attention to the winter flush for 2015/16.

Root growth dynamics were quantified using a mini-rhizotron approach. Acrylic butyrate tubes (1.05 m) were installed parallel to the tree row, 40 cm from the tree base at an angle of approximately 45° of individual trees. A root scanner (CI-600, CID Bioscience, Inc, Camas, WA USA) was used to obtain images of root growth in four segments/windows per tube. In the EGVV area, the phenological phases were recorded as well. Yield (two seasons) and photosynthesis (one season) was recorded for the ‘Golden Delicious’, and photosynthesis for one season for ‘Corder Gala’ orchards and related to white root peaks. Continuous logging capacitance probes were installed in these sites to quantify soil temperatures and moisture.

The timing of root growth flushes were distinctly periodic and similar for the bearing orchards, irrespective of the different scions and soil types, but the duration and magnitude of the flushes differed. The main peak of white roots occurred in winter, with a smaller peak early summer.

The white root growth pattern of the young, non-bearing orchard clearly differed from the mature, bearing orchards and trees produced roots in various quantities throughout the growing season with no consistent white root growth during winter.

Minimum winter soil temperatures, especially at 60 cm soil depth (5 – 12°C), are conducive to root growth and is reflected in a peak growth period from May to August in the bearing trees.

As the establishment of the cover crops only occurred late during 2015 – the effect of cover crops on white root development was never observed.

Ceres data confirmed white root growth during winter for both the young and bearing trees.

The white root growth peak in winter in the EGVV and Ceres area is unique and differs from literature. Soil winter temperatures at these sites were conducive to continuous white root growth during May to August. The effect of this energy requiring activity should be investigated further with reference to a possible impact on dormancy release of apples and nutrient application