Spatial and temporal distribution of root activity of Annona squamosa ('Sugar apple') and A. reticulata ('Bullock's Heart') seedlings and their grafts with 'Arka Sahan' scion using isotopic technique

In 4-year-old healthy trees of Annona squamosa L. and A. reticulata L. seedlings and in situ grafts with 'Arka Sahan', an inter-specific hybrid of A. atemoya [A. cherimola Mill.xA. squamosa] and A. squamosa raised on a red sandy loam (Typic Haplustalf), A. squamosa showed significantly higher intensity of root activity (18 138 dpm/g dry matter) compared to A. reticulata (15 880). Similarly, the seedlings showed higher intensity (20 625) compared to that of the grafts (13 394) owing to dilution of the isotope in the volumetric biomass of the tree. A. squamosa showed a smaller volume of the plant (5.724 and 27.328 m3 in seedling and graft, respectively) compared to that of A. reticulata (19.406 and 30.536 m3). On an overall basis, the intensity of root activity increased from 15 826 dpm/g during early rainy season to the highest of 23 435 during late rainy season. Winter showed a drastic reduction of root activity (9 809) which significantly inqeased to 18 967 during summer. This trend was highly pronounced in A. squamosa but during winter, between seedlings and grafts, the intensity of root activity was at par (9 416 and 9 742, respectively). In A. reticulata however, early rainy season showed the highest intensity of 19 921 dpm/g which progressively declined to 10 039 during winter. The former showed an intensity of 20 624 dpm/g dry matter while the latter, 13 394. Root activity distribution showed that during early rainy season, the active roots were predominantly surface oriented in both seedlings and grafts (61.6-68.2%). From late rainy to summer season, the root activity distribution tended to be well-distributed throughout the rooting volume. The root activity distribution of A. reticulata tree closely conformed to that of A. squamosa. The grafts of both the species showed a relatively uniform distribution of active roots throughout the year compared to the seedlings that may promote a better anchorage, drought tolerance and utilization of native soil moisture and nutrients from the rooting volume.