Aboveground shifts in carbohydrate partitioning are reflected by greater selectivity of root growth in nutrient-rich patches
Carbohydrate (C) limitation above ground can restrict C availability below ground and suppress root growth, respiration, and exudation. In fruit crops, a majority of C is used for reproduction which may limit C availability below ground. For example, dwarf apple trees in well managed orchards partition 50-80% of annual C to fruit and only about 2% to roots. Tree productivity and health is dependent on efficient root processes; however, the consequences of limited C availability to root and mycorrhizal fungal foraging in heterogeneous soils are unknown. To investigate this, ‘Golden Delicious’ apple trees (Malus domestica) on M.9 rootstock were used to study root and mycorrhizal fungal foraging responses in a nutrient-rich patch when C partitioning was shifted by adjusting crop load to no crop or full crop. Root growth was observed using 20 rhizotrons between trees with 10 replications per crop load. In each rhizotron, there were three nutrient patches: mineral nitrogen (N), an organic source of N, or an unfertilized water control. New root growth was traced weekly over two observation periods from May 31 to August 8, 2013 and August 15 to October 25, 2013. Roots were harvested at the end of each observation period to microscopically quantify mycorrhizal and non-mycorrhizal fungal colonization.
Root growth was suppressed in full-crop trees when C availability below ground was limited; however, root proliferation of cropped trees was enhanced in mineral and organic N patches compared to the control patch. Root proliferation was not affected by N treatment in no-crop trees. Cropping had no effect on arbuscular mycorrhizal fungi (AMF) but increased colonization by non-mycorrhizal fungi (NMF) (P=0.02). In contrast, root colonization by AMF was suppressed by the organic N treatment while N treatment had no effect on NMF. These results suggest that when C availability below ground was limited by cropping, roots more selectively foraged for N in nutrient-rich patches than when C is more available to roots. Selective proliferation may increase foraging efficiency, but these roots may also be compromised by NMF infection.