Recovery of citrus surface roots following prolonged exposure to dry soil

The effects of prolonged exposure to dry surface soil on the capacity of roots to take up water and phosphorus were examined in mycorrhizal sour orange (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In the first experiment, upper portions of the pots were either irrigated every 2-3 d, droughted for 14 d, droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrigated and fertilized every 2-3 d. Phosphorus uptake capacity was estimated in excised roots using ³²P in aerated 50, 750, and 1500 μM P solutions. Exposure to dry soil had no appreciable effect on P uptake capacity. In the second experiment, the ability of intact roots to acquire water and P in the 8 d following rewatering after roots were exposed to localized drought for 14 and 43 d was examined. Roots were observed non-destructively using small transparent tubes (2 cm diameter) and a rigid borescope. Soil water depletion was monitored using time-domain reflectrometry. Phosphorus (³²P) was added at various depths in the soil in the upper compartment and uptake was assessed by non-destructively counting beta particle emissions from leaves using a scintillation probe. Similar to the first experiment, localized drought had no effect on P uptake and soil water depletion in citrus roots compared to continuously irrigated plants. Water and P uptake in the first few days apparently occurred from existing roots because of delayed production of new roots in the droughted treatment. Thus, citrus roots exposed to extended periods of dry soil apparently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an overall rooting strategy where essentially no surface roots are shed following prolonged exposure to dry soil.