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ently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an The effects of prolonged exposure to dry surface soil overall rooting strategy where essentially no surface on the capacity of roots to take up water and phos- roots are shed following prolonged exposure to dry phorus were examined in mycorrhizal sour orange soil. (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In Key words: Drought, split-root, 32P, citrus, uptake kinetics, the first experiment, upper portions of the pots were phosphorus uptake, water uptake, sour orange, Citrus either irrigated every 2‐3 d, droughted for 14 d, aurantium. droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrig- Introduction ated and fertilized every 2‐3 d. Phosphorus uptake capacity was estimated in excised roots using 32 Pi n The soil surface layers, with their accumulation of roots, aerated 50, 750, and 1500 mM P solutions. Exposure organic matter and nutrients, commonly undergo large to dry soil had no appreciable effect on P uptake fluctuations in temperature, water content, and nutrient capacity. In the second experiment, the ability of intact availability. These fluctuations promote pulses of nutrient roots to acquire water and P in the 8 d following release by increasing microbial biomass turnover (Reid, rewatering after roots were exposed to localized 1974; Stark, 1994; Cui and Caldwell, 1997). Moisture drought for 14 and 43 d was examined. Roots were and temperature fluctuations may cause opportunities for observed non-destructively using small transparent nutrient uptake by roots in the litter, organic and surface tubes (2 cm diameter) and a rigid borescope. Soil water mineral layers to be brief and unpredictable (Grime, depletion was monitored using time-domain reflectro- 1994). There are at least two basic strategies for exploiting metry. Phosphorus (32P) was added at various depths resources in soil surface layers that periodically dry and in the soil in the upper compartment and uptake was which may develop high temperatures. One strategy is to assessed by non-destructively counting beta particle grow rapidly relatively thin inexpensive roots that have emissions from leaves using a scintillation probe. high absorptive capacity when the soil is wet, and then Similar to the first experiment, localized drought had rapidly shed these roots when the soil becomes dry or no effect on P uptake and soil water depletion in citrus otherwise unfavourable. This kind of root strategy is roots compared to continuously irrigated plants. Water commonly found in hot desert climates with desert succuand P uptake in the first few days apparently occurred lents (Huang and Nobel, 1992) and in certain desert from existing roots because of delayed production of half-shrubs like Cryptantha (personal observation). In new roots in the droughted treatment. Thus, citrus addition, many annual crops seem to exhibit this strategy ( Eissenstat and Yanai, 1997). roots exposed to extended periods of dry soil appar

Recovery of citrus surface roots following prolonged exposure to dry soil