ROOT RESPIRATION IN CITRUS ACCLIMATES TO TEMPERATURE AND SLOWS DURINGDROUGHT

Citrus seedlings were grown in soil columns in which the root system w as hydraulically separated into tyro equal layers; this enabled us to maintain roots in the upper layer without water for 110 d, The columns were placed into waterbaths modified so that soil temperatures in the top layer could be maintained at 25 degrees C or at 35 degrees C, whi le temperature in the bottom layer was maintained at 25 degrees C, We hypothesized that, if citrus plants were grown in dry soil for an exte nded period, root mortality would increase if the cost of maintaining the roots was increased by elevating the soil temperature. However, du ring the drought period we did not observe any root mortality, even at the higher soil temperature, Moreover, we did not find that root resp iration was increased by prolonged exposure to drought and higher soil temperature, We did find that root respiration rates slowed in dry so il, Furthermore, when the soil columns were switched from one temperat ure treatment to another, root respiration rates in met soil rapidly i ncreased when moved to a higher temperature or rapidly decreased when moved to a lower temperature, But after only 4 d, respiration rates re turned to their original level; root respiration in dry soil was not a ffected by either short-or long-term shifts in soil temperature, Root respiration in citrus appears to acclimate rapidly to changes in soil temperature.