Root conductivity was measured in decapitated 18-month-old citrus rootstocks using a modified pressure chamber apparatus. Carrizo citrange [Citrus sinensis (L.) Osbeck × Poncirus trifoliata L. (Raf.)], a frost-hardy species, exhibited a log-linear decrease in conductivity over a temperature range from 40 to 10°C. Rough lemon (C. jambhiri Lush.), a frost-susceptible species, also exhibited decreasing conductivity from 40 to 20°, but conductivity was similar at 10 and 20°. No endogenous diurnal cycling of root conductivity was observed in Carrizo citrange root-stocks.
The influence of root temperature on whole-plant water relations and cold hardiness in seedlings of 2 citrus rootstocks—rough lemon (Citrus jambhiri Lush.) and Carrizo citrange [C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.]—and ‘Valencia’ scions on both rootstocks was examined. Plants were exposed to root temperatures of 5°, 10°, or 15°C for 5–8 weeks, while shoots were exposed to a nonacclimating air temperature of 30°. Root temperatures of 5° decreased leaf xylem water potential and increased cold hardiness. Statistical differences in diffusive resistance and transpiration were observed only at the 5° root temperature. Root temperature did not significantly alter leaf relative water content in either seedlings or budded plants. A decrease in soil and root temperature alone, without a simultaneous reduction in air temperature, can provide an effective cold-acclimating environment for citrus.
Tipburn is considered a major limiting factor to lettuce production in greenhouses and controlled environment agriculture facilities. Conditions which promote optimal growth also result in high levels of tipburn incidence. It has been reported that air flow directed at inner leaves of rapidly growing lettuce can prevent tipburn without a concurrent reduction of growth, assumedly due to increased transpiration with increased air movement over leaf surfaces.
Lettuce was grown in the greenhouse in nutrient film technique, with additional lighting providing total of 17 to 19 mol m-2 d-1 of PAR. Control plants developed tipburn 20 to 25 days after seeding. Plants with air supplied to inner leaves by a perforated plastic sleeve did not develop tipburn up to 35 days after seeding. Diurnal changes in physiological parameters were measured starting one week prior to harvest. Leaves of control plants had significantly higher stomatal conductance and transpiration than did those of air-supplied plants, although diurnal patterns of control and air-treated plants were similar. Air flow treatment had no significant effect on the rate of photosynthesis. However, air-supplied plants had a slightly lower percentage of dry matter than control plants. The apparent growth reduction resulting from the air flow treatment evidently reduced the demand for calcium.