Growth and mineral nutrition of cucumber seedlings as affected by elevated air and root-zone temperatures

Relative contributions of supraoptimal air and root-zone temperatures on the growth inhibition of cucumber (Cucumis sativus L.) during hot seasons were evaluated. Seedlings of a Chinese cultivar, Sangoh (Ji huang gua 3) and a Japanese cultivar, Sharp I were grown in water culture for 8 days in controlled environment rooms. The environmental conditions were : 26/26 degrees C, 26/37 degrees C, 37/26 degrees C or 37/37 degrees C daytime air/root-zone temperature regimes (air temperature in the night was 5 degrees C lower than that in the day), 80/90 % relative air humidity, and a 15-hr photoperiod of ca. 480 mu mol . m(-2). s(-1) PPFD. Temperature of leaves was approximately equal to air temperature during the daytime. Although plants thrived best at 26/26 degrees C and worst at 37/37 degrees C combinations, when growth at 26/37 degrees C and 37/26 degrees C regimes was compared, high air temperature was more inhibitory than high root-zone temperature. Root growth, leaf expansion rate, leaf water content, photosynthetic rate and leaf mineral concentrations were reduced more severely by high root-zone temperature. Seedlings of 'Sangoh' were significantly more tolerant to high root-zone temperature than were those of 'Sharp I'. Possible causes of growth inhibition by high air temperature compared to that by high root-zone temperature and the mechanisms of the cultivarietal difference in thermotolerance of cucumber plants are discussed.