Relationship between leaf temperature, compatible solutes and antitranspirant treatment in some desert plants

Three desert perennials from the Wahiba Sands, Oman were studied for their ability to withstrand high air temperatures combined with low water availability. Plants were also sprayed with antitranspirant to reduced water loss. This limited the potential for evaporative leaf cooling and resulted in high leaf temperatures in Prosopis cineraria (Druce) and Zygophyllum qatarense (Hadidi). Diurnal measurements were made of leaf and air temperature and of of photosynthesis and transpiration. Antitranspirants did not significantly reduce transpiration in Heliotropium kotschyi (Guerke), which contained high concentrations of the quaternary compound betaine. Transpiration continued through the hottest periods of the day and appeared to be essential for leaf cooling. All species appeared to have a threshold leaf temperature above which photosynthesis was severely impaired. Samples were taken for analysis of amino acids, quaternary ammonium compounds (QACs) and low molecular weight carbohydrates in conjunction photosynthesis and transpiration in order to examine any possible protective functions. In Prosopis cineraria the background concentration of the cyclitol, pinitol was high but there was no increase in response to the antitranspirant induced increase in leaf temperatures. Proline concentration increased in leaves of Zygophyllum qatarense throughout the day but this did not appear to be related to leaf temperature.