Root Cytokinin Level In Relation To Heat Tolerance Of Phaseolus-Acutifolius And Phaseolus-Vulgaris

Heat stress influences plant growth and crop production. This research was conducted to determine the importance of root cytokinin level in plant heat tolerance. Tepary bean (Phaseolus acutifolius Gray) and heat tolerant (GNUI 59) and heat sensitive (BBL 47) genotypes of common bean (P. vulgaris L.) were used to determine root and leaf cytokinin levels under 45 degrees C air temperature and either 45 or 25 degrees C root temperature. The concentrations of root zeatin riboside (ZR) were about two- to three-fold higher in tepary bean and GNUI 59 than in BBL 47 under all conditions. Root isopentenyl adenosine ([9R]iP) was much loner than root ZR. High shoot/root temperature (45/45 degrees C) for 5 h reduced the root ZR and [9R]iP concentrations in all three genotypes. Heat treatments did not alter leaf ZR and [9R]iP. Pretreatment with exogenous ZR (1 mu M) did not show any alleviation from heat Injury in terms of photosynthetic activity, even though endogenous ZR levels increased, compared with those without ZR pretreatment. Reciprocal grafting of BBL 47 shoots on tepary roots increased leaf ZR level at high temperature but did not increase photosynthetic activity. Results suggest that root ZR level is not important in determining plant heat tolerance.