Growth, transpiration, root-born cytokinins and gibberellins, and nutrientcompositional changes in sesame exposed to low root-zone temperature underdifferent ratios of nitrate : ammonium supply

The growth of sesame (Sesamum indicum L.) was studied at three root tempera ture regimes (25/25, 20/10 and 15/15 degrees C day/night) factorially combi ned with three NO3-: NH4+ ratios (mM ratios, 10:0, 8:2, or 6:4), as a sourc e of nitrogen (N), in the irrigation solution. The air temperature was kept constant at 30 degrees C. Transpiration, nutrient composition, and level o f root-born cytokinins and gibberellins in the xylem exudate were monitored . The two low root temperature regimes, 15/15 and 20/10 degrees C, restrict ed the growth of sesame, reduced transpiration and increased the accumulati on of soluble carbohydrates in the shoot and in the roots compared to the 2 5/25 degrees C regime. The NO3-:NH4+ ratios had no effect on growth. Nutrie nt contents in the shoot at low root temperatures, particularly K+, NO3-, a nd H2PO4- were decreased markedly, but Na+ increased relative to it's conte nt in the 25/25 degrees C regime. Increasing NH4+ proportion in the irrigat ion solution raised total N concentration in the plant tissues at all root temperatures. The amounts of cytokinins and gibberellins in the xylem exuda te decreased at the low root temperature regimes relative to the 25/25 degr ees C regime. Low root temperature reduced xylem transport of nutrients and root born-phytohormones, most probably because of reduced water flow throu gh the plant relative to the 25/25 degrees C regime.