RESPONSE OF MELON PLANTS TO SALT .2. MODULATION BY ROOT-GROWTH TEMPERATURE - THE ROLE OF ROOT MEMBRANE-PROPERTIES

The proposition that the physical state of root membranes is critical in plant response to salinity was tested by utilizing temperature as a physical modulator of fluidity. Simultaneous studies of melon (Cucumi s melo L.) seedling morphology and root membrane properties as a funct ion of root growth temperature, alone and in combination with salinity , were carried out. Melon seedlings were grown hydroponically in the a bsence (control) and presence (salt-grown) of 100 mM NaCl. The growth media were set at various temperatures ranging from 20 to 30 degrees C . Growth of salt-grown seedlings was considerably inhibited at 20 degr ees C. At higher temperatures growth was enhanced more in the salt-tre atment than in the control. Consequently, the morphological difference s between control and salt-grown seedlings, which were substantial at 20 degrees C, were diminished at 30 degrees C. In control seedlings, c hemical analysis of isolated root membranes revealed that the growth t emperature affected their composition, primarily the content of lipids . As a result, the weight ratio of lipid to protein (L/P) in the membr anes was lower at higher temperatures. A parallel change in membrane f luidity (the inverse of viscosity) was measured by fluorescence de-pol arization techniques, suggesting that the changes in membrane L/P were related to the plant mechanism of <<homeoviscous adaptations>>, i.e. maintenance of a constant membrane viscosity. In salt-grown seedlings, the effect of temperature was primarily on the membrane protein conte nt. Membrane L/P decreased between 20 and 25 degrees C, but did not ch ange significantly above 25 degrees C. A comparison between root membr anes from control and salt-grown seedlings disclosed that at 20 and 25 degrees C L/P values were significantly higher in the control. Howeve r, the magnitude of the difference decreased as the growth temperature increased, and at 30 degrees C it was practically null. Thus, raising the temperature from 20 to 30 degrees C resulted in smaller effects o f salinity on both growth and membrane composition. Further support fo r the role of membrane L/P in the plant response to salt came from exp eriments with control seedlings of different root-membrane L/P. The di fferent seedling groups, produced by growth at various temperatures, w ere potted, transferred to the greenhouse (27/17 degrees C, day/night temperatures) and tested for their salt sensitivity by irrigation with salt containing medium. Seedlings pre-grown at the lowest temperature , thus, having the highest root membrane L/P, were the most tolerant t o salt treatment. The results substantiate the prominent role of root membranes in the response of melon seedlings to the growth temperature alone and in combination with salinity. Furthermore, they support the proposition that membrane composition, in particular L/P, plays an im portant role in salt-tolerance, probably due to its effect on membrane physical properties.