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 (Cucumis melo L.) seedling morphology and root membrane properties as a function of root growth temperature, alone and in combination with salinity, were carried out. Melon seedlings were grown hydroponically in the absence (control) and presence (salt-grown) of 100 mM NaCl. The growth media were set at various temperatures ranging from 20 to 30 °C. Growth of salt-grown seedlings was considerably inhibited at 20 °C. At higher temperatures growth was enhanced more in the salt-treatment than in the control. Consequently, the morphological differences between control and salt-grown seedlings, which were substantial at 20 °C, were diminished at 30 °C. In control seedlings, chemical analysis of isolated root membranes revealed that the growth temperature affected their composition, primarily the content of lipids. As a result, the weight ratio of lipid to protein (UP) in the membranes was lower at higher temperatures. A parallel change in membrane fluidity (the inverse of viscosity) was measured by fluorescence de-polarization techniques, suggesting that the changes in membrane UP were related to the plant mechanism of «homeoviscous adaptation», i.e. maintenance of a constant membrane viscosity. In salt-grown seedlings, the effect of temperature was primarily on the membrane protein content. Membrane UP decreased between 20 and 25 °C, but did not change significantly above 25 °C. A comparison between root membranes from control and salt-grown seedlings disclosed that at 20 and 25 °C UP values were significantly higher in the control. However, the magnitude of the difference decreased as the growth temperature increased, and at 30 °C it was practically null. Thus, raising the temperature from 20 to 30°C resulted in smaller effects of salinity on both growth and membrane composition. Further support for the role of membrane UP in the plant response to salt came from experiments with control seedlings of different root-membrane L/P. The different seedling groups, produced by growth at various temperatures, were potted, transferred to the greenhouse (27/17 °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 UP, were the most tolerant to 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 UP, plays an important role in salt-tolerance, probably due to its effect on membrane physical properties.