DIRECT MEASUREMENT OF WATER AVAILABILITY IN GELLED PLANT-TISSUE CULTURE MEDIA

Water constitutes nearly 100% of the volume and 95% of the mass of gel led plant tissue culture media. Even so, plant growth and development responses observed to occur with relatively small changes in gelling a gent concentration (0.1% of media total mass) have been attributed to changes in media water availability. Measurements with three alternati ve direct techniques, specific for measuring physiochemical water avai lability indicated the effects of a change of this magnitude in gellin g agent concentration negligibly affected the media water potential an d water conductivity. Sensitive pressure membrane measurements indicat ed that incremental gelling agent concentration increases of 0.1% (of media total mass) within the range normally used for plant tissue cult ure media, depressed water matric potential only 1-2 cm H2O (1-2 x 10( -4) MPa (mega pascal)); these values were confirmed with equally sensi tive tensiometer measurements. Moreover, no effect of concentration on water movement could be detected with a precise constant-head permeam eter over a broader range of gelling agent concentrations. These resul ts indicate that either in vitro plants are extremely sensitive to sub tle shifts in water status, or other physiochemical factors that also change with gelling agent concentration are contributing to the report ed effects on plant growth and development.