Effects of Photoperiod on the Apparent Viscosity of Leaf Cytoplasm in Kalanchoe blossfeldiana. I. A Method for Quantitative Estimation and its Sources of Error
An adaptation of the centrifuge method of determining cytoplasmic viscosity is described, suitable for the examination of leaves of higher plants exposed to different photoperiodic conditions. Leaf segments are centrifuged for several standard periods, usually i, 2, and 4 minutes, followed by immediate fixation, microscopic examination of hand-microtome sections, and the determination of the proportions of cells in the mesophyll region whose chloroplasts have been displaced. Among factors which may cause systematic errors, variations in cell size, chloroplast size and chloroplast starch content have been found to cause relatively little error. Similarly, changes in water content which cause up to 14 per cent loss of fresh weight have no large or consistent effect. Significant effects may arise from diurnal changes, or from differences in ambient and in growth temperature, as well as in leaf age and the region within the leaf that is tested; they can all be minimized by appropriate sampling techniques. Any treatment effects revealed by the method are clearly not ascribable to viscosity differences alone, but they are likely to represent real changes in the physical state of the cytoplasm; they are therefore referred to as representing changes in ‘apparent viscosity’.