SEASONAL-CHANGES IN FLUORESCENCE INTENSITY OF KIWIFRUIT NUCLEI STAINED WITH PROPIDIUM IODIDE AND MEASURED BY FLOW-CYTOMETRY

Nuclei were extracted from kiwifruit (Actinidia deliciosa) axillary bu ds and shoot apices at intervals during two growing seasons. Nuclei fr om fruit tissues were also examined. Extracted nuclei were stained wit h propidium iodide, which intercalates into double stranded nucleic ac ids, and the intensity of the staining reaction (fluorescence channel number) was measured by flow cytometry. Fluorescence intensity (mean c hannel number) of 2C nuclei varied during seasonal growth in both 1991 and 1992. In 1991, channel numbers of nuclei from shoot apices of pis tillate and staminate vines were highest at the time of bud break.and then declined. A similar change was observed in nuclei from pistillate shoots grown in a glasshouse. In 1992, however, nuclei extracted duri ng bud break from axillary buds and shoot apices had low channel numbe rs. In this year, highest channel number values occurred during mid su mmer and the values for nuclei from fruit tissues were also high at th is time. Lower channel number values were obtained in late summer for both axillary buds and fruit tissues. When nuclei with low channel num bers were treated with 0.1 N hydrochloric acid for 1.0-1.5 min, channe l numbers were increased. In 1991, the acid treatment fully restored c hannel numbers to maximum values, but, in 1992, complete restoration o nly occurred for nuclei sampled during late winter and early spring. C hanges observed in channel number values of nuclei from fresh tissues most likely resulted from conformation changes in the nucleus which al tered the intercalation of propidium iodide. However, altered propidiu m iodide intercalation did not fully explain lowered channel numbers d uring spring and summer in 1992. Changes found in the nuclei are discu ssed in relation to periods of cell differentiation in shoot and fruit tissues.