SOMACLONAL VARIATION AND IMPROVEMENT OF CHILLING TOLERANCE IN RICE - CHANGES IN CHILLING-INDUCED CHLOROPHYLL FLUORESCENCE

Chilling is one of the major constraints for rice (Oryza sativa L.) cu lture at high altitude. The use of chlorophyll fluorescence to screen genotypes for chilling tolerance has been widely documented. Within a program dealing with improvement of chilling tolerance in rice, somacl onal variants were obtained previously and chilling tolerance was sele cted for during four successive generations. To evaluate the implicati on of the efficiency of photosystem II (PSII) photochemistry (Fv/Fm) a t low temperature in the increased chilling tolerance of these somaclo nes, the chlorophyll fluorescence of the somaclones and their original parental cultivars was measured with a portable fluorometer (PEA Anal yser, Hansatech, England). First, chilling tolerance of four rice cult ivars was investigated in phytotron trials by subjecting excised leave s to at 10/5 degrees C day/night for a 72-h period before returning to 28/22 degrees C. Fv/Fm was determined after 0, 3, 6, 12, 24, 36, 48, 60, 72, 75, 144, and 147 h and compared with measurements performed on control leaves maintained throughout at 28/22 degrees C. The chilling treatment induced a marked decrease in Fv/Fm during the day, which wa s only partially compensated for during night. The extent of this decr ease was related to cultivar chilling sensitivity. Second, leaves of s omaclonal families improved for chilling tolerance and arising from th ree of these cultivars were chilled during 6 d under the same conditio ns as described above and then returned to control conditions. Large d ifferences were found between somaclonal families and their cultivar o f origin. Fv/Fm was higher in the Kirundo 3 somaclonal families, but l ower in the somaclones originating from Facagro 76 and Kirundo 9. Chlo rophyll fluorescence allowed detection of changes in chilling toleranc e in several, but not all, somaclonal families. Different mechanisms a re implicated in chilling-tolerance improvement.