Evolutionary significances of cryptic variation in the reaction norm of photoperiod sensitivity of rice (Oryza sativa L.)

Patterns of photoperiodic-response curve (PRC) of heading time in rice were examined using the five near-isogenic lines (NILs) of Taichung 65, in which insensitive alleles was replaced by photosensitive alleles through backcrossings. In the first experiment, the NILs showed the diversified patterns of PRC, depending on genotype, under the controlled conditions (9-, 10-, 12- and 14-h daylengths). In addition, the two NILs with different Se1 alleles showed the different patterns although they showed similar heading times under natural daylength (NDL) conditions. The cryptic difference gives some clues as to the relationship between the patterns and heading-time behaviors under NDL conditions. In the second experiment, behaviors of vegetative and reproductive developments under 20-h daylength condition with high- and low-nutrient conditions were examined to know whether or not they behave as expected from their PRC obtained in the first experiment. Under the extremely long-day condition, the photosensitive alleles brought about long-growth durations where the internal changes for senescence might drastically occur with increases of total number of phytomers (TNP) and decrease potentials for reproductive developments. The nutrient levels might not affect the pattern of internal changes but might affect TNP, depending on genotype, and as a result, the nutrient levels might bring about different developmental fates in the same genotype under the long-day condition. The present results suggest the importance of genotype-by-environment interaction in relation to the cryptic variation of PRC in rice.