High Cross Pollination Frequency in Rice ( Oryza sativa ) Landraces in Field Condition

Flower opening time (FOT) and the duration of florets remaining open (= flower exposure duration, FED) are crucially important for successful cross pollination between neighbouring rice plants. When the florets of the ovary parent (or pollen recipient) open even a few seconds after the closure of all the florets of the the pollen parent of a rice cultivar, the chances of cross pollination (CP) would obviously be zero. Conversely, as shown by our previous study, cross pollination frequency (CPF) would be very high (>80%) if the overlap of FEDs of the PP and OP is considerably long. In continuation of our previous study conducted in 2019, we subsequently conducted crossing experiments with 19 pairs of rice landraces during short-day flowering season and 8 pairs during long-day flowering season. The results show that with a long (>20 min) duration of FED overlap between OP and PP, the CPF tends to exceed 60%, often reaching 100%, although in some cases successful CP resulted in variable degrees of F1 sterility. These data confirm our previous report; and contradict the century-old ‘received wisdom’ based on a series of crossing experiments, that CP in rice rarely exceeds 2%, as widely published in all textbooks and official literature in rice biology. Our experimental evidence show the actuality of naturally high CPF in rice under circumstances of considerable FED overlap between parental cultivars, and indicates the necessity of further exploration into genetic interactions of sterile grain production and assortments of polygenic traits in the offspring from crosses between different varietal genotypes.