INTRINSIC EARLINESS AND BASIC DEVELOPMENT RATE ASSESSED FOR THEIR RESPONSE TO TEMPERATURE IN WHEAT

The related concepts of basic vegetative period, intrinsic earliness and basic development rate in wheat are examined. These concepts have the common assumption that, if plants are vernalised fully and then grown at long daylength in order to remove any responses to vernalisation and photoperiod, the calendar or thermal time then taken to anthesis will be a characteristic of a genotype that will be heritable. Thus, regardless of temperature, early genotypes will always be earlier than late genotypes (providing there are no vernalisation and photoperiod responses). Using four genotypes, exposed to 50 days of vernalisation, and then grown at 18 h photoperiod under six temperature regimes ranging between 10 and 25 degrees C, it is shown that: (1) no genotype had an absolute basic period as, depending on temperature, durations to anthesis for any one genotype varied by more than 50 days; (2) no genotype had an absolute value for intrinsic earliness (to anthesis), ranging for any genotype by more than 300 degrees Cd depending on temperature; (3) basic development rate was not a single value for a genotype but varied with stage of development; (4) some genotypes changed their ranking for earliness depending on the temperature regime; and (5) genotypes were differentially sensitive to temperature for the subphases prior to anthesis. We conclude that the earliness descriptors should not be considered as static genotypic characteristics but as the result of the interaction between the genotype and temperature. Thus, the apparent earliness trait is likely to be related to temperature sensitivity.