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

The related concepts of basic vegetative period, intrinsic earliness a nd basic development rate in wheat are examined. These concepts have t he common assumption that, if plants are vernalised fully and then gro wn 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, r egardless of temperature, early genotypes will always be earlier than late genotypes (providing there are no vernalisation and photoperiod r esponses). Using four genotypes, exposed to 50 days of vernalisation, and then grown at 18 h photoperiod under six temperature regimes rangi ng between 10 and 25 degrees C, it is shown that: (1) no genotype had an absolute basic period as, depending on temperature, durations to an thesis for any one genotype varied by more than 50 days; (2) no genoty pe had an absolute value for intrinsic earliness (to anthesis), rangin g for any genotype by more than 300 degrees Cd depending on temperatur e; (3) basic development rate was not a single value for a genotype bu t varied with stage of development; (4) some genotypes changed their r anking for earliness depending on the temperature regime; and (5) geno types were differentially sensitive to temperature for the subphases p rior to anthesis. We conclude that the earliness descriptors should no t be considered as static genotypic characteristics but as the result of the interaction between the genotype and temperature. Thus, the app arent earliness trait is likely to be related to temperature sensitivi ty.