Aims Relative growth rate (RGR) is an indicator of the extent to which a species is using its photosynthates for growth and it is affected by environmental factors, including temperature. Nevertheless, most of plant growth studies have been carried out at a single growth temperature or at different temperature treatments, resulting in the lack of information on the relationship between RGR and changing mean daily air temperature. We analyzed the temporal changes in RGR during early growth stages in three Cistus species grown outdoor in a common garden from seeds of different provenances. Moreover, we wanted to define the relationship between daily changes in RGR and mean daily air temperature for the considered provenances. The hypothesis that intra-specific temporal variations in RGR can reflect differences in the behavior to maximize RGR (RGR max) in response to temperature was tested.
Methods Seedlings of C. salvifolius, C. monspeliensis and C. creticus subsp. eriocephalus were grown outdoor in the experimental garden of the Sapienza University of Rome under a Mediterranean climate. We analyzed early growth with non-linear growth models and calculated function-derived RGRs as the derivative with respect to time of the parameterized functions used to predict height divided by current height. The relationships between function-derived RGRs and mean daily air temperature were analyzed by linear and non-linear models, which were ranked according to their standard errors and correlation coefficients. The temperature dependency of RGR max per each provenance was evaluated through the relationship between RGR max and the coefficients of the best regression model obtained.
Important findings A parameter that could summarize the temperature dependency of RGR up to RGR max during the early growth stages for the selected provenances was defined. This allowed us to highlight that a greater RGR temperature responsiveness was related to a delay in the time to reach RGR max independently by the species. Nevertheless, a greater temperature sensitivity of RGR lead to a reduced maximum height which reflects a negative trade-off between the length of the developmental phases and the extent of RGR temperature responsiveness. Thus, variations in temperature responsiveness of RGR up to RGR max have a significant role in shaping the early growth for the investigated species. Our findings quantitatively define provenance dependent strategies by which the selected species cope with daily air temperature variations during early growth.