A model estimating the degree of floral transition in an evergreen perennial exposed to warm periods during winter

Increasing winter temperatures jeopardize the yield of fruit trees requiring a prolonged and sufficiently cold winter to flower. Assessing the exact risk to different crop varieties is the first step in mitigating the harmful effect of climate change. Here, we focus on olive (Olea europaea) – a traditional crop in the Mediterranean basin. Olive flowering depends on the sufficiency of cold periods and the lack of warm ones during the preceding winter. Yet, a satisfactory quantitative model forecasting its expected flowering under natural temperature conditions is still lacking. We empirically tested the effect of different temperature regimes on olive flowering intensity and flowering-gene expression. We constructed a dynamic model, describing the response of a putative flowering factor to the temperature signal. The crucial ingredient in the model is an unstable intermediate, produced and degraded at temperature-dependent rates. Our model accounts not only for the number of cold and warm hours but also for their order. We used empirical flowering and temperature data to fit the model parameters, applying numerical constrained optimization techniques, and successfully validated the model outcomes. Our model more accurately predicts flowering under winters with warm periods yielding low-to-moderate flowering and is more robust compared to previous models.

Methods
Hourly temperature data in the different experiment sites starting from 15/10 each autumn and ending at floral evocation (dates vary between the sites).