Phenotypic plasticity to altered apical bud temperature in Cucumis sativus: more leaves-smaller leaves and vice versa

Many studies investigated temperature effects on leaf initiation and expansion by relating these processes to air temperature or the temperature of a specific organ (e.g. leaf temperature). In reality plant temperature is hardly ever equal to air temperature or spatially uniform. Apical bud temperature (Tbud), for example, may greatly differ from the temperature of the rest of the plant (Tplant) dependent on the environment. Recent research in Cucumis sativus showed that Tbud influences leaf initiation independent of Tplant. These findings trigger the question if such spatial temperature differences also influence leaf expansion and plant phenotype. In a 28 day study, we maintained temperature differences between Tbud and Tplant ranging from −7 to +8 °C using a custom-made bud temperature control system. Leaf expansion did not only depend on leaf temperature but also on the difference between bud and leaf temperature. Differences between Tbud and Tplant considerably influenced vertical leaf area distribution over the shoot: increasing Tbud beyond Tplant resulted in more and smaller leaves, while decreasing Tbud below Tplant resulted in less and larger leaves. The trade-off between leaf number and leaf area resulted in phenotypic alterations that cannot be predicted, for example, by crop models, when assuming plant temperature uniformity.