Cool Temperature and Water Deficit Interact during Floral Induction in Citrus

Abstract

Cool temperatures (<20 °C) and water deficits have long been known to induce flowering in citrus trees. However, there have been no reports of the combined effects of these two factors in the literature in spite of cool temperatures and water deficit usually occurring naturally together in many citrus-producing regions of the world. We characterized the flowering responses of two citrus species to combined floral-inductive cool temperatures and water deficits under growth chamber and field conditions. Interactions of cool (inductive) and warm (non-inductive) temperatures with water deficit were tested in two separate growth chamber experiments. In both experiments a statistical interaction between the effects of temperature and water deficit was detected. As a result of this interaction, mild water deficit increased the number of inflorescences induced on trees under a thermo-period of 23/18 °C (day/night) compared to trees at 15/10 °C (day night) whereas moderate water deficit increased the number of inflorescences formed at 15 °C (no day/night variation) compared to trees at 23 °C. In the field, water deficit was induced by withholding irrigation or by covering the soil with an impermeable cover during the fall/winter in three different seasons in sweet orange and grapefruit trees. Field trees under water deficit consistently produced more inflorescences than well-irrigated trees. Our results support the hypothesis that low temperature and water deficit interact during floral induction in citrus and that water deficit could be used to manipulate flowering in field trees.