Vulnerability of Xylem Vessels to Cavitation in Sugar Maple. Scaling from Individual Vessels to Whole Branches

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Abstract
The phytochromes are one of the means via which plants obtain information about their immediate environment and the changing seasons. Phytochromes have important roles in developmental events such as the switch to flowering, the timing of which can be crucial for the reproductive success of the plant. Analysis of phyBmutants has revealed that phyB plays a major role in this process. We have recently shown, however, that the flowering phenotype of thephyB monogenic mutant is temperature dependent. A modest reduction in temperature to 16°C was sufficient to abolish thephyB mutant early-flowering phenotype present at 22°C. Using mutants null for one or more phytochrome species, we have now shown that phyA, phyD, and phyE, play greater roles with respect to phyB in the control of flowering under cooler conditions. This change in the relative contributions of individual phytochromes appears to be important for maintaining control of flowering in response to modest alterations in ambient temperature. We demonstrate that changes in ambient temperature or photoperiod can alter the hierarchy and/or the functional relationships between phytochrome species. These experiments reveal new roles for phyD and phyE and provide valuable insights into how the phytochromes help to maintain development in the natural environment.