Experiment 1: ploidy effects on leaf physiology and its
response to stress
Differences in ploidy strongly affected leaf physiological traits (Fig.
3a, Appendix 3, left panels). Averaged across defoliation treatments,
well-watered triploids exhibited higher photosynthetic rates (11%),
stomatal conductance (8%), V cmax (23%) and
estimated levels of the quantum efficiency of photosystem II (14%) than
diploid trees.
Genotype significantly affected variation of all physiological traits,
except for V cmax (Appendix 6). Diploid and
triploid trees differed in net photosynthesis, the quantum efficiency of
photosystem II, and rubisco activity even after accounting for genotypic
variation (Fig. 3a, Appendix 3, right panels).
For trees that were well-watered, none of the leaf physiological traits
was affected by defoliation and no ploidy × defoliation interactions
were observed (Fig. 3a, Appendix 3). For trees that were exposed to
drought stress, we found similar vulnerability curve progressions for
diploid and triploid trees of the same defoliation treatment. No
significant impact of ploidy or ploidy × defoliation interactions was
detected for any curve parameter (Fig. 3b). However, defoliation
treatments per se significantly affected net photosynthesis responses to
drought. Photosynthesis rates of undefoliated trees plummeted when
experiencing ΨLEAF of -7 bars or lower. In contrast, photosynthesis
rates of defoliated trees decreased more gradually. Defoliated trees
were still photosynthetically active, even when experiencing severe
drought stress. Correspondingly, we found significant differences
between curve parameters describing the defoliated and undefoliated tree
curves. These differences were observed in the model not accounting for
(results not shown), as well as in the model accounting for, genotypic
variation (Fig. 3b).