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).