Turgor limitation of tree height: no evidence from leaf epidermal cell anatomy
HighlightLeaf cell size distributions failed to show key signatures of the turgor limitation of tree height; instead, it is more likely that natural selection favors the cell sizes plants display. The turgor limitation hypothesis (TLH) predicts that reduced turgor caused by gravity and hydraulic resistance at the treetop makes cell expansion beyond a certain size impossible. This environmentally-imposed upper limit on cell size should be manifest as an abrupt right-hand threshold in cell size distributions. Selection is "limited" from reaching the favored cell size, which remains on the right-hand, inaccessible, side of the developmental threshold. The TLH thus predicts that as trees grow taller, treetop leaf cell size distributions should become increasingly narrow-as selection pushes as close as possible toward the inaccessible, favored values- with a right-hand threshold. To test this, we sampled 58 individuals of two tree species across different heights, along with 24 canopy-dominant rainforest species. We measured guard, subsidiary, and pavement cell lengths from apicalmost leaves. Contrary to TLH predictions, leaf cell dimension distributions were symmetrical, without right-hand thresholds, even in the tallest trees. Regression analyses mostly revealed positive or non-significant relationships between cell size, skewness, variance, and tree height, rather than the predicted negative trends. Our findings suggest that, instead of turgor-imposed "limitation," it is more plausible that trees produce the heights, leaf sizes, and cell dimensions favored by selection in a given environment.