From fossils to physiology: testing the functional significance of leaf shape

Abstract

Leaf morphological diversity has captured the attention of botanists for hundreds of years and while several correlative relationships between leaf form and climate have been confirmed through experimental testing, much of the adaptive significance is still an open question. We consider this link to be essential to the fields of plant physiology, cell and developmental biology, paleobotany, and abiotic stress physiology. Gaining a more complete understanding of how plants inherently acclimate and over time adapt to new environments can be approached by merging the science of two fields: 1) the paleobotanical approach of identifying morphological traits from fossils for climate reconstruction, and 2) the physiological approach for testing mechanistic hypothesis suggested from correlations. This merged approach will reveal how leaf shape contributes to overall plant performance and could yield considerable insight into species range-shifts and ecosystem flux due to future climate instability. It is also a novel way to confront inefficiencies and stress resiliencies within crops. Leaf shape is largely determined by venation patterning, hardwired in many species, due to biomechanical limitations determining structure and physiological limitations determining resource movement. A subset of wild and domesticated tomato leaflets with a range of margin shapes (toothed, entire) was used to test the hypothesis that this morphological form provides a functional boost in the form of increased gas exchange and resilience during applied stress from water deficit. Here we show that leaf teeth in tomato do provide a disproportionately higher photosynthetic rate per area, and this pattern persists even as the water potential gradient across the leaf increases in magnitude suggesting margin shape, and not the species, underlies this pattern. Lastly, we show that leaf hydraulic conductance and its coordinated response with gas exchange and vein structural traits give insight into plant adaptations to abiotic stress. The leaf shape, as determined by its vascular pattern and structural traits therein, is a critical point of resistance along the water pathway through the plant, and here we show that form matches functional potential as the venation traits, particularly vein surface area, is a strong predictor of maximum rates of gas exchange and hydraulic conductance.