Measurement of photosynthesis in excised leaves of ornamental trees: a novel method to estimate leaf level drought tolerance and increase experimental sample size

Abstract

Measuring gas exchange on excised ornamental tree leaves may be a rapid and accurate method to study leaf-level drought tolerance, and greatly increase experiment sample size. Drought is a common phenomenon in arid and semi-arid urban environments. Because water is valuable and limited in these environments, it is crucial to study drought tolerance of ornamental tree species used in urban forests. In a first of its kind study, we observed leaf-level drought tolerance using photosynthetic decline curves in excised leaves of seven ornamental urban tree species: Shantung maple (Acer truncatum), Mexican redbud (Cercis canadensis mexicana), Texas redbud (C. canadensis texensis), White redbud (C. canadensis texensis ‘Alba’), Oklahoma redbud (C. canadensis texensis ‘Oklahoma’), Chinquapin oak (Quercus muehlenbergii), and English oak (Q. robur). For both excised and non-excised treatments, two leaves from the same tree were simultaneously clamped into cuvettes of two Li-Cor LI 6400 XT portable photosynthetic systems. For the excision treatment, the leaf was excised from the branch at 120 s. By comparing photosynthetic decline curves from excised leaves with their non-excised counterparts, we found Mexican redbud and White redbud leaves take a longer period of time (450 s. and 360 s., respectively) compared to English oak leaves (90 s.) to show a significant decline in photosynthetic gas exchange. This technique also proposes a solution to a common problem faced by many plant researchers. While using the leaf excision technique on large trees, we were able to significantly increase sample size with rapidity and accuracy. Thus, we suggest measuring gas exchange on excised leaves of select ornamental tree species as a rapid and accurate method to estimate leaf-level drought tolerance, and greatly increase the number of samples measured in a given time period.