Hard and tough: the coordination between leaf mechanical resistance and drought tolerance

Leaves of terrestrial plants vary tremendously in size, shape, and stiffness. Hard leaves with high mass per unit area (LMA) occur more frequently in water- and nutrient-limited habitats, where stress-tolerance is key to survival. Coordination between leaf mechanical properties and drought tolerance can arise because some (but not all) of the anatomical modifications leading to high LMA are mechanistically correlated to physiological traits conferring tolerance to dehydration. Thick cell walls are frequently coupled with low turgor loss point values, and also prevent cell shrinkage and collapse after turgor loss has occurred. This also protects the leaf against shrinking-induced loss of water transport capacity in the extra-vascular pathway. In turn, vascular water transport in the leaf veins is at risk of embolism-induced disruption under drought, but higher length of major veins per unit area can provide alternative pathways for water delivery to the mesophyll, while contributing to increased leaf stiffness. Water shortage in dry and warm habitats implies the risk of reduced photosynthetic rates, apparently favouring species that produce hard and drought-tolerant leaves, to warrant long leaf life span and to maintain gas exchange and positive carbon gain when conditions become harsh.