Ecological stress memory: Evidence in two out of seven species through the examination of the relationship between leaf fluctuating asymmetry and photosynthesis

Increased morphological fluctuating asymmetry is considered as an indication that the extent of a natural stress has surpassed the ability of the genotype to maintain the normal symmetry in the body plan. Young leaves can suffer by a variety of stresses, such as nutrient deficiencies, drought, heat and cold in combination with high light. Therefore, increased morphological fluctuating asymmetry in mature leaves may be considered as an ecological indicator of such environmental pressures. Hence, deviations from symmetry in a mature leaf may be a reminder of an adversity having occurred when the leaf was young and developing. We argued that a past stress may stigmatize future photosynthesis and we searched for correlations between leaf fluctuating asymmetry and photosynthetic parameters obtained from fast chlorophyll a fluorescence rise curves in mature leaves from seven tree species. The results were species-specific, with four species (Arbutus unedo, Cercis siliquastrum, Platanus orientalis and Populus alba) showing both maximum quantum yield of photosystem II and photosynthetic performance index (PItotalsensu Strasser) to be independent of the alleged juvenile stress, while in one species (Ceratonia siliqua) the two parameters were negatively correlated to leaf fluctuating asymmetry. Interestingly, in two species (Olea europaea and Pistacia lentiscus), higher leaf asymmetry was linked with lower risk of chronic photoinhibitory damage and higher photosynthetic capacity. An ‘ecological stress memory’ may be inferred for some species improving their tolerance to future adversities and global change.