Stomatal and non-stomatal limitations to photosynthesis in seedlings and saplings of Mediterranean species pre-conditioned and aged in nurseries: Different response to water stress

The aim of the present work was to compare the physiological responses to water stress and recovery of seedlings and saplings of three different Mediterranean species (Olea europaea var. sylvestris, Rhamnus alaternus and Cneorum tricoccon), pre-conditioning and aged in nursery and presenting different ages and pot sizes. Our hypothesis was that the ratio of plant size to soil volume (which constrains root development leading to low root-to-shoot ratios) rather than any of the two factors separately determines the seedling response to water stress. Seedlings (1-y) and saplings (3 to 4-y) were transplanted into pots bigger than those used during growth in the nursery and irrigation was stopped to each species × age/size combination. Leaf water potential (Ψ), net CO2 assimilation (AN), stomatal (gs) and mesophyll (gm) conductances, and the rate of photosynthetic electron transport (ETR) were determined every few days. Plants were re-watered when AN dropped below 70% of control values.Saplings of each species presented larger total leaf area (TLA) and reached lower Ψ than seedlings. Even under irrigation, saplings showed lower AN, which was not related to gs but to lower gm and ETR. During water stress, AN decreased slowly in seedlings due to stomatal limitations, while in saplings it decreased fast and mainly associated to non-stomatal limitations (gm and ETR). Upon re-watering, seedlings recovered maximum AN within a few days, while recovery was slow and incomplete in saplings. At the end of the experiment, significant leaf die-back occurred in saplings but not in seedlings except for Cneorum. The minimum Ψ achieved during water stress was strongly linearly related to TLA when pooling all species and ages, and leaf die-back was strongly dependent on Ψ and on the appearance of non-stomatal limitations to photosynthesis. Therefore, we conclude that the total amount of leaf area for a given volume of substrate (i.e., maximum water availability), rather than plants pre-conditioning in nurseries or plant age, determines seedling/sapling responses to water stress and re-watering in Mediterranean species.

► Photosynthesis response to water stress differs in young and old seedlings.
► In young, mainly stomatal limitations lead to fast recovery after re-watering.
► In old non-stomatal limitations lead to slow and incomplete recovery.
► The worst performance of old seedlings is due to increased leaf area to root volume.
► Unavoidable plant ageing during hardening has to be considered in afforestation.