Daily changes in photoprotective mechanisms were studied in sun leaves of Quercus suber L., Quercus ilex L., Olea europaea L. and Eucalyptus globulus Labill. trees during the summer in Portugal. Even though stomatal closure explained most of the diurnal variation in carbon assimilation along the summer, a decline in the photochemical yield of photosystem II (F′v/F′m) also occurred, as a result of an excess of intercepted solar radiation when carbon assimilation is limited by stomatal closure due to high vapour pressure deficits and/or soil water deficits. These changes were accompanied by the conversion of violaxanthin to antheraxanthin and zeaxanthin which were correlated with thermal dissipation of excess photon energy. In spite of a common general response, differences between species were observed ‐Olea europaea, which is a slow‐growing tree, had the lowest net photosynthetic rates, the highest proportion of carotenoids in relation to chlorophyll and the highest rates of de‐epoxidation of violaxanthin. This enabled a large thermal dissipation of the excess intercepted radiation but led to rather small values of light utilisation for photochemistry (ca 20%). In contrast, in E. globulus, a fast‐growing tree, photosynthetic rates were the highest, thermal dissipation of absorbed radiation the lowest and maximal values of light utilisation for photochemistry reached ca 50%. The two Quercus species exhibited an intermediate response. A high degree of co‐ordination is apparent between stomatal behaviour, photosynthetic capacity and photoprotection mechanisms.

We review the applications of REL test as a technique for detecting injury and thereby forecasting survivability of transplanted seedlings. The objectives of the present review are to present the fundamentals of this method, assess the relevant literature, present evidence of seasonal variations of REL, describe how REL responds to different stress conditions, suggest priorities for future research as well as practical recommendations for REL testing, and assess leakage of organic compounds as an indicator of root damage. Seasonal changes in REL may be connected with root freezing tolerance that varies among plants of different seed sources and species and thus does not always indicate health state of seedlings. REL technique can be used for assessing frost hardiness of roots under certain conditions (e.g. roots should be sampled into the test tubes prior to the freezing test). It can be used for studying heat stress with certain prerequisites (e.g. series of high temperatures or different exposure times should be used to find a threshold for heat tolerance). In desiccation tolerance assessment, REL presents high variation depending on species. In case of rough handling or hypoxic conditions, the effect depends on certain developmental stages. Additionally, though REL may be useful in quantifying damage caused by cold storage, it should be considered as a relative index of plant quality only since the relationship between REL and survivability could vary depending on cultural and handling practices prior to planting as well as on post-planting environmental conditions. In some cases, REL is correlated with field performance of seedlings, but in other cases the correlation is weak. Factors as species, seed lots, developmental stage of root tissue, season, and bud dormancy intensity may affect REL. Thus, REL must first be calibrated to these factors before it can be reliably used to predict the field performance of all types of seedling stock. Ambient storage, ageing and amino acids and protein leakage are also discussed. Limitations and questions for future research are suggested (e.g. speciesdependence and decreasing variation). In general, REL is casually and statistically related with root damage and survivability of seedling. However, it depends on many other factors apart from root damages, and thus it remains a great challenge to improve its reliability.

The effects of warm storage (15 °C) on the physiological responses and subsequent field performance of 2-year-old oak (Quercus robur L.) seedlings that had been cold (1–2 °C) stored until May were assessed. Shoot xylem pressure potential, shoot water content, shoot tip dry mass, root electrolyte leakage, and root growth potential were evaluated at regular intervals during the 21-day warm storage period. The date of lifting to cold storage had little effect on the physiological responses to warm storage. Root electrolyte leakage increased after 7 days of storage, despite the fact that root growth potential and survival were unaffected. Values then increased further before decreasing again by the end of the 21-day storage period to values similar to those recorded after 0 or 7 days of storage. Storage for up to 21 days greatly reduced root growth potential and shoot quality after planting in the field. The shoot xylem pressure potential of seedlings declined during the early stage of warm storage, but then increased to values similar to those recorded after cold storage. Shoot water content increased during the early storage period, then declined slightly, but values were higher after 21 days of storage than at the time of placement in warm storage. While most physiological responses fluctuated in a consistent manner, these changes were not necessarily predictable. Root growth potential was a good indicator of plant field performance potential, but the other physiological parameters were less useful to this end.

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