Periodic flooding of perennial crops such as lucerne (Medicago sativa, L) is a major cause of lowered productivity and leads in extreme cases to plant death. In this study, effects of waterlogging and subsequent recovery on plant nutrient composition and PSII photochemistry were studied to gain a better understanding of the mechanisms of recovery as they relate to leaf photochemistry (chlorophyll fluorescence) and nutrient dynamics. Three lucerne cultivars and one breeding line were flooded for 20 d, drained and left to recover for another 16 d under glasshouse conditions. Leaf and root nutrient composition (P, K, Ca, Mg, B, Cu and Zn) of waterlogged lucerne was significantly lower than in freely drained controls, leaf N concentrations were also significantly lower in waterlogged lucerne. At the same time, there were significantly (5-fold) higher concentrations of Fe in waterlogged roots and Na in leaves (2-fold) of stressed plants. PS II photochemistry, which was impaired due to waterlogging, recovered almost fully after 16 d of free drainage in all genotypes. Alongside fluorescence recovery, concentrations of several nutrients also increased in recovered plants. Growth parameters, however, remained suppressed after draining. The latter was due to both the smaller capacity of CO 2 assimilation in previously waterlogged plants (caused in part by nutrient deficiency and associated inhibition of PSII) and the plant's need to re-direct available nutrient and assimilate pools to repair the damage to the photosynthetic apparatus and roots. It is concluded, that for any lucerne-breeding program it is important to determine not only the degree of tolerance to waterlogging but also the potential for recovery of different genotypes, as well as look for 'outstanding individuals' within each population.Abbreviations: Ci -substomatal cavity CO 2 concentration; ETR -electron transport rate; Fo -minimal fluorescence; Fm -maximal fluorescence; Fv -variable fluorescence; NPQ -non-photochemical quenching; qP -photochemical quenching; qN -non-photochemical quenching; PAM -pulse amplitude modulation; WLwaterlogging; SDW shoot dry weight; RDW -root dry weight.

Waterlogging is a serious environmental stress on lucerne (Medicago sativa) affecting its agronomic performance. To facilitate the breeding process, efficient tools to screen a population of lucerne cultivars are needed. In this study, a comparative analysis of waterlogging effects on leaf photosynthesis, pigment composition, PSII photochemistry, and plant growth characteristics was undertaken using four different lucerne cultivars (Aurora, Hunter River, L153 and Sequel HR). Two-month-old plants, grown in half-strength Hoagland nutrient solution, were waterlogged for 16 days, and plant physiological characteristics were monitored at regular intervals (every few days). All cultivars had significantly reduced fresh and dry weight for both shoots and roots after 16 days of waterlogging. Root biomass showed a greater percentage of reduction than did shoot biomass. As waterlogging stress developed, chlorophyll content, CO2 assimilation rate, transpiration rate, stomatal conductance and maximal quantum efficiency of PSII (Fv/Fm) decreased significantly. Chlorophyll a and b content gradually decreased over the time of the experiment in the stressed cultivars, and leaf chlorosis became increasingly evident. Although most of the parameters showed significant changes as waterlogging progressed, limitations render some of them inapplicable for screening. It is concluded that for practical screening purposes, the Fv/Fm ratio is the most appropriate. A significant difference between control and waterlogged plants became evident as early as day 7. Possible physiological mechanisms involved are discussed.

Salinity tolerance is a complex trait inferring the orchestrated regulation of a large number of physiological and biochemical processes at various levels of plant structural organisation. It remains to be answered which mechanisms and processes are crucial for salt tolerance in lucerne (Medicago sativa L.). In this study, salinity effects on plant growth characteristics, pigment and nutrient composition, PSII photochemistry, leaf sap osmolality, changes in anatomical and electrophysiological characteristics of leaf mesophyll, and net ion fluxes in roots of several lucerne genotypes were analysed. Salinity levels ranged from 40 to ~200 mm NaCl, and were applied to either 2-month-old plants or to germinating seedlings for a period of between 4 and 12 weeks in a series of hydroponic, pot and field experiments. Overall, the results suggest that different lucerne genotypes employ at least two different mechanisms for salt tolerance. Sodium exclusion appeared to be the mechanism employed by at least one of the tolerant genotypes (Ameristand 801S). This cultivar had the lowest leaf thickness, as well as the lowest concentration of Na+ in the leaf tissue. The other tolerant genotype, L33, had much thicker leaves and almost twice the leaf Na+ concentration of Ameristand. Both cultivars showed much less depolarisation of leaf membrane potential than the sensitive cultivars and, thus, had better K+ retention ability in both root and leaf tissues. The implications of the above measurements for screening lucerne germplasm for salt tolerance are discussed.

Salinity affects many physiological processes at all levels of plant structural organization. Being a physiologically and genetically complex trait, salinity tolerance implies a coordinated contribution of multiple mechanisms, making plant screening for salt tolerance extremely difficult. In this work, we show how the use of excised leaves can fulfill that task. We argue that, by adding NaCl directly to the transpiration stream, the protective effects of several mechanisms regulating Na(+) delivery to the shoot are eliminated, enhancing PSII exposure to salinity treatment and resulting in a significant decline in leaf photochemistry (Fv/Fm characteristics). We suggest that measuring Fv/Fm characteristics on excised salt-treated leaves provides an opportunity to evaluate the efficiency of vacuolar Na(+) compartmentation, arguably the most important feature for salt tolerance. We also explain the observed decline in Fv/Fm values as salt-induced structural damage to chloroplasts caused by oxidative stress.