Polyphasic chlorophyll a fluorescence represents a promising tool for detection of plant tolerance to various environmental stresses. In pot vegetation experiments, plants of seven winter wheat varieties were screened for their drought tolerance. The drought stress was initiated in plants by withholding water at the beginning of anthesis. While water content was measured continuously as relative water content (RWC), fast chlorophyll a fluorescence kinetics was measured and analysed on dehydrating intact leaves by the JIP-test (analysis of O-J-I-P fluorescence transient). Maximum quantum efficiency of PS II photochemistry (F V /F M ) parameter was almost unaffected by dehydration until the severe water stress occurred. In contrast to this a continuous decrease of performance index (PI abs ) parameter (Strasser et al. 1995) was observed from the very beginning of dehydration following the decrease of RWC. Statistically significant differences were also found in the PI abs parameter among all tested varieties. The results show that PI abs may serve as an index of plant/variety vitality and/or sensitivity to water stress reflecting their different drought tolerance.

Nitrogen deficiency influences importantly the plant photosynthetic capacity and crop productivity. Here, we employed the rapid, non-invasive measurements of chlorophyll a fluorescence kinetics for calculation of the integrative fluorescence parameters related to the leaf photosynthetic performance. In pot experiments with winter wheat (Triticum aestivum L.) we cultivated plants during the whole growing period in the soil substrate supplied with four different doses of nitrogen. The leaf nitrogen and chlorophyll content as well as the plant dry mass were analyzed after chlorophyll fluorescence records in three growth stages. Our results indicate that the commonly used parameter F v /F m (the maximum quantum yield of photochemistry) was almost insensitive to nitrogen treatment. In contrary, the performance index (PI abs ) and total performance index (PI tot ) were much more responsive and significant differences among plants of different nitrogen treatments as well as between the youngest and third leaf from the top were observed. Parameter PI tot was shown to express only small diurnal changes, thus being more reliable and more useful for comparison of different samples in field conditions than more frequently used parameter PI abs .

Photosynthesis limitation by CO2 flow constraints from sub-stomatal cavities to carboxylation sites in chloroplasts under drought stress conditions is, at least in some plant species or crops not fully understood, yet. Leaf mesophyll conductance for CO2 (gm) may considerably affect both photosynthesis and water use efficiency (WUE) in plants under drought conditions. The aim of our study was to detect the responses of gm in leaves of four winter wheat (Triticum aestivum L.) genotypes from different origins under long-term progressive drought. Based on the measurement of gas-exchange parameters the variability of genotypic responses was analyzed at stomatal (stomata closure) and non-stomatal (diffusional and biochemical) limits of net CO2 assimilation rate (AN). In general, progressive drought caused an increasing leaf diffusion resistance against CO2 flow leading to the decrease of AN, gm and stomatal conductance (gs), respectively. Reduction of gm also led to inhibition of carboxylation efficiency (Vcmax). On the basis of achieved results a strong positive relationship between gm and gs was found out indicating a co-regulation and mutual independence of the relationship under the drought conditions. In severely stressed plants, the stomatal limitation of the CO2 assimilation rate was progressively increased, but to a less extent in comparison to gm, while a non-stomatal limitation became more dominant due to the prolonged drought. Mesophyll conductance (gm) seems to be a suitable mechanism and parameter for selection of improved diffusional properties and photosynthetic carbon assimilation in C3 plants, thus explaining their better photosynthetic performance at a whole plant level during periods of drought.

An effect of nitrogen rates (0.0 g, 1.0 g, 2.0 g N per pot) on NRA (nitrate reductase activity) in leaves of spring barley (cultivar Kompakt) was investigated in a pot experiment. Plants were grown under optimum moisture regime and drought stress was induced during the growth stages of tillering, shooting and earing. Before and after respective stress period plants were grown under optimal water regime. In all the fertilized and unfertilized treatments, NRA was significantly higher under optimal water regime than in drought stress conditions. Nitrogen fertilization alleviated adverse effects of drought stress on the yields of grain; the rate of 1 g N per pot increased the grain yield of plants stressed during tillering 3.73 times compared to unfertilized and stressed treatment. When the stress was induced during shooting or earing grain yields declined by over 50% compared to optimal water regime; when compared with stressed and unfertilized treatment, the rate of 1 g N however increased yield by 29% (stress at shooting) and 55% (stress at earing). NRA values were significantly higher when plants were grown under optimum water regime than under stress conditions as well as when fertilized with nitrogen compared to unfertilized control both under optimum water regime and drought stress.

Nitrogen deficiency strongly influences crop photosynthetic performance and biomass production. In this study, we applied rapid, non-invasive records of chlorophyll a fluorescence kinetics for evaluation of wheat (Triticum aestivum L.) leaf photosynthetic performance. Plants were grown during the whole season in big pots filled with the soil substrate and they were supplied by different doses of nitrogen nutrition. The plant nitrogen and leaf chlorophyll content as well as the plant aboveground biomass were analyzed after chlorophyll fluorescence records in three growth stages. Our results confirmed that the maximum quantum yield of photochemistry frequently used in previous studies was almost insensitive to nitrogen treatment, if measured in young, non-senescent leaves. On the contrary, the performance index was much more responsive. The performance index values derived from parallel measurements in the youngest and third leaf from the top were used to calculate performance index leaf ratio, which correlated well with the plant nitrogen content. Moreover, this parameter was not sensitive to midday depression, which significantly affected the values of other chlorophyll fluorescence parameters. Although the optimum performance index leaf ratio values decreased continuously in the second part of the growing season, the optimum and critical values for each growth stage can be easily identified. Thus, we suggest performance index leaf ratio as the simple parameter for the rapid evaluation of the wheat photosynthetic performance and an early indicator of insufficient nutrition level. Unlike some other approaches, suggested method seems to be useful also in conditions of a moderate nutrient deficiency.