PHOTOSYNTHETIC ACTIVITY OF SOME DOMESTIC AND FOREIGN CULTIVARS OF LOLIUM PERENNE L . UNDER CONDITIONS OF SODIUM CHLORIDE SALINITY

In pot experiments carried out in 2005 and 2006, tolerance to sodium chloride salinity of 4 cultivars of perennial ryegrass (Lolium perenne) (‘Nira’, ‘Stadion’, ‘Ronija’, ‘Darius’) was studied. Three concentrations of NaCl in medium (earth + sand): 0.0 mM (control); 50 mM; 100 mM, were used in the investigations. In three successive crops of grass, fresh weight yield of leaves, stomatal conductance, photosynthesis intensity, content of chlorophyll and PS II maximum quantum yield (Fv/Fm) were determined. The obtained results showed that perennial ryegrass is a species tolerant to NaCl salinity. Among the studied cultivars, cv. ‘Ronija’ showed the highest tolerance, whereas cv. ‘Nira’ showed the lowest. The growth of perennial ryegrass plants under salinity conditions was limited by low stomatal conductance of leaves and photosynthesis, but not by the photosynthetic activity of chlorophyll and its contents.


INTRODUCTION
In Poland a mixture of sand and sodium chloride has been used for ice and snow control on roads for several dozen years with good effect.As a result of that, vegetation used for the establishment of lawns within cities and of green belts along transport routes outside cities should exhibit increased resistance to NaCl salinity.S t a w i c k a et al. (2006) report that salinity on urban lawns in Warsaw ranged between 0.3 and 1.2 dS x m -1 , and among monocotyledonous species, red fescue (Festuca rubra), perennial ryegrass (Lolium perenne) and smooth-meadow grass (Poa pratensis) proved to be most useful for the establishment of lawns.The high share of these grass species in the fl oristic composition of Warsaw's lawns is also confi rmed by W y s o c k i (1994).
same time, all these three authors found distinct differences in resistance to NaCl in the studied grass species.Because plants which tolerate a particular type of stress are generally also resistant to other stresses, the present paper compares the response to salinity of two ryegrass cultivars ('Nira' and 'Stadion'), quite commonly used in domestic mixtures, to a Swedish cultivar 'Ronija' used in a more severe climate and a Dutch cultivar 'Darius' used in a milder climate.The sensitivity of these cultivars to salinity was evaluated based on the photosynthetic activity of plants and biomass production.

MATERIALS AND METHODS
The experiments were carried out in a plant house in the period April -October in the years 2005 and 2006.Seeds of two Polish cultivars of Lolium perenne ('Nira', 'Stadion'), the Swedish cultivar 'Ronija' and the Duch cultivar 'Darius' were sown in the amount of 30 seeds of each (3 seeds in 10 points) into 2.0 dm 3 pots fi lled with a mixture of garden mould and river sand in a 3:1 volume ratio.The used earth contains less than 85% of organic substance, its pH in H 2 O is 5.4 -6.0, and it is manufactured by the company Hollas from Pasłęk, Poland.Each of the four cultivars was grown in 15 pots.After emergence, unnecessary seedlings were removed from the pots, leaving 10 plants of each cultivar for further growing, as well as the plants were supplemented with a half-concentration of Hoagland's medium.The second dose at the same amount was applied a week later.At the same time, for each cultivar 3 experiment series were set up which differed in the degree of sodium chloride salinity of the substrate, i.e. 1) control -0.0 NaCl 2) 50 mM dm -3 NaCl 3) 100 mM dm -3 NaCl.Sodium chloride was applied 8 times at 3-day intervals, depending on the level of substrate salinity at a dose of, respectively, 12.5 and 25 mM per pot in the form of aqueous solution.In the years 2005 and 2006 the last dose of NaCl was given, respectively, on 6 and 9 June, and the fi rst grass crop was harvested on June 13 and 16, respectively.The next four crops of grass were harvested at 30-day intervals.Throughout the growth period, the plants were watered with distilled water up to 70% of substrate water-holding capacity.
The present paper presents fresh weight yield of leaves of the studied Lolium perenne cultivars from the fi rst, second and third crop (Tab. 1) as well as results of measurements of photosynthesis intensity (Tab.2), stomatal conductance (Tab.3) and chlorophyll content (Tab.4).Data on maximum quantum yield of PS II in leaves are not included due to the absence of a signifi cant effect of the experiment factors on the value of F v /F m .Since a similar reaction of the investigated culti-vars to the applied salinity levels was observed in both years of study, data presented in the tables are means for the years 2005 and 2006.Photosynthesis intensity and stomatal conductance of leaves were determined using a portable infra-red gas analyser LCA-4 attached to a narrow leaf chamber.Measurements were each time made in the middle parts of leaf blades, 1-3 days before the harvest at noon hours, at an irradiance of 1500-1700 μmol m -2 s -1 .The content of chlorophyll "a+b" was determined using the spectrophotometric method described by A r n o n (1949).Maximum quantum yield of photosystem II (PS II) was determined in the same parts of leaves as for photosynthesis using the Handy PEA fl uorimeter (manufactured by Hensatech Instruments Ltd) after the investigated portions of leaf blades had been earlier obscured for 15 minutes using special clips.Measurement and analysis results were statistically verifi ed using analysis of variance, whereas the signifi cance of differences was determined by Tukey's test at p=0.05.

RESULTS AND DISCUSSION
The results collected in Tab. 1 show that increased substrate salinity resulted in a signifi cant decrease in fresh weight yield of leaves in all three crops of grass.At the salinity of 50 mM, the mean decrease in yield for the four investigated cultivars was 19.1%, compared to the control, and at the salinity of 100 mM as much as 51.2%.Such response of Lolium perenne to the applied stress is confi rmed by numerous authors conducting research both on grasses  2003).However, the mean decrease of biomass yield for 3 grass crops harvested, compared to the control, was similar for the investigated cultivars at the salinity of 50 mM and distinctly different at the salinity of 100 mM, and it was 59.1% for cv.'Nira', 'Stadion' 54.8%, 'Ronija' 37.6%, 'Darius' 47.5%.Signifi cant differences in yielding of cv.'Ronija', compared to the other cultivars, are also confi rmed by statistical analysis.Other authors conducting research on cultivars other than those used in the experiment (Yo n g q i n et al.Irrespective of the level of substrate salinity, fresh weigh yields of grass signifi cantly increased in successive crops.It is undoubtedly associated with the process of adaptation of plants to stress conditions.Because salinity stress reduced substrate water potential, Table 1 Fresh weight yield of leaves (g pot -1 ) of some domestic and foreign cultivars of Lolium perenne from the fi rst, second and third crop.Sodium chloride salinity of ryegrass plants did not also have any effect on primary photosynthetic reactions taking place at the chloroplast level.The measurement of maximum quantum yield of PS II, being the ratio of variable chlorophyll fl uorescence to maximal chlorophyll fl uorescence (F v /F m ), shows that none of the factors applied in the conducted experiment exhibited a signifi cant effect on the value of the trait in question (F v /F m ranged between 0.802 and 0.824).Other authors' studies conducted on different plant species also confi rm the absence of a signifi cant effect of salinity stress on the F v /F m ratio value (M i s h r a et al. 1991; H a v a u x , 1992; J i m e n e z et al. 1997; L i et al. 1999).In their studies conducted on Phillyrea latifolia, T a t t i n i et al. ( 2002) also found that salinity-induced water stress in leaves of this plant did not produce irreversible damage in the photosynthetic apparatus.Therefore, the results demonstrated in this paper show that the negative effect of sodium chloride salinity stress on the growth of perennial ryegrass is primarily associated with a reduction in stomatal conductance of leaves, which affects adversely the photosynthesis and, as a consequence, the biomass increase in plants.

CONCLUSIONS
1. Perennial ryegrass (Lolium perenne) is a species tolerant to sodium chloride salinity; it is manifested by low noxiousness of average concentrations of NaCl in the substrate (50 mM), as well as by a high degree of adaptation to stress conditions during growing (successive grass corps).2. The studied ryegrass cultivars showed different tolerance to salinity stress -the most tolerant was cv.'Ronija', medium tolerant cv.'Darius' and 'Stadion', and cv.'Nira' proved to be the least tolerant.
which was associated with the hindered uptake of water by roots, the plants, on the one hand, increased the osmotic potential of cell sap by increased accumulation of hydrophilic ions (Na + , Cl -) (B o r o w s k i , 2007), and on the other hand, they limited transpiration (unpublished data) through reduced stomatal conductance of leaves.Data contained in Tab. 2 show that the mean stomatal conductance of the control plants (0 mM NaCl) was 0.13 mol m -2 s -1 , whereas at the salinity of 50 mM NaCl it decreased to 0.09, and at the salinity of 100 mM NaCl down to 0.05 mol m -2 s -1 .Hence, in effect, the mean intensity of transpiration of the control plants was The obtained results demonstrated that mean stomatal conductance of ryegrass leaves from the fi rst crop was signifi cantly lower (0.08 mol m -2 s -1 ) than that from the next two crops (0.10 mol m -2 s -1 ), which should be recognised as the effect of adaptation of grass to growth under stress conditions, as well as that mean stomatal conductance of leaves of cv."Ronija" was signifi cantly higher than in the other studied cultivars (Tab.2).The degree of pore opening in Lolium perenne leaves had an effect not only on the rate of diffusion of water vapour particles, but also on the rate of diffusion of CO 2 into the leaf interior, which was one of the conditions for effi cient photosynthesis.CO 2 assimilation was the fastest in the control plants (8.98 μmol CO 2 m -2 s -1 ), at the salinity of 50 mM NaCl there was an almost 19% decrease in intensity of this process, and at the salinity of 100 mM the decrease was over 50%.The CO 2 fi xation rate in plants is affected not only by the degree of stomata opening, but also by the chlorophyll content in leaves and the rate of primary photosynthetic reactions at the photosystem level.The results collected in Tab. 4 show that the growing level 3.The growth of perennial ryegrass plants under salinity conditions is limited by a decrease in stomatal conductance of leaves and photosynthesis, and not by the photosynthetic activity of chlorophyll (F v /F m ) and its content in leaves.
(H o r s t and D u n n i n g , 1989; R a s m u s o n and A n d e r s o n , 2002; K h a l e g h i and R a m i n , 2005; A k r a m et al. 2007) and on other cultivated plant species (S e em a n n and C r i t c h l e y , 1985; A l d e s u q u y and I b r a h i m , 2001; D a s et al. 2002; B o r o w s k i , 2003; H u j u n et al. 2001; P a w l u s k i e w i c z , 2000) also confi rm clear differences in tolerance to salinity of various ryegrass cultivars.
Many other authors conducting research on both grasses (R a m u s o n and A n d e r s o n , 2002; A k r a m et al. 2007) and dicotyledonous plants (S e e m a n n and C r i t c h l e y , 1985; L i et al. 1999; D a s et al. 2002; B o r o w s k i , 2003) found a clear decrease in the photosynthesis process under conditions of plant salinity.Irrespective of the degree of salinity and dates of harvesting, CO 2 was fi xed most effi ciently in leaves of cv.'Ronija' ryegrass plants, signifi cantly weaker in cv.'Darius' and 'Stadion', and the weakest in cv.'Nira' (Tab.3).
Lolium perenne from the fi rst, second and third crop.

Table 3
Photosynthesis intensity of leaves (μmol CO Lolium perenne from the fi rst, second and third crop.) in leaves of some domestic and foreign cultivars of Lolium perenne from the fi rst, second and third crop.