Nitrogen Accumulation in Lucerne ( Medicago sativa L . ) Under Water Deficit Stress

In order to study nitrogen accumulation in aboveground and root dry mass in lucerne (Medicago sativa L.) under water deficit stress, a pot experiment was carried out at the Institute of Forage Crops, Pleven, Bulgaria. The plants were grown under optimum water supply (75-80% FC) and 10-days water deficit stress was simulated at the stage of budding by interrupting the irrigation until soil moisture was reduced to 37–40% FC. Mineral nitrogen fertilization (ammonium nitrate) at the doses of 40, 80, 120 and 160 mg N kg soil was applied. It was found that nitrogen accumulation in dry aboveground mass was reduced to 18.0%, and in dry root mass to 26.5% under water deficit stress. Mineral nitrogen fertilization contributed to easily overcome the stress conditions of water deficit stress in lucerne.


Introduction
Lucerne (Medicago sativa L.) is a nitrogen-fixing crop with a capability to accumulate significantly more nitrogen in their biomass ( Jarvis 2005).The application of additional nitrogen is debatable in the literature due to its nitrogen-fixing ability (Oliveira et al. 2004, Werner & Newton 2005).Unfavourable factors limit multiplication frequently.Although potential to tolerate acute conditions of water deficit stress (Busse & Bottomley 1989, Solanki & Patel 2000), due to the deeper root system, seed yield in lucerne is considerably influenced by lack of moisture (Shock et al. 1989, 2001, Frame et al. 1998) and decreased from 18 to 33% (Vasileva et al. 2006).Because of greater necessity of C and N assimilates during the reproductive stage nitrogen accumulation is an important indicator and one of the limiting factors for plant growth and seed yield formation (Serraj at el. 1999).The aim of this work was to study nitrogen accumulation in lucerne under water deficit stress.
Samples were taken when lucerne was harvested for seeds.Dry aboveground mass and dry root mass were recorded (dried at 60 о C).Nitrogen in aboveground and root dry mass was calculated as a product of aboveground and root dry mass yield and total nitrogen content (determined by standard Kjeldahl method in dry mass) (Nelson & Sommers 1980).Total nitrogen accumulation was calculated as the sum of that in aboveground and root mass.Reduction in percentage of nitrogen accumulation after water deficit stress was determined using the following formula: The data were statistically processed by the LSD using SPSS 10.0 computer program.

Results and Discussion
Lucerne used soil or fertilizer nitrogen, since nitrogen assimilation required lower rates of СО 2 and energy than nitrogen fixation in starting development (Hartwig & Soussana 2001).This reflects on the nitrogen accumulation in plants.
In our study nitrogen accumulation in dry aboveground mass under optimum water supply increased with the increase of doses of mineral nitrogen fertilization (Table 1).For the experimental doses of 40 and 80 mg N kg -1 soil, the increase as compared to unfertilized control was by 6.7 and 9.5% and for the doses of 120 and 160 mg N kg -1 soil by 22.3 and 24.2%, respectively.Doses of 40 and 80 mg N kg -1 soil did not significantly affect nitrogen accumulation in dry aboveground mass under water deficit stress.For doses of 120 and 160 mg N kg -1 soil there was an increase in nitrogen accumulation as compared to unfertilized control by 6.1 and 13.8%, respectively.
Nitrogen accumulation in dry root mass decreased to 35.1% with increase of doses of mineral nitrogen fertilization under optimum water supply (Table 2).It is related to the directing of assimilates to leaf mass and needs of nitrogen for seeds with high protein content (Tufenkci et al. 2006).
Under the conditions of water deficit stress for the doses of 120 and 160 mg N kg -1 soil, nitrogen in dry root mass decreased by 24.9 and 30.7%, respectively.
Our results showed that plants insured with nitrogen easily overcome water deficit stress.In confirmation of that are the findings of Radeva (1991) and Antolin et al. (1995) according to which lucerne plants fertilized with mineral nitrogen and subjected to water deficit stress showed better growth and productivity as compared to these depend only on the nitrogen of symbiotic N 2 fixation.Nitrogen is important for growth and formation of seeds with high protein content and water deficit affects supply of plants with nitrogen.Total nitrogen accumulation (N in dry aboveground + N in dry root mass) varied in narrow range and was less affected by mineral nitrogen fertilization under both conditions of water supply (Figure 1).
Although lucerne is more tolerant than most of forage legumes, some characteristics like nitrogen in yield are influenced negatively by the water deficit stress (Serraj et al. 1999, Zahran 1999, 2001, Humphries & Auricht 2001).Root mass of the plants was more sensitive to water deficit stress as compared to aboveground mass.Nitrogen accumulation in dry aboveground mass decreased to 18.0%, but in dry root mass to 26.5% (Table 3).
Some authors (Cihacek 1994, Petkova 1994) consider lucerne needs nitrogen despite its nitrogen-fixing ability.This is particularly true for the stress conditions of water deficit.Water deficit stress strongly reduced nitrogen in dry root mass in the plants of unfertilized control (by 23.3%).The decrease for doses of 80 and 120 mg N kg -1 soil was 13.0 and 16.0%, respectively.Under experimental doses of 80 and 120 mg N kg -1 soil lucerne showed a slightly overcome the negative effect of water deficit stress in terms of nodulation and dry root mass (Vasileva 2010).Water deficit stress reduced total nitrogen accumulation (N in dry aboveground +N in dry root mass).This decrease was lowest (11.9%) in treatment subjected to 80 mg N kg -1 soil.

Conclusions
Water deficit stress decreased nitrogen in dry aboveground mass to 18.0% and nitrogen in dry root mass to 26.5%.Mineral nitrogen at a dose of 80 mg N kg -1 soil contributed to easily overcome the stress conditions of water deficit in lucerne.

Table 1 .
Nitrogen accumulation in dry aboveground mass in lucerne

Table 2 .
Nitrogen accumulation in dry root mass in lucerne

Table 3 .
Percent of reduction in nitrogen in dry mass yield in lucerne after water deficit stress