MINERAL NUTRITION OF Cymbopogon nardus ( L ) Rendle : PART II . EFFECTS OF MAGNESIUM AND PHOSPHORUS NUTRITION ON THE FRACTIO ~ AL COMPOSITIO ~ OF ESSENTIAL OIL .

Results obtained from a fertilizer experiment conducted with thirteen treatments indicate the positive effect of phosphorus and magnesium nutrition on the yield and the fractional composition of essential oil of Cymbopogon nardus. Phosphorus nutrition has increased the "total geraniol" (total acetylisable compounds) in oil. Magnesium in combination with phosphorus fertilizers significantly increased the total geraniol at the second harvest. The increase of total geraniol was always accompanied by the decrease of total hydrocarbons, which indicates the better utilization of monoterpenes in biosyntheis of constituents of total geraniol in oil as a result of phosphorus nitrition. Phosphorus content in leaves has positive relationships with total geraniol (r=+O.5), essential oil yield (r= +0.58) and output of total geraniol (r=+0.59). The optimum phosphorus concentrations in leaves after four months of planting that produced maximum yields of essential oil and total geraniol by this crop were found to be in the range of 0.23-0.26 %.


Introduction
Citronella oil is the essential oil obtained from citronella grass which is botanically classified into two species: Cymbopogon winterianus Jowitt and Cymbopogon nardus (L) Rendle (Abeywikreme, 1959;Guenther, 1950).Sri Lanka is one of the major citronella oil producing countries and the "Ceylon citronella" species, Cymbopogon nardus has been cultivated commercially in Sri Lanka for over a century (Brown and Matthews, 1951).
These two citronella species are distinguished morphologically, anatomically (Iruthayathas et. al, 1977) and chemically (Jowitt, 1908;Wijesekera, 1973).One of the striking differences observed was the presence of monoterpene hydrocarbons amounting to more than 20% of the oil in C. nardus as against 3-4 % in C. winterianus.Cymbopogon winterianus contained around 85 % total acetylisable compounds expressed as "total geraniol" which determines the market quality of citronella oil (Brown and Matthews, 1951).Cymbopogon nardus on the other hand was reported to contain 55-65% total alcohols (Wijesekera, 1973).
Several posphorylation reactions are involved in the biosynthesis of essential oils in plants.For example' the conversion of mevalonic acid to squalene in yeast was shown to be dependant on adenosine triphosphate (ATP) and reduced pyridine nucleotides and magnesium ions (Hanson, 1967).Possible operation of an essential oil biosynthetic pathway via neryl or geranypyrophosphate in Cymbopogon nard us has been suggested (Wijesekera, 1973).
The positive effect of magnesium and phosphorus nutrition on the essential oil production by Cymbopogon nardus has been reported (Ranaweera and Thilakaratne, 1992).However, no recorded information is available on the effects of phosphorus and magnesium nutrition on the fractional composition of citronella oil.As such, this study was undertaken to examine the effects of phosphorus and magnesium nutrition of Cymbopogon nardus, on the fractional composition of essential oil in view of improving its market quality.

Materials and Methods
Experimental Procedure: A fertilizer experiment was conducted according to the scheme indicated in Table 1.The experiment had thirteen fertilizer treatments and eight replicates.The surface 20 cm. of Yellow podsolic soil (pH=5.1)with low phosphorus (5.7mg P 2 0S/lOOg soil) and magnesium (lOmg Mg/IOOg soil) content, was air dried, sieved and mixed with appropriate fertilizers according to different fertilizer treatments.Each pot was filled with 3kg of soil and planted with two slips of Cymbopogon nardus and later thinned down to one.The crop was harvested twice.The first harvest was made after four months and the second harvest after seven months of planting.For harvesting, the plant parts above ground was cut.The total phosphorus content in leaves was determined colorimetrically by vanadomolybdate procedure (Peterburski, 1968).The Essential oil content of leaves was determined by steam distillation, using a Cleavanger distillation apparatus and the oil content was expressed in c1eavanger units (Brain and Turner, 1975) and the output of total geraniol was expressed in milli litres per 100 plants.

Statistical methods:
Experimental data were analysed using ANOVA method at the 5 % level of significance.Regression analysis of data was carried out by fitting the data to non-liniar regression equations with the aid of MINITAB statistical package,

Results
The "total geraniol" content of oil was significantly affected by phosphorus and magnesium nutrition.Results presented in Table 1, indicate that the application of P 3 and P 4 levels have significantly increased the total geraniol at the first harvest from 46.81 % in control to 50.48% and 65.57% respectively.The effect of phosphorus fertilizers on the total geraniol was more marked at the second harvest.At the second harvest all phosphorus levels have significantly increased the total geraniol over the control.The P level has prcduced the highest geraniol content of 69.17%As shown in Figure 1 and Table 3, a positive relationship was observed between the leaf phosphorus and the total geraniol in oil (r = +0.5).As observed in Table 1, this increase of total geraniol has occured mainly due to the increased accumulation of borneol, geraniol, citronelool, geranyl butarate, elamol, linalooJ, nerol and methyl iso -euginol constituents in oil.As indicated in Table 3, the phosphorus content in leaves is correlated well with the accumulation of linalool (r= +0.71), borneol (r=+0.53)and methyl iso-euginol (r=-t 0.51) in oil.
The positive effect of magnesium on the total geraniol was observed at the first harvest when it was applied in combination with PI level.Magnesium tend to decrease the total geraniol with other phosphorus levels.However, at the second harvest, magnesium (Mg 2 level) showed a positive effect on the total geraniol content.
Fertilizer combinations, P I +Mg 2 and P 4 +Mg 2 increased the total geraniol by 4.58 % and 4.65 % over their respective controls.
As shown in the Fig. 4, the borneol content in oil was further increased when magnesium was applied together with phosphorus fertilizers.
The increase of total geraniol due to magnesium and phosphorus nutrition.was accompanied by the decrease of hydrocarbon content in oil.As indicated in Table 1, total hydrocarbon content in oil was decresed from 38.51 % in control to 27.13, 22.08, 4.01 and 2.61 % as a result of the application of PI' P 2 , P 3 and P4 levels respectively.The same tendency was observed at the second harvest.The negative relationship was observed between the total geraniol and total hydrocarbon contents in oil (r= -0.6) at the first harvest and (r= -0.9) at the second harvest (Fig. 2).Output of total geraniol was affected positively by all four levels of phosphorus nutrition (Table 2).As shown in Figure 3, a high positive correlation was observed between the phosphorus content in leaves and the output of total geraniol (r=+0.59)and essential oil yield (r=+0.58).

Discussion
Our results have shown that phosphorus and magnesium nutrition of Cymbopogon nardus has increased the total geraniol content in its oil.The market quality of citronella oil is determined by its total geraniol content (Wijesekera, 1973).This increased production of total geraniol was mainly due to the increase of its constituents : borneol, linalool and methyl iso-euginol.Positive correlations were observed between leaf phosphorus and linalool (r= +0.71), borneol (1'= +0.53) and methyl iso-euginol (r= +0.637) contents in oil (Table 3).Geranyl pyrophosphate has been shown to act as a specific precurssor of linalool (Hanson, 1967).The positive effect of phosphorus nutrition on geraniol (constituent of total geraniol) content in Cymbopogon winterianus oil has been previously reported (Chandra, 1972).
The combined effect of magnesium with phosphorus fertilizers on the total geraniol content in oil was significant at the second harvest.Fertilizer treatments with magnesium-phosphorus combinations have enhanced the production of borneol, linalool and eIemol contents in citronella oil.
From our results, it is clearly observed that phosphorus nutrition had a positive effect on the output of total geraniol (Table 2).Magnesium has enhanced the oil production, when it was applied in combination with phosphorus fertilizers.This positive effect of phosphorus and magnesium nutrition on the essential oil production of citronella can be due to involvement of these ions in the biosynthesis of essential oils.Several phosphorylation reactions of precurssor compounds are involved in the essential oil biosynthesis and they are shown to be dependant upon adenosine triphosphate (ATP), reduced pyridine nucleotides and magnesium ions (Am dur et aI, 1957).As observed in Fig. 1, the negative relationship of hydrocarbons and the positive relationship of total geraniol with the phosphorus content in leaves of C. nardus, indicate the involvement of phosphorus in the bio-synthesis of constituents of total alcohols, through better utilization of hydrocarbons by this plant.The conversion of mineral phosphorus into organic phosphorus compounds in soybeans has shown to be enhanced by magnesium fertilizers (Ranaweera & Threshov, 1977) The optimum concentrations of leaf phosphorus that produced maximum yields of essential oil and total geraniol were found to be in the range of 0.23 to 0.26 %, at the first harvest, four months after planting (Fig 3).
It is clear from our experimental results that phosphorus and magnesium nutrition is effective on the production of essential oil by Cymbopogon nardus and also it improves the market quality of oil by increasing its total geraniol content.Further extensive field experimentation is needed, in order to recommend optimum fertilizer dozes for citronella crop,

Table 1
Fractional compostlon of essential oils of Cymbopogon nardus as affected by phosphorus and magnesium nutrition. ;:;.

Table - 2
Phosphorus content In leaves, essential 011yield, total geraniol and ODtputor geraniol or Cymbopogon nardus as affected by P and Mg nutrition

Table - 3
Correlation values for some constituents of Cymbopogon nardus oil and phosphorus concentration in leaves (%) at the first harvest.