Prasad. M.N.V. and J. Raieswar. School of Life Science, University of Hyderabad, Hyderabad 500 134, India

CARBON DIOXIDE ASSIMILATION IN LEAVES AND DEVELOPING PODS OF LEUCAENA LEUCOCEPHALA WITH REMARKS ON ITS

PRODUCTIVITY AND PHYTOMASS COMPOSITION

Introduction. Studies on the productivity of multipurpose trees gained prominence recently to augment renewable resources. L. leucocephala (Lam.) de Wit cv. K8 was selected for the present study, as its productivity on saline soils of the semiarid tropics was not known. However, a few reports were available on selected aspects of its productivity, culture and utilization from other parts of the world (Oakes, 1968; Ferraris, 1979; Hegde, 1987; Mishra et al, 1986; Natarajan et aL,1988).The effect of temperature and salinity on productivity, phytomass composition, and the role of developing pods on carbon economy are reported in this paper.

Materials and methods. Study site: The experiment was conducted at the University of Hyderabad campus, situated at 17°26’ N and 78°27’ E at an altitude of 540 m.a.s.l. L. leucocephala cv. K8 was raised in a net house. Three month old seedlings were transplanted to 1 x 1 x 1 ft pits spaced at 1 x 1 m in 50 x 50 m plots. 1/2 kg of farm-yard manure was added to each pit. Watering was done weekly for 3 months, and thereafter every two weeks throughout the study period.

Plant and soil chemical analyses. After 10 months, 10 plants were randomly harvested for productivity studies. The leaf fresh, dry, and total dry weights were determined (excluding roots) after drying at 80+. 5° C to a constant weight. Ten soil samples collected at random from the 0-10 cm horizon were pooled to give one composite sample. Plant material and soil were chemically analyzed (Allen et aL, 1974). Total nitrogen (TN) was measured by Kjeldahl method, Na, P, Ca, were estimated, and pH and electrical conductivity (EC) determined. Cation exchange capacity (CEC) was determined by distillation and titration with M/140 HCL. The heat of combustion of 9 months old stems was determined with a bomb calorimeter.

Gas exchange measurements. Carbon dioxide exchange rates were determined using an open- circuit gas system. C02 concentration was measured with an LCA-2 infrared gas analyzer. Leaf and air temperatures were measured with a Parkinson broad leaf chamber (PLC-B). Quantum yield and efficiency of water use were calculated following Long and Hallgren (1985).

Results and discussion: This study was conducted on saline soil with high sodium (2.4 mg/g and electrical conductivity (568 uS cm'1) (Table 1). Growth parameters and leaf phytomass characteristics are shown in tables 1 and 2 respectively. Like leaves, L. Leucocephala pods also possessed stomata (Figs 1,2). The carbon dioxide assimilation rate was maximum in April under high solar radiation (1800 umol m'2 S'1) and temperature 30-33°C. (Fig. 3). Stomatal conductance correlated with C02 assimilation rate was reported earlier (Wong et aL, 1987). However, with increasing temperature, the C02 assimilation rate and stomatal conductance decreased (Fig. 3). During day, pods showed respiration up to 10 days after anthesis. Subsequently, pods performed photosynthesis and once again showed respiration from fourth week onwards (Fig. 4). The pods matured by about 10-12 weeks (Fig. 5). During the course of pod maturation there was an inverse relationship between the dry matter content of the pod walls and seeds (Fig. 5).

Temperature was found to be the major controlling factor in productivity; 33-38°C was found to be optimum. Transpiration rate varied in different months due to differences in stomatal conductance (Table 3) and temperature (Fig. 3). Water use efficiency was largely related to stomatal conductance and transpiration rate. In May, stomatal conductance, transpiration rate, quantum yield, and water use efficiency were low due to high temperatures (40-44° C).

Figure 1. Pod wall stomata x 250. Figure 2. Pod wall stomata x 1000

Figure 3. The effect of air temperature on C02 uptake of leaves during early summer andsummer months in full irradiance (1800 u mol m V ) (Feb.-May, 1988) ( o ). The values presented are averages of 5 day measurements. The temperature ranges for February 30-34°C. March 32-38°C, April 33-38°C and May 40-44°C. It also shows the influence air temperature on stomatal conductance ( • ).

DAYS AFTER ANTHESIS IN WEEKSFigure 4. Net C02 exchange (photosynthesis or respiration) in pods during light in full irradiance at different stages of development.

Figure 5. Diy weight percentage in developing pods from March to May 1988 (—...... ). Relative dry weight percentage of podwall and seeds at different stages of development.

Biomass yield was relatively lower compared to other agroclimatic zones of India (Table 4). Previous studies on productivity revealed that salinity caused a greater reduction in whole plant fresh weight, especially in experiments dealing with leucaena seedlings (Gorham et aL, 1988). Nodulation alters the effects of salinity, however. In this study nodulation was not observed. Plants observed high concentrations of calcium (about 6 times of soil concentration) in leaves possibly as an adaptational strategy of salt stress.

Legume pods are known to be photosynthetically active and are considered to play a major role in the carbon economy of plants (Grover and Shinha, 1985; Sheoran et al., 1987). Gas exchange studies conducted in conifers revealed that cones contribute appreciably to the carbon budget of the tree (Koppel et aL, 1987. A positive correlation was observed between stomatal conductance and C02 assimilation rate (r= 0.91, P< 0.001). Air temperature exhibited a negative correlation with C02 assimilation rate (r=-0.61, P<0.05). Air temperature also influenced stomatal conductance and showed negative correlation (r=-0.92, P< 0.001).

Carbon dioxide exchange studies of developing pod walls revealed a net C02 release during the first week after anthesis, followed by a net C02 fixation in the next two weeks. From the fourth week a continuous increase in the rate of net C02 release (Fig. 4) was observed. During the early stages of pod development, in spite of having cholorophyllous walls, COz release was observed (Fig. 5). However, during the second and third weeks the photosynthetic activity of pod walls could meet its energy demand. The fixed C02 was found to contribute to about 20% to seed diy weight (Fig. 5). From 4-7 weeks, there was a constant increase in the rate of net C02 release which may be due to translocation of photosynthate to the developing seeds, and gradual maturation of pod wall rendering it photosynthetically inactive.

There was a constant increase of dry matter (DM %) during pod development (Fig. 5). A portion of DM was contributed by net C02 fixation by green pod walls, and the rest by leaves. A relative increase in dry weight.(%) of the seeds and corresponding decrease in pod wall during development (Fig. 5) was also observed. This may be due to translocation of photosynthate from pod wall to seeds during development. Thus, the developing pods reduce metabolic cost of the tree considerably and contribute to biomass production.

Table 1. L. leucocephala cv,Mean height 4 m (range 1.75 to 5 m)Basal diameter of the bole 5.3 cm (range 2.5 to 5.6 m)Diameter at breast height 3.7-4 cm (range 3 to 4 cm)Mean crown diameter 1.4 m (range 0.8 to 1.6 m)Mean crown length 2.5 m (range 1.6 to 3 m)Above ground mass 22 t/haLeaf area/plant 5.75 m2Leaf dry mass/plant 0.58 kgNumber of trees/ha 10,000Plot size 50 x 50mSpacing of plants 1 x 1 mWood volume/plant 0.0034 m3Calorific value 3811 Cal/kgTotal leaf chlorophyll 2.8 mg/g F.wt

Table 2. Chemical analyses.Leaf phytomass composition inic matter 90.9%

Soil chemistry* 7.0%93.0%0.12%

Organic matter Ash Total nitrogen Crude protein Phosphorus Sodium Potassium CalciumIn vitro digestibility Metabolizable energy

9.1%2.38%14.87%0.9%4.6 mg/g 7.5 mg/g6.4 mg/g

55.6%neiaooiizaoie energy 9.4 M J /K g _______ ;Soil pH 8.35; electrical conductivity 569 u S cm'1; CEC 1.72 me/lOOg.

0.008% 2.4 mg/g 4.6 mg/g 1.1 mg/g

Table 3. Stomatal conductance, transpiration rate, quantum yield, and efficiency of water use J from February to May 1988 (Averages of 5 days in each month).__________ _ «■

1988 Stomatal Conductance (mole m V1)

Transpiration rate (m mol H20 mV*

quantum yield (« mol mV1)

water use efficiency

February 0.410 8.1 0.0045 1.01March 0.460 11.6 0.0076 1.18April 0.400 12.2 0.0068 1.00May 0.012 6.6 0.0019 0.52

Table 4. Productivity of 1 x 1 m spaced L. leucocephala cv. K-8 at different agroclimatic zones in

Place Agroclimatic zone Biomass yield ReferenceMaduraiUrlikanchanKanpurHyderabad

Southern India Western India Northern India Central India

27-30 t h a y 1 33-35 t ha V 1 35-38 t ha'V"1

25 t ha'*yr'1

Natatajan et al., 1988 Hegde 1987 Mishra et al., 1986 Present study

Acknowledgements. Financial aid from the Department of Nonconventional Energy Sources and Indian National Science Academy, New Delhi, is gratefully acknowledged. J. Rajeswar is grateful to the University of Hyderabad for a scholarship award.

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