The impact of CO₂ enrichment on fiber dimension and lignocellulose properties of three varieties of kenaf (Hibiscus cannabinus L.)

 

A. Mahdi Khalatbari¹, H.Z.E. Jaafar¹* and A. Ali Khalatbari¹

¹Department of Crop Science, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malasia, 43400

*Corresponding author: magnificent_pal@yahoo.com

---

Abstract

The effects of two different carbon dioxide levels on fiber yield, fiber dimension and lignocelluloses properties of three varieties of kenaf (Hibiscus cannabinus L.) namely Fuhong (FH991), V36 and Kohn-Kaen60 (KK60) were assessed in a growth house experiment at faculty of Agriculture, Universiti Putra Malaysia. Seeds were sown in polyethylene bags containing top (loamy soil). Carbon dioxide enrichment treatment started when the seedlings reached four weeks and plants were exposed to 400 and 800 µmol mol^-1 of CO₂. A factorial experiment was arranged in a split plot using a randomized complete block design (The CO₂ chamber is perpendicular to sunrise and sunset) with CO₂ levels as the main plot, and different varieties as sub-plot replicated three times. Different CO₂ levels had significant impact on fiber dimension, fiber yield and lignocellulose properties of bast and core fiber for all three varieties. Result indicated that increasing CO₂ concentration from 400 µmol mol^-1 to 800 µmol mol^-1 positively affected fiber of all varieties under study. Increase in fiber length and slight reduction in fiber diameter at 800 mol^-1 resulted in higher fiber quality for paper production purposes. These results provide significant insights into opportunities for growing of kenaf under enriched CO₂ concentration.

Keywords: Carbon dioxide levels, bast fiber, core fiber, fiber dimensions, fiber length, fiber diameter.

---

1. Introduction

Carbon dioxide is considered as one of the most prominent and limiting factors in photosynthesis. The CO₂ concentration in atmosphere is currently about 400 µmol µmol^-1 and it is expected to increase by 600 µmol·mol¹־ CO₂ by the end of 2050 (Taylor and Llyold, 1992). For many years, there have been many interests among scientists about prospect of photosynthesis improvement in crops through CO₂ enrichment (Porter and Grodzinski, 1985). An increase in plant growth, morphology, development, and yield of many crops has been observed when they were subjected to enriched levels of CO₂. This type of response depends on CO₂ rate and duration of exposure (Brevoort, 1998). Crops under higher levels of CO₂ acquire positive attributes e.g. an increase in plant height which is important for higher fiber yield and higher photosynthesis rate, with enhanced plant adaptation and growth.

CO₂ is an essential substrate for photosynthesis which links the atmosphere to the biosphere. Enriched CO₂ level would stimulate photosynthesis that will lead to increased carbon (C) uptake and assimilation and eventually results in higher plant growth. Plants with a C3 photosynthetic pathway (like kenaf) often show higher growth rate compared to those with a C4 pathway (Amthor and Loomis, 1996; Bowes, 1993; Poorter et al, 1992; Rogers et al, 1996). Considering C3 plants, positive responses at ambient CO₂ levels are mainly related to competitive inhibition of photorespiration by CO₂ and the internal CO₂ concentrations of C3 leaves due to being less than the Michaelis-Menton constant of ribulose bisphosphate carboxylase/oxygenase (Amthor and Loomis, 1996). Many reports have stated increased photosynthesis under elevated CO₂, this increase varies for plants with a C3 (33% to 40% increase) versus a C4 (10% to 15% increase) (Kimball and Idso, 1983; Prior et al., 2003).

Elevated CO₂ can change C partitioning/allocation beside photosynthesis and aboveground growth stimulation. Increased C supply which is caused by elevated atmospheric CO₂ could profoundly induce the distribution ofphotosynthate belowground (Ceulemans and Mousseau, 1994; Prior et al, 1997; Rogers et al., 1996). It was observed in many cases that plants tend to allocate photosynthate to tissues which is needed in order to acquire the most limiting resources such as water and nutrients (Chapin et al., 1987).

It has been reported from many studies that there would be lower stomata conductance and lower transpiration rate due to higher CO₂ concentration. (Rogers and Dahlman, 1993). In fact, improved plant water relations were observed by both C3 and C4 species. Elevated CO₂ induces the partial closure of leaf stomatal guard cells that results in slowing transpiration (Jones and Mansfield, 1970). Experiments and studies conducted in growth chambers and glasshouses under controlled condition have indicated that enriched CO₂ declines transpiration for C3 plants (Allen et al, 1994; Jones et al., 1984, 1985). Reduction in transpiration rate and an increase in photosynthesis can contribute to improve the water use efficiency the ratio of carbon fixed to water transpired, (Baker et al., 1990; Morison, 1985; Sionit et al., 1984) as it was observed in this experiment on young kenaf varieties.

Kenaf plant absorbs CO₂ from the atmosphere more than any other crop, about 1.5 tons of CO₂ seems sufficient in order to produce 1 ton of dry matter of Kenaf. It means that each hectare of Kenaf consumes 30-40 t of CO₂ per growing cycle (Kimball and Idso, 1983). Growth of kenaf is precisely indispensible due to its importance for fiber production and fiber content. The most prominent advantage of CO₂ enrichment is the vegetative growth of young plants (Kimball and Idso, 1983). The greenhouse industry has proven the advantages of manipulating CO₂ and light in order to benefit proper grown crops in controlled conditions for extended periods. When plants are young, they grow swiftly and somehow exponentially while older plants grow more in a linear way (Lindhout and Pet, 1990).

Due to lack of documentation on the impact of CO₂ enrichment on growth and fiber development of kenaf and knowing that CO₂ enrichment studies need optimization of the plant microclimatic conditions such as temperature, relative humidity, irrigation and nutrients, the present study is important in order to gain needed information on fiber development of kenaf under CO₂ enrichment. The objective of this study was i): to evaluate the growth and histochemical responses of selected varieties of kenaf under different CO₂ concentration and ii): to choose the best variety and best CO₂ level to improve the fiber quality.

2. Materials and Methods

2.1. Experimental location, plant materials and treatments

The experiment was carried out under growth house at Ladang 2, Faculty of Agriculture Glasshouse Complex, University Putra Malaysia (longitude 101° 44' N and latitude 2° 58'S, 68 m above sea level) with a mean atmospheric pressure of 1.013 kPa. One-month old kenaf seedlings of three varieties namely Fuhong (FH991), V36 and Kohn-Kaen60 (KK60) were selected for this experiment. Seeds were obtained from the Institute of Tropical Forestry and Forest products. Seeds were sown in polyethylene bags with 16 cm diameter and 24 cm height containing approximately 12.5 kg of top soil (loamy soil-fine loamy, siliceous, isohyperthermic). Carbon dioxide enrichment treatment started when the seedlings reached four weeks old and the plants were exposed to 400 and 800 µmol mol^-1 CO₂. The experiment was arranged in a split plot using a randomized complete block design with CO₂ levels being the main plot, and varieties as sub-plot replicated three times. Each treatment consisted of twelve seedlings (Figure 1).

2.2. Growth house microclimate and CO₂ enrichment treatment

The seedlings were kept in specially constructed growth houses receiving 12-hour photoperiod and average photosynthetic photon flux density of 300 ìmol m^-2 s^-1. Daily and night temperatures were recorded at 30±1.0°C and 20±1.5°C, respectively, and relative humidity at about 70% to 80%. Vapor pressure deficit ranged from 1.01 to 2.52 kPa. Carbon dioxide at 99.8% purity was supplied from a high-pressure CO₂ cylinder and injected through a pressure regulator into fully sealed 2 m x 3 m growth houses at 2-hour daily and applied continuous from 8 to 10a.m. (Ibrahim et al., 2010). The CO₂ concentration at different treatments was measured using Air Sense ™CO₂ sensors (high-pressure CO₂ cylinder) designated to each chamber during CO₂ exposition period. Plants were irrigated three times a day for 5 min using drip irrigation with emitter capacity of 2 L hr^-1. The experiment lasted 10 weeks since CO₂ treatment was applied.

2.3. Fibre dimensions and values

Stem samples for the fiber studies were obtained from the second internode counting from the base. Small slices or slivers (fiber bundles) were obtained from the bast and from the central wood core were macerated in 67% HNO₃ and then (suspended) boiled in water bath for 5 minutes (the time of boiling has been modified from 10 minutes to 5 minutes in order to prevent any fiber slivers deterioration). Fiber suspension that had been dyed with toluidine was finally placed on microscope slides. Fiber samples were teased on slides and viewed under calibrated microscope (MEIJI® light microscope with Nikon® camera attached) and following fiber dimensions were recorded. A total of eight fibers were measured for each sample and mean values calculated for fiber length (fl), fiber diameter (fd), fiber lumen diameter (fld) and fiber wall thickness. The optimum fiber length for bast is with range of 1.8 to 4.0 mm and this range for core is 0.4 to 1.0 mm. Considering fiber diameter, for bast it is within the range of 14 - 27 µm and for core is 18 - 37 µm (fwt) (Ogbonnya et al., 1997).

From these above measurements, the following values were calculated: coefficient of suppleness (CS) or flexibility coefficient as fld/fd (Petri, 1952), slenderness ratio (SR) as fl/fd (Rydholm, 1967) and runkel ratio (RR) as 2 x fwt/fld, (Okereke, 1962).The runkle ratio <1 indicates high fiber quality. Lower runkle ratio (> 1) indicates that fiber has lower value of cell wall thickness and fiber lumen diameter which is a positive sign of high quality fiber.

2.4. Chemical analysis

The proportions of chemical properties in these three varieties of kenaf bast and core fibres were determined based on approved standard of Technical Association of Pulp and Paper Industry (TAPPI standard). The percentage of lignin was determined by TAPPI standard T 222 os-74 using 72% sulphuric acid. For holocellulose, it was based on the method of Wise et al., (1946) using sodium chlorite, 10% acetic acid and acetone with a slight modification. The percentage of á- cellulose was determined by TAPPI standard T 203 os using 8.3% NaOH, 17.5% NaOH and 2N acetic acid. á-cellulose extractive was derived from holocellulose.

2.5. Experimental design and statistical analysis

The factorial experiment was arranged in a split plot using a randomized complete block design (RCBD) with CO₂ levels being the main plot, and varieties as subplot replicated three times. Each treatment consisted of twelve seedlings. Data were analysed using analysis of variance by Statistical Analysis System computer package (SAS Institute Inc., 2007). Mean separation test between treatments was performed using Duncan New Multiple Range Test (DNMRT) and p-value of < 0.05 was regarded as significant.

3. Results

3.1. Fiber dimension

The effect of different CO₂ and different varieties levels on the fiber dimensional properties of three kenaf varieties are presented in Table 1. Different levels of CO₂ had significant impact on fiber dimension of bast and core fiber. Different varieties showed significant effect on fiber attributes except for core lumen diameter and core cell wall thickness. The interaction between different varieties and different CO₂ levels had no significant impact on fiber dimension parameters except for bast lumen diameter and core fiber diameter (Table 1).

The highest bast fiber length was recorded for 800 µmol mol^-1 CO₂ level with value of 3.10 mm whereas CO₂ level of 400 µmol mol^-1 recorded bast fiber length of 2.68 mm (Table 2). Higher bast fiber diameter, bast lumen diameter and bast cell wall thickness were recorded at ambient CO₂ level (400 µmol mol^-1) with value of 23 µm, 10.1 µm and 4.1 µm, respectively. CO₂ of 800 µmol mol^-1 had bast fiber diameter of 21.3 µm, bast lumen diameter of 8.9 µm and bast cell wall thickness of 3.5 (Table 2). For core fiber attributes, the highest core fiber length were at elevated CO₂ (800 µmol mol^-1) with value of 0.98 mm whereas at ambient CO₂ the core fiber length was 0.92 mm (Table 2). It was observed that CO₂ level of 400 µmol mol^-1 attained higher core fiber diameter of 24.3 µm, core lumen diameter of 16.9 µm and core cell wall thickness of 3.7 µm compared to elevated CO₂ (Table 2). These results for bast and core fiber diameter indicated that elevated CO₂ of 800 µmol mol^-1 would produce higher fiber and this is because of higher fiber length and lower fiber diameter when the plants are subjected to CO₂ enrichment.

Considering the impact of different varieties on fiber dimension attributes, variety FH991 recorded the highest bast fiber length with value of 3.1 mm, followed by varieties V36 and KK60 with bast fiber length of 2.8 mm and 2.7 mm, respectively (Table 3). Variety FH991 also attained the highest bast fiber diameter of 23.0 µm. Variety KK60 presented a bast fiber diameter of 22.0 µm, followed by variety V36 with value of 21.4 µm (Table 3). For core fiber length and diameter, variety FH991 recorded the highest core fiber length of 0.96 mm and core fiber diameter of 23.0 µm. For core fiber length and core fiber diameter, there was no significant difference between varieties V36 and KK60 (Table 3). Variety V36 attained core fiber length of 0.95 µm whereas variety KK60 had value of 0.94 µm. For core fiber diameter variety V36 and KK60 presented similar values of 21.8 µm (Table 3).

3.2. Fiber values

The effects of different CO₂ levels on the fiber derived values of three kenaf varieties are presented in Table 4. For bast fiber, slender ratio was affected only by CO₂ levels. The runkle ratio and coefficient of suppleness were affected by varieties and the interaction between varieties and CO₂ levels. For core fiber, slender ratio showed significant difference for CO₂ levels and the interaction between varieties and CO₂ levels. Varieties, CO₂ and the interaction between treatments had significant impact on core coefficient of suppleness. Core runkle ratio showed no significant difference among all treatments (Table 4).

Considering the interaction of varieties with levels of CO₂ in this experiment, FH991 under CO₂ concentration of 400 µmol mol^-1 recorded the highest coefficient of suppleness for bast (52.2) which was followed by interaction of FH991 with CO₂ level of 800 µmol mol^-1 (45.20) and KK60 subjected to 800 µmol mol^-1 (43.9). The lowest bast coefficient of suppleness was recorded for V36 exposed to CO₂ level of 800 µmol mol^-1 that was 36.3. This is due to lower lumen diameter compared to fiber diameter (Table 5). Variety FH991 under ambient CO₂ level (400 µmol mol^-1) and elevated CO₂ level (800 µmol mol^-1) presented the lowest runkle ratio of 0.56 and 0.52. The highest runkle ratio belonged to variety KK60 subjected to CO₂ level of 400 µmol mol^-1 (Table 5). The highest core slender ratio was recorded by varieties FH991, KK60 and V36 subjected to elevated CO₂ with ratio of 49.9, 49.2 and 46.9 respectively whereas variety FH991 under ambient CO₂ level of 400 µmol mol^-1 had the lowest core slender ratio of 36.3 (Table 5). Variety KK60 under 800 µmol mol^-1 attained the highest core coefficient of suppleness of 76.0 followed by variety FH991 under CO₂ concentration of 800 µmol mol^-1 with coefficient of suppleness of 74.4. The lowest suppleness ratio belonged to varieties FH991 (65.8) and KK60 (69.6) when these varieties were subjected to 400 µmol mol^-1 (Table 5).

3.3. Chemical properties

The effects of different CO₂ levels on the fiber chemical properties of three kenaf varieties are presented in Table 6. Bast and core lignocellulose properties of three kenaf varieties were affected by different CO₂ concentrations. Varieties had no significant impact on these properties except for core holocellulose. There was no significant interaction observed for lingnocellulose properties (Table 6).

Chemical composition of kenaf lignocellulose would give great deal of information on feasibility of plant material for paper making purposes. The highest bast and core lignocelluloses were observed for plants under elevated CO₂ (Table 7). The highest bast holocellulose, á-cellulose and lignin were observed at enriched CO₂ of 800 µmol mol^-1 (87.3%, 57.9% and 14.3% respectively), whereas the lowest holocellulose of 85.75, á-cellulose of 56.54 and lignin of 13.45 were recorded for plants exposed to 400 µmol mol^-1 (Table 7). Core lignocellulose attributes attained highest percentages under elevated CO₂ while holocellulose was 84.2%, a-cellulose 47.5% and lignin 21.6%. Lower core lignocelluloses parameters were determined for varieties under ambient CO₂ with holocellulose of 82.5%, a-cellulose of 46.0 and lignin of 20.0 (Table 7).

3.4. Fiber yield

The bast, core and total fiber yield of the three kenaf varieties were affected by CO₂ concentrations and varieties. The interaction between varieties and CO₂ level had significant impact on bast, core and total fiber yield (Table 8).

The optimum value of bast fiber yield was obtained by FH991 when it was subjected to enriched CO₂ level of 800 ìmol mol^-1 (14.3 g plant^-1) followed by variety V36 (14.0 g plant^-1) and KK60 (13.5 g plant^-1) at the end of experimental period while the lowest value for bast fiber yield was recorded by variety KK60 under ambient CO₂ level of 800 µmol mol^-1 with value of 12.58 g plant^-1 (Table 9).

The highest core fiber yield of 24.2 g plant^-1 was recorded by FH991 under elevated CO₂, followed by variety V36 with value of 23.2 g plant^-1 and variety KK60 with value of 22.5 g plant^-1. KK60 under ambient CO₂ level attained the lowest value 20.6 g plant^-1. Varieties V36 and FH991 under ambient CO₂ level showed a core fiber yield of 21.9 g plant^-1 and 22.2 g plant^-1 respectively (Table 9).

Enriched CO₂ level of 800 ìmol mol^-1 increased total fiber yield measured for all varieties. Variety FH991 subjected to elevated CO₂ level showed the highest total fiber yield of 38.5 g plant^-1, followed by varieties V36 with total fiber yield of 37.2 g plant^-1 and KK60 with value of 36.0 g plant^-1. All varieties under ambient CO₂ attained lower total fiber yield. Variety KK60 presented a total fiber yield of 33.2 g plant^-1 which was the lowest value followed by varieties V36 (35.0 g plant^-1 ) and variety FH991 (35.4 g plant^-1) (Table 9).

4. Discussion

One factor to increase fiber production and quality is to speed up the growth process (Ogbonnya et al., 1997). There is no documentation on the impact of enriched CO₂ level on physiological aspects and lignocelluloses attributes of kenaf as a fibrous plant. This could be plausible by using carbon dioxide at different concentrations (400 and 800 µmol mol^-1) at seedling growing stage.

It was observed that most of fiber dimension and lignocellulose attributes were influenced by the CO₂ concentrations of different varieties. These parameters showed higher value when plants were subjected to elevated CO₂ concentration of 800 µmol mol^-1. Fiber dimension parameters showed higher quality for paper production purposes when all three varieties were under enriched CO₂ level. The first quantitative review of crop yield based on responses to elevated CO₂ was conducted by Kimball and Idso, (1983). He reported an average increase of 33% among 37 agricultural species, which included fiber, root/tuber, and leaf crops. Cure and Acock, (1986) also collected data on total yield of 10 major crops (grain, leaf, tuber and fiber) and found an average increase of 41% when CO₂ concentration was doubled.

Plants show many differences, namely morphology, variation in photosynthesis, respiration, biochemical composition and even at reproductive phase when are subjected to different concentrations of CO₂. Plants with C3 pathway like kenaf have two different physiological processes which are directly affected: photosynthesis and transpiration. Net photosynthesis rate increases as CO₂ concentration increase, probably due to a decrease in photorespiration and partly due to an increase in substrate supply. A lower stomatal conductance will reduce transpiration rate as well (Cure and Acock, 1986; Amthor and Loomis, 1996). One of possible reasons of increase in photosynthesis rate is the lower of water plant losses that leads to higher growth rate. Most of plants exposed to higher CO₂ concentrations have plant growth stimulation (Kimball and Idso, 1983). There have been reports on cotton (Gossypium hirsutum L.) as a good source of fiber production in which elevated CO₂ level (almost twice of ambient) enhanced total biomass and harvestable yield (Reddy et al., 1999; Tischler et al., 2000). One of the most important benefits of elevated CO₂ is in increment of leaf gas exchange capacity, especially when plants are under undesirable climatic conditions (Mark and Jackson, 2000).

In present experiment kenaf varieties under enriched CO₂ level had higher fiber yield and support the result of Mark and Jackson (2000). Enhancement of plant growth and fiber yield subjected to enriched CO₂ has been reported before (Norby et al., 1996; Ceulemans and Mousseau, 1994; Idso and Idso, 1994; Saxe et al., 1998). This is due to an increase in total leaf area and leaf photosynthetic rate coupled with a decline in shoot respiration rate (Ceulemans and Mousseau, 1994). It has been stated that the relative area of mesophyll cells is more closely related to photosynthesis rate than are the epidermis and the vascular bundles (Parkhurst, 1986; Evans, 1999; Roderick et al, 1999).

It has been reported by Ceulemans and Mousseau, (1994) and Norby et al, (1999) that enriched CO₂ level often results in increasing in the total leaf area, leaf weight and leaf weight compared to shoot weight. Leaf area development is a key factor for determination of total plant productivity and could be different in a range of environmental conditions (Taylor and Llyold, 1992). Total leaf area of kenaf varieties especially FH991 under elevated CO₂ of 800 ìmol mol^-1 had higher value compared to the plants subjected to ambient CO₂ (400 ìmol mol^-1) (data not shown). This higher total leaf area probably led to a higher total biomass and total fiber yield. Many studies have reported that crop growth and yield are favored by elevated CO₂ (Kimball and Idso, 1983; Amthor and Loomis, 1996). Elevated atmospheric CO₂ would increase leaf and canopy photosynthesis, especially in C3 crops like kenaf and this is the reason why the presence of CO₂ is not enough to saturate Rubisco. Higher CO₂ concentration can inhibit the competing process of photorespiration by suppressing RuBP oxygenase activity increasing carbon assimilates for plant growth and development (Lawlor and Mitchell, 2000).

There has been a debate that CO₂ may also have impact on cell soluble/fiber composition of plant tissue. Plant with higher growth rate would have higher fiber fractions, whereas increasing in carbon assimilation may result in higher cell soluble content due to storage of excess carbohydrates. (Akin et al., 1994, 1995; Owensby et al, 1996; Frehner et al, 1997; Carter et al., 1999; Fritschi et al, 1999). Morgan et al., 2004 stated that plant subjected to elevated CO₂ concentration would obtain larger yield which is an indication of greater plant growth. Higher plant growth can result in a higher lignin component. This statement is in accordance with our findings in which kenaf varieties under enriched CO₂ level recorded higher total biomass and therefore higher fiber yield and greater lignocelluloses properties compared to plants under ambient CO₂ of400 ìmol mol^-1. But in other studies conducted in other microclimate, small impact of CO₂ on cell solubles, hemicellulose-cellulose, lignin and fiber fractions was observed (Akin et al, 1994, 1995; Soussana and Loiseau, 1997; Booker, 2000; Fritschi et al, 1999).

5. Conclusion

A doubling ambient CO₂ concentration leads to higher fiber content for kenaf (as a C3 plant and fibrous plant, responds strongly to elevated CO₂ level) that could be highly considered for paper production.

Varieties subjected to elevated CO₂ level attained higher value for fiber dimension (especially fiber length) and lignocelluloses properties that are prominent attributes related to paper producing quality. Among varieties, it was variety FH991 that attained the highest values for most attributes. Based on this study result, increment in fiber length and slight reduction in fiber diameter caused by enriched CO₂ level of 800 ìmol mol^-1 will result in higher fiber quality for paper production purposes.

Acknowledgment

The authors gratefully acknowledge Universiti Putra Malaysia (UPM ) their contribution for this project number of 5523867 FRGS (Fundamental Research Grant Scheme).

Refrences

Allen, L.H., Valle, R.R. Jr., Mishoe, J.W., Jones, J.W. 1994. Soybean leaf gas-exchange responses to carbon dioxide and water stress. Agron. J. 86, 625-636.

Amthor, J.S., Loomis, R.S. 1996. Integrating knowledge of crop responses to elevated CO₂ and temperature with mechanistic simulation models: Model components and research needs. In: Koch, G.W., Mooney, H.A (eds). Carbon dioxide and terrestrial ecosystems. Academic Press., San Diego, CA, pp: 317-346.

Baker, J.T., Allen, L.H., Boote, K.J. Jr., Jones, P., Jones, J.W. 1990. Rice photosynthesis and evapotranspiration in subambient. ambient, and superambient carbon dioxide concentrations. Agron. J. 82, 834-840.

Bowes, G. 1993. Facing the inevitable: Plants and increasing atmospheric CO₂. Annu. Rev. Plant Physiol. Plant Mol. Biol. 44, 309-332.

Brevoort, P. 1998. The blooming United State botanical market: A new overview. Herbalgram. 44, 33-46.

Ceulemans, R., Mousseau, M. 1994. Effects of elevated atmospheric CO₂ on woody plants. New Phytol. 127, 425-446.

Chapin, F.S., Bloom, A.J., Field, C.B., Waring, R.H. 1987. Plant responses to multiple environmental factors. Bioscience. 37, 49-55.

Ibrahim, M.H., Jaafar, H.Z.E., Haniff, M.H., Raffi, M.Y. 2010. Changes in growth and photosynthetic patterns of oil palm seedling exposed to short term CO₂ enrichment in a closed top chamber. Acta Physiol. Plant. 32, 305-313.

Jaafar, H.Z.E. 1995. Impact of Environmental Stress on Reproductive Development in Sweet pepper (Capsicum anuum). Ph.D. Thesis, University of Nottingham, Nottingham, UK.

Jones, P., Allen, L.H., Jones, J.W. Jr., Boote, K.J., Campbell, W.J. 1984. Soybean canopy growth, photosynthesis, and transpiration responses to whole-season carbon dioxide enrichment. Agron. J. 76, 633-637.

Jones, P., Allen, L.H., Jones, J.W. Jr., Valle, R. 1985. Photosynthesis and transpiration responses of soybean canopies to short- and long-term CO₂ treatments. Agron. J. 77, 119-126.

Jones, R.J., Mansfield, T.A. 1970. Increases in the diffusion resistances of leaves in a carbon dioxide-enriched atmosphere. J. Expt. Bot. 21, 951-958.

Kimball, B.A., Idso, S.B. 1983. Increasing atmospheric CO₂: Effects on crop yield, water use, and climate. Agr. Water Manage. 7, 55-72.

Lindhout, P., G. Pet. 1990. Effects of CO₂ enrichment on young Plant growth of 96 genotypes of tomato (Lycopersicon esculentum). Euphytica. 51(2), 191-196.

Morison, J.I.L. 1985. Sensitivity of stomata and water use efficiency to high CO₂. Plant Cell environment. 14, 467 - 474.

Ogbonnaya, C.I., Nwalozie, M.C., Roy-Macauley, H., Annerose, D.J.M. 1997. Growth and water relations of Kenaf (Hibiscus cannabinus L.) under water deficit on a sandy soil. Ind. Crops Prod. 8, 65-76.

Okereke, O.O. 1962. Studies on the fiber dimensions of some Nigerian timbers and raw materials. Part 1: Research Report No. 16: Fed. Ministry of Commerce and Industry Lagos Nigeria.

Petri, R. 1952. Pulping studies with African tropical woods. TAPPI 35, 157-160.

Poorter, H. Gifford, R.G., Kridelman, P.E., Wong, S.E. 1992. A quantitative analysis of dark respiration and carbon content as factors in the growth response of plants to elevated CO₂. Australian Journal Botany. 40, 501 - 513.

Porter, M.A., Grodzinski, B. 1985. CO₂ enrichment of protected crops. Hort. Res. 7, 345-398.

Prior, S.A., Runion, G.B., Mitchell, R.J., Rogers, H.H., Amthor, J.S. 1997. Effects of atmospheric CO₂ on longleaf pine: Productivity and allocation as influenced by nitrogen and water. Tree Physiol. 17, 397-405.

Prior, S.A., Torbert, H.A., Runion, G.B., Rogers. H.H. 2003. Implications of elevated CO₂ induced changes in agroecosystem productivity. J. Crop Prod. 8, 217-244.

Rogers, H.H., Dahlman, R.C. 1993. Crop responses to CO₂ enrichment. Vegetatio. 105, 117-131.

Rogers, H.H., Prior, S.A. Runion, G.B., Mitchell, R.J. 1996. Root to shoot ratio of crops as influenced by CO₂. Plant Soil. 187, 229-248.

Rydholm, S.A. 1967. Pulping process. New York, Wiley and Sons, 1167p.

Sellers, T., Miller, G.D., Fuller, M.J. 1993. Kenaf core as a board raw material. Forage Prod. J. 43, 69-71.

SAS. 2007. SAS/STAT User's Guide, Version 9.2. SAS Institute Inc., Cary, NC, USA.

Sionit, N., Rogers, H.H., Bingham, G.E., Strain, B.R. 1984. Photosynthesis and stomatal conductance with CO₂-enrichment of container- and field-grown soybeans. Agron. J. 76, 447-451.

Taylor, J.A., Llyold, J. 1992. Sources and sinks of atmospheric CO₂. Aust. J. Bot. 40, 401-418.

Wise, L.E., Murphy, M., D'Addieco, A.A. 1946. Chlorite holocellulose, its fractionation and bearing on summative wood analysis and studies on the hemicelluloses. Pap. Trade J. 122(2), 35-43.