Pulp and paper from kenaf bast fibers

Samples of kenaf (Hibiscus cannabinus) grown in Malaysia were examined to determine the kraft pulp and papermaking properties of their bast (or bark) fibers. Using kraft pulping process showed that bast fibers were relatively easy to cook resulting good pulp yields in the range of 45–51%. The bast pulp produced sheets with great density, tear index and dry zero-span breaking length. Kenaf bast fiber is considered promising for production of high-grade printing, writing and specialty papers.     

Population density and row spacing effects on dry matter yield and bark content of kenaf (Hibiscus cannabinus L.)

Efforts to bring kenaf from experimental crop status to an accepted alternative in established cropping systems have been ongoing for years. To compete with existing crops and maximize monetary returns, the effects of agronomic practices on yield and crop quality must be better understood. Row spacing and population density are implicated in dry matter yield, and therefore, bast fiber production. Four row spacings, four population densities, and two cultivars were examined to determine these effects on dry matter accumulation and bark content. Of all factors evaluated, only row spacing significantly affected dry matter yield. Yield was greatest at the 35.5-cm row spacing, and lowest at the 71- and 101.6-cm row spacings. With regard to bast yield, calculated as a percentage of total sample dry weight, a significant cultivar × row spacing interaction occurred. However, data indicate that manipulating row spacing to maximize total stalk yield per hectare resulted in the highest bast fiber yield for the two cultivars tested. The narrowest row spacing of 35.5 cm, gave the greatest biomass yield as well as the highest bark yield per hectare.     

Comparison of various pulping processes for producing pulp from vine shoots

The results of characterization for four different vine shoot varieties, grown using two different methods, revealed no significant differences in composition among vine varieties or between growing methods. The holocellulose content of vine shoots (67.14%) is lower than those of other non-wood raw materials (e.g. wheat straw, sunflower stalks, cotton stalks, etc.), but similar to those of pine and higher to those of olive trimmings. On the other hand, their lignin content (20.27%) is similar to those of eucalyptus and the non-wood raw materials. Also, their contents in cold-water, hot-water and 1% soda solubles, and ethanol–benzene extractables, are higher than for pine and eucalyptus.Soda, kraft, ethanol and ethylene–glycol pulping processes have low yields (29–47%) relative to the pulping processes applied to olive trimmings and wheat straw. Kraft pulp is that exhibiting the best properties, including a higher α-cellulose content (73.74%) than pulp from wheat straw or olive trimmings, but a higher lignin content (17.18%). Also, kraft pulp provides paper with the highest breaking length, burst index and tear index (viz. 1316 m, 1.63 kN/g and 1.59 mN m²/g, respectively); these values, which are intermediate among those for olive trimmings, wheat straw and sunflower seeds, are low and can be substantially improved by appropriate refining of the pulp.     

Evaluation of harvesting time effects on kenaf bast lignin by pyrolysis-gas chromatography

A Chinese kenaf (Hibiscus cannabinus L.), variety Sekko-ichi, was planted in May 1996, and harvested at 76, 116, 152, 185, and 226 days after planting. The hand-separated bast fibers were analyzed for lignin content and in situ lignin composition to clarify their variations with maturity. Bast fibers increased in lignin until 152 days after planting, and did not increase thereafter. Variations in in situ lignin compositions with maturity were monitored using pyrolysis-gas chromatography. The volatile pyrolysis products released at 500 °C for 4 s were identified by gas chromatography-mass spectrometry. Pyrolysis products derived from guaiacyl and syringyl lignin units were revealed. The yield of guaiacyl lignin-derived pyrolysis products decreased with maturity, while the yield of the syringyl analogous increased with maturity. The ratio of yield of syringyl lignin-derived products to that of guaiacyl ones (S/G) increased from 1.86 to 3.16 with maturity, but remained constant at 152 days after planting. High S/G ratio showed that mature bast lignin is of syringyl type. Quantitative pyrolysis results suggested that harvesting of kenaf at age of 5-months growth period provides a raw material with an ease of delignification.     

Studies on Hibiscus cannabinus and Hibiscus sabdariffa as an alternative pulp blend for softwood: An optimization of kraft delignification process

Hibiscus cannabinus and Hibiscus sabdariffa, agro-based residues consist of bast and wood fibers which resemble to those of softwood and hardwood, respectively. The runkel ratio of core fibers H. cannabinus and H. sabdariffa is comparable to that of Picca abies, whereas, it is much less than those of hardwood like Eucalyptus tereticornis. The slenderness ratio of H. cannabinus is much closer to P. abies in comparison to H. sabdariffa whereas, it is 7.7 and 13% less than E. tereticornis. The flexibility coefficient of H. cannabinus and H. sabdariffa are slightly lower than that of P. abies but it is 59.6 and 57.0% are more than that of E. tereticornis. It indicates that morphological characteristics of core fibers of H. cannabinus and H. sabdariffa closely resemble to that of softwood except fiber length which can be compensated by long bast fibers. Due to identical pulping conditions, H. cannabinus and H. sabdariffa can be delignified together by kraft pulping process. The optimum cooking conditions for H. cannabinus and H. sabdariffa were found to be as, active alkali 16%, sulfidity 20%, temperature 160 °C, time (at temperature) 120 min and wood to liquor ratio of 1:4.5. An anthraquinone (AQ) dose of 0.05% at an active alkali dose of 13% (as Na₂O) produces the screening rejects and kappa number similar to that obtained by using 15% active alkali (as Na₂O). The reaction kinetics study indicates that delignification is of first order. Low sulfidity AQ additive kraft pulping at constant H-factor produces better strength properties compared to non-additive kraft cooks.     

Characterization of hairs and pappi from Cynara cardunculus capitula and their suitability for paper production

The capitula of Cynara cardunculus contain hairs and pappi representing 7% of the total plant biomass. These low density biomass components could be mechanically separated without apparent losses using a whole-plant processing prototype. Hairs and pappi are filamentous structures made up of longitudinally aligned fibre cells, without intercellular voids or pitting, with the following dimensions regarding length, width and wall thickness: 1.35 mm, 19.8, and 4.8 μm for hairs and 1.78 mm, 10.4, and 2.9 μm for pappi. Chemically hairs and pappi have low content of ash (1.9% and 1.1%, respectively), extractives (5.4% and 6.0%) and lignin (10.6% and 17.8%), and high content of holocellulose (77.5% and 72.8%) and α-cellulose (55.2% and 46.8%).Pulps could be produced using a conventional kraft process with high yields and low residual lignin, e.g. 63% at Kappa 7 for hairs and 48% at Kappa 11 for pappi, low coarseness values (0.04 and 0.03 mg m^?1) and adequate pulp properties for paper (40 and 42 N mg^?1 tensile index; 3.6 and 3.4 kPa m² g^?1 burst index in unrefined pulps of hairs and pappi, respectively). The results also indicated that there is scope for improving pulp quality by optimising pulping conditions to this type of new raw materials. The differences between hairs and pappi may also be further exploited namely the lower lignin content of hairs and the higher slenderness and wall thickness of pappi fibres.The utilization of hairs and pappi may strengthen the differentiated use of biomass fractions of the Cynara plant and its potential as a bioenergy crop.     

Antifungal activity in vitro of Aloe vera pulp and liquid fraction against plant pathogenic fungi

The leaf pulp of Aloe vera, designated as the gel, and the bitter, yellow liquid fraction have been tested against pathogens (bacteria and fungi) affecting human and plants. However, their activity for fungal control in commercial industrial crops has not been determined. The objectives of this study were to evaluate the inhibitory effect of Aloe pulp and liquid fraction on the mycelial growth of three phytopathogenic fungi and to determine the extract concentrations that can inhibit mycelial development. A. vera leaves were cut from plants grown under greenhouse conditions at the University Antonio Narro, disinfected with sodium hypochlorite, and separated in two groups. In the first group, the pulp was manually scraped out; in the second, a laboratory roll processor was used for the pulp and liquid fraction separation. Both types of extracts were pasteurized. Antifungal activity of pulp and liquid fraction was evaluated on the mycellium development of Rhizoctonia solani, Fusarium oxysporum, and Colletotrichum coccodes that were isolated from a potato crop by the hyphae point and monosporic techniques. The concentrations of the plant extract ranged from 0 to 10⁵ μl l⁻¹. Fungal plugs 0.4 mm in diameter were placed in Petri dishes with a potato–dextrose–agar (PDA) culture media, and treated with various concentrations of pulp or liquid fraction. The cultures were incubated at 24±2 °C and the radial growth of mycelia measured daily for 7 days. The antifungal effect was measured under a totally random design with four replications. The results showed an inhibitory effect of the pulp of A. Vera on F. oxysporum at 10⁴ μl l⁻¹ and over a long period. For the two types of Aloe fractions the activities were similar. Besides the liquid fraction reduced the rate of colony growth at a concentration of 10⁵ μl 1⁻¹ in R. solani, F. oxysporum, and C. coccodes. This is the first report of any Aloe liquid fraction activity against plant pathogenic fungi.     

Acetosolv delignification of depithed cardoon (Cynara cardunculus) stalks

Modelling of the Acetosolv treatment of the cardoon bark (Cynara cardunculus) was accomplished using a second-order face-centred factorial design. We considered as independent (experimental) variables: cooking time (60–180 min), acetic acid concentration in the cooking liquor (60–90%) and hydrochloric acid concentration in the cooking liquor (0.20–0.80%); as well as dependent variables: pulp yield, kappa number and viscosity.Empirical models were deduced to satisfactorily fit experimental data with the values of the independent variables and allow quantifying the effects of each variable.An optimisation with constraints led to the calculation of the region of the experimental domain (time = 180 min, acetic acid concentration ≥ 71.3% and HCl concentration > 0.41%) leading to pulps with kappa numbers < 25 at a maximal pulp yield and viscosity, giving us maximum possible values for pulp yield (46.3%) and viscosity (557 mL/g).     

Optimization for enzyme-retting of flax with pectate lyase

Flax (Linum usitatissimum L.) is an important commercial crop that supplies both linseed and bast fibers for multiple applications. Retting, which is a microbial process, separates industrially useful bast fibers from non-fiber stem tissues. While several methods (i.e., water- and dew-retting) are used to ret flax, more recently enzymes have been evaluated to replace methods used currently. Alkaline pectate lyase (PL) from the commercial product BioPrep 3000 and ethylenediaminetetraacetic acid (EDTA) from Mayoquest 200 as a calcium chelator were used in various formulations to ret flax stems. Retted stems were then mechanically cleaned through the USDA Flax Fiber Pilot Plant and passed through the Shirley Analyzer. The PL and chelator effectively retted flax from both fiber flax and linseed stems, and the use of enzyme plus chelator retted flax stems better than either component alone. Fiber yield and strength were greater than retting with a mixed-enzyme product that contained cellulases. Retting with PL and chelator was optimized based on fine-fiber yield, remaining shive content, and fiber properties. PL at levels of about 2% of the commercial product for 1 h at 55 °C followed by treatment with 18 mM EDTA for 23–24 h at 55 °C provided the best fibers based on these criteria. Yield and fiber properties determined by these tests were not improved with PL levels of 5% of the commercial product.     

Paper from olive tree residues

An experimental design was performed to study the influence of process variables (135–175 °C for temperature, 60–120 min for pulping time and 15–25% for active alkali) on the properties of pulps (yield, Kappa index, viscosity, 1% NaOH solubles, alcohol–benzene extractives holocellulose, lignin and α-cellulose contents and brightness) and paper sheets (stretch index, burst index, and tear index) obtained from olive trimming residues. Obtaining pulps with acceptably high physical and chemical properties entails operating at a temperature of 175 °C for 90 min and 25% of active alkali. The paper sheets obtained from olive trimming residues pulps that were produced in different degrees of refining are characterised for their stretch index, burst index, and tear index. An increase in the different parameters for the paper sheet upon increasing the degree of refining is found. All pulps reached between 33 and 39 kN m/kg in the stretch index, between 1.5 and 2 kN/g for the burst index and 0.7–2.5 N m²/g for the tear index and not in excess of the refining degree (<45 °SR).     

The influence of rainfed and limited irrigation conditions and early vs. late plantings on kenaf as a potential industrial crop in the southern High Plains,USA

Kenaf (Hibiscus cannabinus L.) is a nonwoody fiber source with many uses. To evaluate the dry matter yield potential of kenaf at two locations in the southern High Plains of the USA and the effects of late planting/late emergence as a possible replacement for hail damaged cotton, four varieties were grown in 2004 and 2005 at New Mexico State University's Agricultural Science Centers at Clovis, under limited irrigation, and Tucumcari, under rainfed conditions. Each study was a randomized complete block design having four replicates. A year × location interaction existed (P < 0.0469) for kenaf yield largely due to precipitation amount and distribution. Either late planting or late emergence due to low soil moisture at planting significantly reduced yields (3.62 vs. 0.63 Mg ha^?1 for mid-May planting and emergence and early July emergence or planting, respectively, P < 0.0001). Consequently, kenaf would not be suitable for recovering input costs when planted after failure of cotton. The very late maturing variety Gregg had consistently lower numeric yields than Dowling, Everglades 41, and Tainung 2 in all comparisons, with the strongest trend within the emergence date comparison (P < 0.0912). Kenaf varieties should be selected for any location that finish blooming approximately three to four weeks prior to the average first autumn freeze to allow for maximum growth.     

Acetosolv delignification of Miscanthus sinensis bark: Influence of process variables

Using a face-centred composite experimental design with three central point replicates, we investigated the effects of cooking time (60–180 min), acetic acid concentration (60–95% of liquor weight) and HCl concentration (0.05–0.15% of liquor weight) on the yield, residual Klason lignin content and total polysaccharide content of pulps obtained from Miscanthus sinensis bark by the acetosolv process. Response surfaces fitted satisfactorily to the experimental results showed the most influential of the independent variables to be acetic acid concentration; increasing acetic acid concentration reduced yield and lignin content, and increased total polysaccharide content (except at the highest concentrations of the acids). The response surface for lignin content was used to design a Box–Wilson steepest-descent optimisation procedure to determine the conditions minimising pulp lignin content; the minimum achieved, 0.5%, was obtained using a cooking time of 147 min, an acetic acid concentration of 93.25% and an HCl concentration of 0.122%, under which conditions pulp yield was 52.6%.     

The role of absorbed hemicelluloses on final paper properties and printability

Hemicelluloses isolated from eucalyptus wood were adsorbed onto both bleached eucalyptus kraft pulp and pine kraft pulp as part of a general strategy to investigate their role on final sheet mechanics and physical properties. The tensile indices increased upon hemicelluloses addition and respectively reached a maximum at 9 mg/g and 10 mg/g hemicelluloses adsorption for bleached eucalyptus kraft pulp and pine kraft pulp, whereas the tear index increased depending on the pulp type. The air permeability and surface roughness of the bleached eucalyptus kraft pulp slightly decreased, however, brightness and opacity did not changed. SEM analysis shows that hemicelluloses adsorption increases bleached eucalyptus kraft pulp fibers interaction, whereas the effect of hemicelluloses addition on the beaten pine kraft pulp is unclear. The hemicelluloses adsorption also affected printability. It was found that print through for the bleached eucalyptus kraft pulp hand sheet slightly decreased, but for the bleached pine kraft pulp hand sheet it was unaffected. Print densities were not influenced.     

Alkaline sulfite–anthraquinone (AS/AQ) pulping of wheat straw and totally chlorine free (TCF) bleaching of pulps

Alkaline sulfite–anthraquinone (AS/AQ) pulping of wheat straw and totally chlorine free (TCF) bleaching of resulting pulps was investigated. Wheat straw was supplied by the Ferdosii University Experimental Station in Northeastern Iran, cleaned and chopped into about 3–5 cm long pieces. In the AS/AQ pulping experiments, the active alkali charge on oven dry wheat straw, heating time to maximum cooking temperature, AQ dose and liquor to straw ratio were kept constant at 16%, 60 min, 0.1% and 4/1, respectively. The alkali ratio, Na₂SO₃/NaOH, calculated as NaOH, was varied from 20/80 to 80/20. Cooking time at maximum temperature was varied from 30 to 90 min and maximum cooking temperature was adjusted between 152 and 160 °C. Reference soda and soda/AQ pulps were produced.AS/AQ and soda/AQ pulps with kappa numbers below 15 were produced easily. Cooking of corresponding soda pulps under comparable conditions stopped at a kappa number of 25. The characteristics of unbleached AS/AQ, soda/AQ and soda pulps were compared. AS/AQ and soda/AQ pulps had similar tensile strength but tear was in favor to AS/AQ. The latter had much higher yield and brightness.TCF bleaching of AS/AQ pulp was performed in O Q (OP) or O Q (OP) P sequences. Oxygen delignification (O) resulted in a delignification degree of approximately one third. Followed by a chelating treatment (Q) and a subsequent (OP) stage under drastic conditions a brightness of 82.7% ISO was achieved in an O Q (OP) sequence. The sequence O Q (OP) P, with 2% H₂O₂ in the final P stage, improved brightness from 77.2% ISO after the OP stage to 83.6% ISO. Bleaching led to only small losses in tensile and burst strength but enhanced tear strength. Short beating in a Jokro mill was sufficient to attain good pulp strength. The results of this study indicate that AS/AQ pulping and TCF bleaching is a convincing alternative to produce high-quality pulp from wheat straw for writing and printing paper.     

Kinetics of organosolv delignification of fibre crop Arundo donax L

Kinetics of ethanol–alkali delignification of fibre crop Arundo donax L. (giant reed) has been studied. The improved approach for determination of the reaction rate constants by accurate quantification of lignin fractions with different reactivity during standard procedure of graphical differentiation was applied. Following to a simplified model, the delignification process was considered as a complex of n-parallel irreversible first-order reactions with similar final product and analysed as a multi-component reaction system. Three kinetically distinguishable lignin fractions of A. donax were revealed and quantified in proportion of approximately 61, 23 and 16% (as initial, bulk and residual lignin, respectively) and their effective degradation rate constants were determined for different pulping conditions. The proportion of lignin fractions was different from that reported for wood, but close to another crop—wheat straw, where the initial lignin fraction was also found as a major fraction (about 90%). The values of apparent activation energy were estimated respectively as 64, 89 and 96 kJ mol⁻¹, and were generally within the range of those reported for wood kraft and organosolv pulping. The simulation of ethanol–alkali delignification using found kinetic parameters showed the high reproducibility of experimental data on lignin removal, providing thereby the adequate test on validation of the suggested kinetic approach. The data reproducibility was substantially higher in comparison with conventional consecutive kinetic model (sum of square residuals (SQR) 0.0036 versus 0.0856).     

Studies on Ipomea carnea and Cannabis sativa as an alternative pulp blend for softwood: An optimization of kraft delignification process

Ipomea carnea Jacq, a common weed known as ‘Beshram’ was introduced in India, as an ornamental plant. Surprisingly, except fiber length, other morphological characteristics like, cell wall thickness, lumen diameter, flexibility coefficient and wall fraction of I. carnea resemble with softwoods like, Pinus kesiya, and Picca abies. Cannabis sativa L., a common weed known as soft hemp consists of both bast fiber as well as core fiber. The unique morphological characteristics of I. carnea like, cell wall thickness and flexibility coefficient, which resembles with softwoods along with long fibers of C. sativa, in which I. carnea deficits in a proper blend ratio, can be used in a better way for the development of high quality value added paper. The optimum kraft cooking conditions for I. carnea and C. sativa were: active alkali 16%, sulfidity 20%, temperature 165 °C, time (at temperature) 120 min and wood to liquor ratio of 1:4 for I. carnea and 1:3.5 for C. sativa. The screened pulp yield of I. carnea and C. sativa were found to be 49.3 and 54.3% at Kappa number of 29, respectively. An anthraquinone (AQ) dose of 0.1% increases the pulp yield to the tune of around 0.6% as well reduce the Kappa number to the tune of 22%. CEHH bleaching of C. sativa produces pulp brightness of 82% (Elrepho) at total chlorine demand of 5.07%. On contrary to this the I. carnea kraft pulp at same chlorine demand produce a pulp with improved brightness of 85.4% (Elrepho).     

Fiber yield and quality of fiber nettle (Urtica dioica L.) cultivated in Italy

To evaluate the production potential of fiber nettle crops in Tuscany (Italy), a German clone of fiber nettle was cultivated during 2006–2007.Although a longer experimentation is essential, the two first years of trials showed that the German clone used also seems to give good results in term of growth and fiber yield in an environment like central Italy, with higher temperatures and generally lower rainfall. Indeed the stalk mean dry matter obtained was about 15.4 Mg ha^?1 with a mean fiber content of about 11% of stalk dry matter, and the resulting fiber yield was 1696 kg ha^?1, comparable to or higher than those reported in the literature.The differences in chemical, physical and mechanical characteristics of fibers extracted from different portions of stalks seemed to indicate an intrinsic heterogeneity of the fibers along the stem. Fiber mean diameter values ranged from 47 to 19 μm and fiber length from 43 to 58 mm moving from stalk bottom to top. Tensile strength of the bottom part of the stalk was much lower than that of the other parts, with mean values of about 24 and 60 cN tex^?1, respectively. More constant mean values along the stalk were found for the elongation parameter (2.3–2.6%). Lignin content decreased moving toward the stalk top from about 4.4% to 3.5%.These physical–mechanical characteristics confirmed the potential of the fibers of nettle cultivated in Tuscany to be used for textile purposes. Indeed they were similar to hemp fibers in diameter, lignin content and elongation, and similar to flax or cotton in tensile strength.     

Radiation interception and the accumulation of biomass and nitrogen by soybean and three tropical annual forage legumes

Field experiments were conducted at Gatton and Dalby in southeastern Queensland to determine parameters associated with radiation interception and biomass and nitrogen (N) accumulation for the ley legume species, phasey bean (Macroptilum lathyroides (L.) Urban) and vigna, (Vigna trilobata (L.) Verdc.). Sesbania (Sesbania cannabina Retz.), a native legume species, and soybean (Glycine max (L.) Merrill)) were included in the study for comparison. The most important differences between species related to differences in radiation interception, radiation-use efficiency (RUE), N-accumulation efficiency and the partitioning of N to plant parts. During early growth, soybean intercepted more radiation than the other species, primarily because of its greater leaf area index (LAI). Sesbania had the highest RUE (1.08 g MJ⁻¹) followed by phasey bean (0.94 g MJ⁻¹), soybean (0.89 g MJ⁻¹) and vigna (0.77 g MJ⁻¹). The efficiency of N-accumulation was greater in soybean (0.028 g N g⁻¹) and phasey bean (0.030 g N g⁻¹) than in vigna (0.022 g N g⁻¹) and sesbania (0.021 g N g⁻¹). In all species, the proportion of N allocated to leaves declined throughout the experimental period, being more rapid in soybean than in sesbania and phasey bean. Despite this decline in total N partitioned to the leaves, both soybean and phasey bean maintained a relatively stable specific leaf nitrogen (SPLN) throughout the experimental periods although sesbania and vigna displayed rapid decreases in SPLN. The large variation between species in RUE and N-accumulation efficiency indicates that the development of ley legume cultivars with a combination of traits for more efficient legume production, water use and soil N-accumulation in the water-limited environments of the grain belt of eastern Australia may be possible. The sensitivity of forage production, water use and soil N-accumulation to variation in RUE and N-accumulation efficiency needs to be quantified using modeling techniques prior to embarking on screening programs to select appropriate germplasm for evaluation studies.     

Variations in chemical characteristics and pulpability within jute plant (Chorcorus capsularis)

The differences in morphological and chemical characteristics and pulpability of core and bark fiber of jute at different position, top, middle and bottom were studied. The fiber length at the middle part of both bark and core was longer than the top and bottom part. With increasing height from bottom to top lignin content is decreased and α-cellulose content increased. The bark had higher α-cellulose and lower lignin than core. The bark produced nitrobenzene oxidation products with larger syringaldehyde/vanillin (S/V) ratios than the core. The yields of nitrobenzene oxidation products in bark were higher than core. The analysis of neutral sugars suggested that xylose is the predominant sugar of hemicellulose in both bark and core. The bark had higher glucose and lower xylose as compared to core. Under identical conditions of pulping, bark gave higher pulp yield and lower kappa number than core. The bark pulp showed better bleachability than core pulp. The higher tensile index in core pulp and higher tear index in bark pulp were observed.     

Intra-specific competition in maize: early establishment of hierarchies among plants affects final kernel set

Reduced plant biomass and increased plant-to-plant variability are expected responses to crowding in monocultures, but the underlying processes that control the onset of interplant interference and the establishment of hierarchies among plants within a stand are poorly understood. We tested the hypothesis that early determined plant types (i.e. dominant and dominated individuals) are the cause of the large variability in final kernel number per plant (KNP) usually observed at low values of plant growth rate (PGR) around silking in maize (Zea mays L.). Two hybrids (DK696 and Exp980) of contrasting response to crowding were cropped at different stand densities (6, 9 and 12 plants m⁻²), row spacings (0.35 and 0.70 m), and water regimes (rainfed and irrigated) during 1999/2000 and 2001/2002 in Argentina. The onset of interplant competition started very early during the cycle, and significant differences (P<0.05) in estimated plant biomass between stand densities were detected as soon as V_4–6 (DK696) and V_6–7 (Exp980). Plant population and row spacing treatments did not modify the onset of the hierarchical growth among plants, but did affect (P<0.02–0.08) the dynamic of the process. For both hybrids, the rate of change in relative growth between plant types was larger at 9 and 12 plants m⁻² (ca. 0.12 g/g per 100 °C day) than at 6 plants m⁻² (ca. 0.07 g/g per 100 °C day). For all treatments, the largest difference in estimated shoot biomass between plant types took place between 350 (V₇) and 750 °C day (V₁₃) from sowing, and remained constant from V₁₃ onwards. Dominant plants always had more kernels per plant (P<0.05) than the dominated ones, but differences between plant types in PGR around silking were significant (P<0.05) only at 12 plants m⁻². Our research confirmed the significant (P<0.01) curvilinear response of KNP to PGR around silking, but also determined a differential response between plant types: the mean of residual values were significantly (P<0.01) larger for dominant than for dominated individuals. Estimated ear biomass at the onset of active kernel growth (R₃) reflected the variation in KNP (r²≥0.62), and was significantly (P<0.01) related to estimated plant biomass at the start of active ear growth (ca. V₁₃). This response suggested that the physiological state of each plant at the beginning of the critical period had conditioned its reproductive fate. This early effect of plant type on final KNP seemed to be exerted through current assimilate partitioning during the critical period.