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Crop Science 42:1930-1936 (2002)
© 2002 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

Development of Redroot Pigweed Is Influenced by Light Spectral Quality and Quantity

Irena Rajcana, Majid AghaAlikhanib, Clarence J. Swantona and Matthijs Tollenaar*,a

a Dep. of Plant Agriculture, Crop Science Bldg., Univ. of Guelph, Guelph, ON, Canada N1G 2W1
b Agronomy Dep., Faculty of Agriculture, Tarbiat Modarres Univ., Tehran, Iran

* Corresponding author (mtollena{at}uoguelph.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Light quantity (photosynthetic photon flux density, PPFD) and quality (red:far-red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was conducted to quantify the effects of incident PPFD and R:FR on development and dry matter accumulation of redroot pigweed (Amaranthus retroflexus L.). Pigweed was grown in growth cabinets from the one-leaf stage to the initiation of seed set under three different PPFD/R:FR treatments: (i) high PPFD (550 µmol m-2 s-1) and high R:FR (1.4) (HH), (ii) low PPFD (180 µmol m-2 s-1) and high R:FR (1.4) (LH), and (iii) low PPFD (180 µmol m-2 s-1) and low R:FR (0.8) (LL). The experiment was undertaken at 12- and 16-h daylengths with three replications. Rate of leaf appearance (RLA) was accelerated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under the 12-h but not under the 16-h daylength. Low PPFD delayed the occurrence of floral primordia, flowering and initiation of seed set. Plant height was a result of the complementary effects of PPFD and R:FR. Total dry matter accumulation and partitioning, with the exception of dry matter accumulation to the stem, were influenced by PPFD only. Results of this study show that both light quality and quantity influence the phenology of pigweed.

Abbreviations: DAE, days after emergence • HH, high PPFD and high R:FR ratio • LH, low PPFD and high R:FR ratio • LL, low PPFD and low R:FR ratio • PPFD, photosynthetic photon flux density • R:FR, red:far-red ratio • RLA, rate of leaf appearance


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
REDROOT PIGWEED is a common weed in many agricultural areas (Weaver, 1984; Horak and Loughin, 2000), and its presence can severely reduce yields of various crops (Weaver and McWilliams, 1980). Research on critical time for weed control together with weed thresholds has highlighted the time of weed emergence relative to the crop as a major determinant of crop yield loss due to weeds (Knezevic et al., 1994; Dieleman et al., 1995; Cowan et al., 1998). It has been shown that phenology and dry matter accumulation of pigweed that emerged after the seven- or nine-leaf stage of maize (Zea mays L.) was retarded in comparison to early emerging dates (three- or five-leaf stage of maize) (McLachlan et al., 1993b). Knezevic et al. (1994) reported no effect of pigweed on yield loss in maize when the pigweed emerged after the seven-leaf stage of maize. Therefore, phenology of weeds (i.e., development and duration of their life cycles) is probably one of the most important factors determining the outcome of crop–weed competition.

Phenology of redroot pigweed has been described in terms of the two most important factors: temperature (McLachlan et al., 1993a) and photoperiod (Huang et al., 2000). McLachlan et al. (1993b) reported reduced rates of leaf appearance of redroot pigweed under canopy-induced shading. Even though retarded development of pigweed under the maize canopy was attributed to the reduced level of PPFD, a potential light-quality effect on pigweed development in the latter study should not be excluded.

Light transmitted through a crop canopy is enriched in far-red radiation (730–740 nm) and depleted in red radiation (660–670 nm) due to selective absorptance of red light and transmittance and reflectance of far-red light by green leaves. Consequently, the R:FR ratio (light quality) is reduced from {approx}1.2 above the canopy to anywhere between 0.1 and 1.0 under the crop canopy (Rousseaux et al., 1999). The R:FR ratio is known as photomorphogenic light, and affects many plant morphological characteristics (e.g., stem elongation, branching, apical dominance, and leaf area distribution) (Salisbury and Ross, 1991; Ballaré and Casal, 2000). Most research into effects of light quality on plants has focused on morphological changes and dry matter distribution (see Ballaré and Casal, 2000, and references therein). The few papers that discussed development responses to PPFD and R:FR have used very limited developmental parameters (i.e., extension rate of stem, duration of one plastochrone, formation of the first and second leaf) (Stuefer and Huber, 1998). Often, information was collected during a short period of a plant's life cycle (e.g., 14 d) (Ballaré et al., 1991). There is no information, however, on the impact of light quantity or quality on phenology during the entire life cycle of a plant.

Many weeds that thrive in various agricultural systems spend most of their life cycles in the lower strata of a crop canopy exposed to low PPFD and low R:FR ratio. Therefore, the effects of PPFD and R:FR on phenology of various weed species can be viewed as important factors in determining (i) the survival strategy of a weed and (ii) the outcome of crop-weed competition. To the best of our knowledge, there is no information in the literature on the effects of light quantity or quality on redroot pigweed development and phenology. Therefore, the objective of this study was to determine effects of incident PPFD and R:FR ratio from the one-leaf stage to seed-set initiation of redroot pigweed on (i) rate of leaf appearance, (ii) phenology, and (iii) dry matter accumulation and partitioning.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cultural Details
Experiments were conducted in a controlled-environment growth room until the one-leaf stage (i.e., first leaf above cotyledons opened and parallel to the surface) and, subsequently, in controlled-environment growth cabinets. Redroot pigweed seed, used in the experiments, were collected in 1999 from Woodstock, ON, Canada, and stored at 4°C. Seeds were planted at the 0.5-cm depth in plastic pots (15-cm diam.) containing a growth medium (55–65% Canadian sphagnum peat moss, perlite, dolomite, calcite, vermiculite, and wetting agent) (LA4 MIX AGGREGATE PLUS, Sun Grow Horticulture Inc., Lameque, New Brunswick, Canada). In the controlled-environment growth room, the day:night temperature regime was 25:20°C, PPFD at the top of the pots was 400 µmol m-2 s-1, and the daylength was 16 h. Pots were watered as required. Plants were supplied with a nutrient solution containing N, P, K, Ca, Mg, and chelated micronutrients prior to their transfer to controlled-environment growth cabinets. In the growth cabinets, nutrients were applied at the first sampling time in both 12- and 16-h daylength experiments and at the second sampling time in the 16-h daylength experiment.

One hundred and twenty pots with one plant per pot were placed in three separate growth cabinets (40 pots in each growth cabinet) programmed for the day:night temperature of 25:15°C. Cabinet temperature was measured using shielded thermocouples positioned near the top apical meristem of the plant. Three light quantity–quality treatments were established: HH, LH, and LL. Details about light sources used in the experiment, PPFD levels, and R:FR ratios are presented in Table 1 . Twice weekly, PPFD was measured using a LICOR Point Quantum Sensor (LI-190SA, LI-COR Inc., Lincoln, NE) and distance between the lights and the canopy was adjusted accordingly. The spectral distribution and R:FR ratios were determined using a LI-COR spectroradiometer LI-1800 with a cosine-corrected sensor on a fiber-optic cable. Both PPFD and R:FR ratios were measured at the top of the canopy at various positions within a growth cabinet. The experiment was conducted under 12- and 16-h daylengths.


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  		Table 1. Characteristics of photosynthetic photon flux density (PPFD) and red:far-red ratio (R:FR) at the top of the canopy in the treatments.

 
Experimental Design and Measurements
The experiment was arranged as a randomized complete-block design with three replications. The experimental unit, that is, a light quality–quantity treatment under one of two photoperiod regimes, was a growth cabinet, and treatments were randomly assigned to a growth cabinet for each of the three replications. Plants within a growth cabinet were periodically rotated to minimize cabinet-position effects. Number of leaves on the main stem was counted on all plants at 3-d intervals. The phenological stages, initiation of floral primordia, flowering, and initiation of seed set were recorded on five plants. Floral primordia initiation was defined as the stage at which the floral bumps on the stem apex were first detectable by a microscope, flowering was defined as the stage when 50% of the flower head was in anthesis, seed set was defined as the stage when all female flowers contained a green seed. All parameters were marked from the day after seedling emergence (cotyledons parallel to the surface). Plant height was measured to the tip of the youngest leaf or tip of the inflorescence at 3-d intervals. Dry weight was sampled at three times during the life cycle. In the 12-h daylength experiment, the first, second, and third sampling was done at the initiation of floral primordia, flowering, and initiation of seed set, respectively. Due to differences in time at which various treatments reached certain developmental stages, sampling was done once all treatments attained a given stage of development. Sampling procedure was slightly changed in the 16-h daylength experiment due to larger differences among treatments in terms of development. Thus, the first sampling was done in all treatments after the initiation of floral primordia occurred in the HH treatment. Second and third samplings were done 10 and 20 d after the first sampling. Ten randomly selected plants in each treatment were sampled for dry matter at each sampling time. Plants were separated into leaves (leaf blades and petioles), stems, inflorescence (where applicable) and roots (after washing). After measuring total leaf area using a LI-3000 area meter (LI-COR), all plant parts were dried until constant weight in a forced-air oven at 80°C and weighed. Data were analyzed using PROC GLM (SAS Institute, 1990).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Development
Rate of leaf appearance of redroot pigweed was affected by PPFD level and R:FR ratio (Table 2) . The RLA of plants grown under high PPFD was 33 and 27% greater than that of plants grown under low PPFD in the 12- and 16-h daylength treatments, respectively, when comparing plants grown under the high R:FR ratio. It has been reported that the RLA of field-grown redroot pigweed was negatively associated with the level of shading provided by a maize canopy (McLachlan et al., 1993a). Tollenaar (1999) reported that the RLA of maize was 16% greater at high PPFD than at low PPFD.


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  		Table 2. Effects of photosynthetic photon flux density (PPFD) and red:far-red ratio (R:FR) on the rate of leaf appearance (RLA) of redroot pigweed grown under a 12- and 16-h daylength. The RLA was calculated as a linear regression between observed leaf number and number of days after emergence (DAE), that is, 20 DAE under the 12-h daylength and 15 and 36 DAE under the 16-h daylength.

 
The effect of light quality (i.e., R:FR ratio) on RLA was observed only under the 12-h daylength treatment, when RLA was 20% greater under the LL treatment than under the LH treatment (Table 2). The effect of low PPFD on RLA relative to the effect of high PPFD was negated by the low R:FR ratio under a 12-h daylength. A similar trend was apparent in the 16-h daylength treatment at 15 d after emergence (DAE), but the difference between the LH and LL treatments was not significant at P < 0.05. Our LL treatment represented the light regime found under a leaf canopy, whereas the LH treatment was an artificial-shading treatment not often found under natural conditions. The effects of light quality on rate of leaf appearance of winter wheat (Triticum aestivum L.) are inconclusive (Casal, 1988; Barnes and Bugbee, 1991). However, to the best of our knowledge, effect of light quality on rate of leaf appearance of redroot pigweed has not been reported previously. Our results indicate that the RLA of redroot pigweed may have adapted to low PPFD by increasing RLA under a low R:FR ratio. Results show that studies carried out under nonnatural conditions (i.e., the LH treatment) may not be applicable for interpreting crop-weed interference under field conditions.

Although RLA was {approx}60% greater in the treatments grown for 36 d under the 16-h daylength than in the treatments grown for 20 d under the 12-h daylength (Table 2), this difference in RLA may not be attributable to the difference in photperiod (i.e., daylength includes effects of both photoperiod and daily accumulated PPFD). First, incident PPFD for the period under study was greater for plants grown under the 16-h daylength than those grown under the 12-h daylength for the following reasons: (i) Plants grown under a 16-h daylength were exposed to greater daily incident PPFD than plants grown under the 12-h daylength regime (i.e., for the HH treatment the incident PPFD was 23.8 and 31.7 mol m-2 d-1 under the 12- and 16-h day treatment, respectively, and for the LH and LL treatments the incident PPFD was 7.8 to 10.4 mol m-2 d-1 at the 12- and 16-h day treatment, respectively), and (ii) the cumulative amount of PPFD was 58% less under the 12-h treatment than under 16-h treatment across the period the RLAs were calculated (e.g., cumulative amount of PPFD was 476 mol during 20 DAE under 12-h and 1141 mol during 36 DAE under 16-h treatment). In order to account for the effect of PPFD accumulation on RLA, RLA in the 16-h daylength treatment was estimated at 15 DAE (i.e., the time at which accumulated PPFD in this treatment was equal to the total accumulated PPFD in the 12-h daylength treatment). The RLA averaged across all treatments was 14% greater under 16-h than under 12-h daylength (difference between the treatments was significant at P < 0.05) when the RLA was calculated for periods with similar cumulative amount of PPFD (Table 2). Second, mean daily temperature was 20.0°C in the 12-h and 21.7°C in the 16-h daylength treatment. McLachlan et al. (1993a) reported that RLA increased linearly with temperature in the range 10 to 35°C at a rate of 0.023 leaves °C-1. Consequently, an estimate of RLA of plants in the 12-h daylength treatment grown at 21.7°C is 0.40 + 1.7 x 0.023 = 0.44 leaves C-1. Hence, RLA adjusted for both differences in PPFD and mean daily temperature would be 0.44 and 0.46 leaves d-1 for the 12-h and 16-h daylengths treatments, respectively. The difference in RLA between the two daylength treatments was not significant, showing that photoperiod did not influence RLA. We distinguish between daylength and photoperiod in that the former includes both quantitative and qualitative effects of light and the latter only includes qualitative effects of light. Cao and Moss (1989) reported that the RLA increased by 17% in wheat and 43% in barley (Hordeum vulgare L.) when the daylength increased from 8 to 16 h, but it was not clear from their results whether the greater RLA was attributable to increased daylength, higher daily accumulated PPFD, or both. Huang et al. (2000) reported the RLA of redroot pigweed of 0.26 and 0.27 leaves d-1 under shorter photoperiods (<=12 h) and 0.31 and 0.57 leaves d-1 under longer photoperiods (14 and 16 h). However, the effect of photoperiod on RLA in their study was also confounded by the effect of total cumulative PPFD on RLA. Tollenaar (1999) reported that the RLA of maize was not affected by photoperiod, which is consistent with results reported herein. Floral primordia initiation, flowering, and seed set initiation were delayed under the 16-h daylength in comparison with the 12-h daylength in all treatments (Table 3) . This was expected because redroot pigweed is a quantitative short-day species (Huang et al., 2000) and its development is delayed under longer days (>12 h). The occurrence of the above-mentioned phenological stages was delayed by low PPFD in comparison with high PPFD under both daylengths (Table 3). The differences among the light treatments were, however, more pronounced under 16-h daylength. The LH treatment was most affected by the daylength extension. For example, plants in the LH treatment took 13.3, 17.0, and 16.7 d longer to reach the initiation of floral primordia flowering and seed set initiation, respectively, under the 16-h than those under the 12-h daylength. The same phenological events under the HH treatment were delayed by 7.0, 9.3, and 9.7 d, respectively, under the 16-h daylength. In addition to low PPFD, the high R:FR ratio further delayed the initiation of floral primordia for 2.0 d under 12-h daylength and 6.3 d under 16-h daylength.


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  		Table 3. Effects of photosynthetic photon flux density (PPFD) and red:far-red ratio (R:FR) on time from seedling emergence to initiation of floral primordia, flowering, and seed set initiation under the 12- and 16-h daylength.

 
The effect of R:FR ratio on flowering and seed set initiation was less pronounced. Under the 12-h daylength the initiation of floral primordia occurred at the same leaf stage (4.5 leaves) regardless of the treatments, whereas under the 16-h daylength, the initiation of floral primordium occurred at 10.4 leaves in HH and LL treatments and at 14.3 leaves in LH treatment (LSD0.05 = 3.3 leaves). Total number of leaves was not affected by the light treatments within a daylength (Table 4) . Tollenaar (1999) also reported that tassel initiation and total number of initiated leaves in maize were not affected by the PPFD level.


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  		Table 4. Effects of photosynthetic photon flux density (PPFD) and red:far-red ratio (R:FR) on final leaf number, height, and branch number at the last sampling time of pigweed plants grown under 12- and 16-h daylengths.

 
Morphology
Plants were taller under high than under low PPFD when the R:FR ratio was high (Table 4). Within the low PPFD treatment, plants were taller when the R:FR ratio was low and the height of plants in the low R:FR treatment did not differ from plants grown under high PPFD (Table 4). Thus, the height of a pigweed plant was influenced by two factors: the PPFD level and the R:FR ratio. The same trend was observed for the 12- and 16-h daylengths. Support for both PPFD and R:FR effects on plant height can be found in the literature, although stem elongation of plants is most commonly associated with low R:FR (Morgan and Smith, 1976; Morgan et al., 1983; Kasperbauer and Karlen, 1994; Smith and Whitelam, 1997). In the study of McLachlan et al. (1993b), redroot pigweed was grown under shade provided by a maize canopy, and results of this study show that the height of redroot pigweed was associated with PPFD transmitted through the maize canopy as pigweed plants grown under more shade (i.e., less PPFD transmitted) were in most cases shorter than the pigweed plants grown outside the maize canopy. The R:FR ratio impinging on the pigweed plants under the maize canopy in this study was probably also associated with the level of shade. In contrast, Begonia et al. (1988) reported considerable increases in height of soybean [Glycine max (L.) Merr.] with decreasing PPFD and constant R:FR ratio. Our results indicate that height of a pigweed plant is a result of interactive (complementary) effects of PPFD and R:FR ratio. Whether these interactions vary across different PPFD levels remains to be determined.

Apart from plant height, apical dominance, and reduced branching in dicots is another characteristic that is strongly affected by a reduction of the R:FR ratio (Smith and Whitelam, 1997). In our study, branch number was more affected by PPFD than by the R:FR ratio. The number of branches was greater at the high than at the low PPFD level for both daylength treatments, whereas no difference in number of branches was found between the two R:FR ratio treatments (Table 4). Decreased branching reported in pigweed plants shaded by a maize canopy was attributed to low PPFD impinging on the pigweed plants (McLachlan et al., 1993b). It is important to notice that in the latter study R:FR ratio impinging on the pigweed plants was probably also reduced and that the response of the number of branches was the result of confounding effects of PPFD and R:FR. Begonia et al. (1988) reported that both low PPFD and low R:FR ratio reduced development of axillary buds into branches in soybeans independent from one another.

Dry Matter Accumulation and Partitioning
The increase in PPFD from low (180 µmol m-2 s-1) to high (550 µmol m-2 s-1) resulted in a greater total dry matter accumulation in both daylength treatments (Fig. 1 and Table 5) . The effect of PPFD on dry matter accumulation was similar throughout all three sampling times. Dry weight of various plant components was affected by PPFD in a similar manner as total dry weight (data not presented). McLachlan et al. (1993b) reported a decrease in total dry matter of redroot pigweed due to a reduction of PPFD that was caused by shading of a maize canopy. The R:FR ratio did not affect total dry matter accumulation at any sampling time under both daylength treatments. However, it was surprising to find that the dry weight of plants in the LL treatment was two times greater than that of the plants in the LH treatment 24 DAE under the 12-h daylength treatment (P = 0.072) (Fig. 1), even though PPFD (Table 1) and leaf area (data not shown) were similar.



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  		Fig. 1. Effect of photosynthetic photon flux density (PPFD) and red:far-red ratio (R:FR) on stem and total plant weight of redroot pigweed under 12- and 16-h daylengths. Statistical analysis is provided in Table 5. DAE = days after emergence; HH = high PPFD and high R:FR ratio; LH = low PPFD and high R:FR ratio; LL = low PPFD and low R:FR ratio.

 

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  		Table 5. Summary of mean comparisons for dry matter accumulation and partitioning of pigweed grown under three different light quality and quantity regimes under the 12- and 16-h daylengths.

 
On the basis of reports in the literature, it was difficult to predict the response of pigweed dry matter accumulation to R:FR treatments. For example, dry weights of Eichhornia crassipes (Mart.) Solms (Méthy et al, 1990) and Impatiens capensis Meerb. (Schmitt and Wulff, 1993) were not affected by R:FR, and Ballaré et al. (1991) also found no difference in dry weight of Amaranthus quitensis Kunth when different R:FR treatments were applied to single plants. Stuefer and Huber (1998), however, reported dry weights of two Potentilla species to be larger when plants were grown under spectral shade (low R:FR) than under neutral shade (high R:FR). Similarly, when the various R:FR treatments were applied to whole canopies of Amaranthus quitensis, plant weight at low R:FR ratio was significantly greater than that of plants at higher R:FR ratio (Ballaré et al., 1991). In the latter case, the difference in dry weight was attributed to the larger stem (sink) of plants at low than at high R:FR ratio. They argued that the larger sink had stimulated leaf photosynthesis through a feedback mechanism (see Ballaré et al., 1991, for further references). We also observed that the stem weight was 3.5 times greater in plants in the LL treatment than in plants in the LH treatment (Fig. 1, Table 5) 24 DAE under 12-h daylength. The possibility of greater stem photosynthesis in LL plants should not be overlooked, as pigweed stems contain photosynthetic tissue. The taller plants would have larger stem area, which, added to the leaf area, may have contributed to the apparent greater whole-plant photosynthesis of LL plants. The difference in total dry weight between LL than LH treatments under the 16-h daylength was smaller in comparison with that under the 12-h daylength (Fig. 1), which may be attributable to the longer period of dry matter accumulation (36 vs. 24 DAE, respectively).

The ratio between the shoot and root was not affected by the light quantity–quality treatments under the 12-h daylength (Table 6) . Shoot:root ratio of maize has been positively associated with incident PPFD (Tollenaar, 1999). Kasperbauer and Karlen (1994) reported that the shoot:root ratio of maize was reduced when the R:FR ratio of light impinging on the leaves increased. However, in the 16-h daylength treatment, shoot:root ratio was smaller in HH plants than in LH and LL plants at all three sampling times (Table 6). There was no effect of R:FR ratio on dry matter allocation between the shoot and root in the 16-h day treatment. Effect of light quantity and quality on dry matter partitioning between the shoot and root under 16-h daylength may be confounded with stage of development, as plants in different treatments were not at the same stage of development when harvested (i.e., 10-d intervals after the HH treatment reached initiation of floral primordia). It has been shown that dry matter partitioning between the root and shoot in maize varies greatly with stage of development (Tollenaar, 1989).


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  		Table 6. Specific leaf weight and shoot:root ratio of pigweed plants grown under three different light quality and quantity regimes under the 12- and 16-h daylengths.

 
Specific leaf weight (amount of photosynthetic tissue per unit leaf area) was a function of plant maturity as well as PPFD level at both daylengths (Table 6). Within each daylength treatment, HH plants had greater specific leaf weights in comparison with LH and LL plants at all sampling times. The R:FR ratio generally did not influence specific leaf weight. In maize, specific leaf weight was strongly affected by PPFD (Tollenaar, 1999) and to a lesser extent by R:FR ratio (Kasperbauer and Karlen, 1994). Response of a plant to R:FR in terms of dry matter partitioning could vary from species to species (Ballaré and Casal, 2000).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study shows that growth and development of redroot pigweed are influenced by both quantity (PPFD) and quality (R:FR) of incident light. Specifically, development (i.e., rate of leaf appearance, total number of initiated leaves, initiation of floral primordia, flowering and seed set) and plant height are affected by both incident PPFD and R:FR. With the exception of dry matter allocation to the stem, dry matter accumulation and partitioning are influenced by incident PPFD only. Hence, studies that attempt to quantify physiological mechanisms that underlie crop-weed interference should take into account effects of both light quantity and quality on the phenology of pigweed.

Received for publication December 3, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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Crop Science 2002 42: 1763-1765. [Full Text]  




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