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CROP ECOLOGY, MANAGEMENT & QUALITY

The Effects of Light, Rehilling, and Mulching on Greenshoulder and Internal Greening in Carrots

Dep. of Plant and Animal Sciences, Nova Scotia Agricultural College, Truro, NS Canada, B2N 5E3

^* Corresponding author (rlada{at}nsac.ca).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Greenshoulder (GS) and internal greening (IG) in carrots (Daucus carota var. sativus) affect root appearance and can lead to significant economic loss. Experiments were conducted to quantify the critical light intensity that may trigger greening by exposing carrot hypocotyls to different light intensities. Chlorophyll a, b, and total chlorophyll were measured. The effects of rehilling on GS and IG were measured in a 2-yr field study with eight treatments imposed at various stages of growth. Another experiment to confirm the effects of rehilling by etiolating various genotypes was also conducted. Leaf area index, internal and external greening, and IG length were measured. Increasing light intensity enhanced chlorophyll a and b linearly and significantly. The highest amount of chlorophyll a, b, and total chlorophyll accumulation in hypocotyl occurred at the highest light intensity tested (320 µmol m^–2 s^–1). Rehilling reduced GS and IG significantly. However, the response varied significantly between seasons. In 2002, rehilling once at the early vegetative stage, initial bulking stage, or active bulking stage was effective in reducing GS and IG. In the 2003 field study, rehilling once at each of the three stages reduced GS and IG. Compared with the control, rehilling reduced GS by 10 to 21% and IG by 13 to 20%. Among the cultivars tested, mulching reduced GS only for Magno. The results suggest that GS and IG are induced at the hypoctoyl stage and that rehilling could reduce GS and IG considerably; however, the etiolation response appears to be genotype specific.

Abbreviations: CI, Chlorophyll index • CIAS, Computer Image Analysis System • DAE, days after emergence • DIFN, differential interception • GS, greenshoulder • IG, internal greening • LAI, leaf area index • PAR, photosynthetically active radiation • PLIG, percentage length of internal greening.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The physiological disorder greenshoulder (GS) occurs due to accumulation of chlorophyll at the crown, while internal greening (IG) is due to chlorophyll accumulation in the core region of the carrot (Daucus carota var. sativus) roots. Although GS and IG appear to occur in the same root, both can occur independently (Ravishankar, 2004). In certain genotypes, GS alone can occur, while in others, only IG can occur. Some genotypes exhibit both GS and IG (Ravishankar, 2004). Greening in carrot roots not only affects appearance but also makes it unfit for consumption because of some yet unknown bitter compounds present in the affected regions. Greenshoulder and IG are serious processing problems leading to significant economic loss for the carrot processors. Annually, in Nova Scotia alone, nearly 452 Mg of the processed carrots are culled due to GS and IG, which accounts for nearly 2% of the processed carrot products. Currently, photoelectronic sorters are being used in the processing line to detect and sort out green carrots. However, they are not effective in completely eliminating GS and IG, resulting in poor quality and reduction in recoverable finished products and profitability (C. Fullerton, personal communication, 2002). The frozen finished products have to be passed through the sorters several times to achieve some control; hence, processing becomes very expensive.

Greenshoulder and IG exhibit a significant relationship with canopy leaf area index (LAI) and differential interception (DIFN) (Ravishankar, 2004), suggesting that light could trigger GS and IG. Differential interception is the fraction of the sky visible under leaf canopy calculated as percentage. Differential interception values give a relative estimate of the quantity of light below canopy. Since hypocotyls transform into crown tissue (Esau, 1940), it is possible that GS and/or IG may be triggered by exposure of hypocotyls to light during the early growth stages. While carrots are epigeal in germination, exposure of hypocotyls to sunlight may be the reason for the development of GS and/or IG. If this is so, preventing exposure of hypocotyls to light will reduce or inhibit GS or IG. The critical light intensity that could trigger chlorophyll synthesis in carrot hypocotyls is not known. In addition, the response of carrot hypocotyls to greening under increasing light intensity has not been reported.

Etiolating the crown by rehilling and/or mulching can protect the crown from exposure to the sun and reduce GS and/or IG. Hilling or "earthing up" is generally recommended for root and tuber vegetable crops. Earlier studies by Van der Zaag (1987) showed that shallow planting of potato (Solanum tuberosum L.) tubers resulted in high tuber greening. Ali (1993) reported that hilling prevents light from reaching the tubers via soil cracks and that significant reductions in greening of potato tubers were achieved by hilling at 6, 10, and 14 wk after planting. Hilling reduces tuber greening even when planted shallow. Although hilling is commonly performed in Nova Scotia for creating a ridge on which slicer carrots are planted, no rehilling (covering the crown with soil) is done at any developmental stage. Rains following seeding or at the early stages of crop development can deteriorate hills and expose crowns to light, possibly triggering GS and IG. In addition, heavy wind and water erosion can lead to hill deterioration. Since canopy growth changes with crop growth, and canopy load has an influence on GS and IG (Ravishankar, 2004), it is possible that the relative effectiveness of rehilling would differ depending on the ontogeny of the crop. Little or no information is available on the relative effectiveness and timing of rehilling on GS and/or IG in carrots.

Several experiments were conducted to test the hypotheses that (i) exposure of hypocotyls to a critical light intensity triggers GS and IG and (ii) etiolating crown either by rehilling or mulching will reduce GS and IG. Thus, the objectives of this study were (i) to understand the relative sensitivity of hypocotyls to synthesize chlorophyll when exposed to light and to identify the minimum light intensity that would trigger greening in hypocotyls, (ii) to assess whether rehilling at different crop growth stages reduces GS and IG, and (iii) to identify the relative effectiveness of hypocotyls etiolation through mulching on GS and IG in various genotypes.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Laboratory Studies
Bioassay Standardization to Determine the Influence of the Length of Exposure Time
A bioassay standardization experiment to determine the time required for chlorophyll synthesis was conducted on 4-d-old, dark-grown hypocotyls by directly exposing the hypocotyls to a light intensity of 190 µmol m^–2 s^–1 in a growth chamber. When hypocotyls emerge, the soil temperature in the field is usually 10°C. Hence, the growth chamber was maintained at this temperature to mimic field conditions. Hypocotyls were sampled every 4 h for chlorophyll estimation over 4 d. There were three replications for the chlorophyll assay.

Preparation of Dark-Adapted Hypocotyls
The slicer cultivar Krakow, which had low GS and low IG in a cultivar evaluation field study conducted in 2002, was selected for preparation of dark-adapted hypocotyls. Seeds were sown at 4 cm from the edge of 24- by 15-cm filter papers. Seeds were placed above a cotton thread that held the seeds in place. The filter paper was then placed on a sheet of black polythene (24 by 15 cm) and rolled up to prevent light penetration. The roll was clamped to keep the set-up intact. This assembly was then placed vertically in a tray that contained 500 mL of distilled water and placed inside a controlled environmental growth room (Conviron, Winnipeg, Canada) that was maintained at 10°C for 90 h. The dark-adapted hypocotyls were then transferred to petri dishes containing 10 mL of distilled water. To identify the lower threshold light intensity required to trigger greening in hypocotyls, 20 hypocotyls were exposed to 75, 150, 200, 280, and 320 µmol m^–2 s^–1 illuminated by fluorescent and incandescent bulbs continuously. The hypocotyls were exposed to five different light intensities independently in growth chambers for 90 h until the first true leaves were formed. Photosynthetically active radiation (PAR) in the growth room was measured using the quantum sensor mounted on the leaf chamber of a LCA4 leaf gas exchange system (ADC, Hoddeson, UK). The experiment was replicated three times for each of the treatments. To prevent heating and desiccation of the hypocotyls, petri dishes containing 30 mL of water were placed over the petri dishes with hypocotyls.

Chlorophyll Estimation
Chlorophyll concentration in the hypocotyls was measured using Arnon's (1949) method after removing the true leaves. Because our preliminary study revealed that hypocotyl greening can be seen at 90 h, at the end of 90-h light period, 200 mg of the hypocotyls per replication were taken and macerated with 80% acetone using a mortar and pestle. The macerated sample was then briefly centrifuged at 3000 rotations min^–1 for 10 min. The supernatant solution was decanted in a 25-mL volumetric flask, and the volume was made up to 25 mL with 80% acetone. Using a spectrophotometer (Spectronic 20, Bausch & Lomb, Canada), the optical density of the solution was recorded at 645, 663, and 652 nm (Arnon, 1949).

Field Experiment
Etiolation Effects of Rehilling on Greenshoulder and Internal Greening
The first field experiment was conducted in Northville, Kings County, NS (45°08' N, 64°49' W) in 2002. In 2003, carrots were grown in Portapique, Colchester County, NS (45°25' N, 63°46' W). The slicer carrot cultivar Oranza (Bejo Seeds, Geneva, NY) was used for both field experiments.

Field Preparation and Cultivation
Soils in Colchester and King's counties contain 2 to 3% organic matter and are classified as Orthic Humo Ferric Podzols and Podzol great group, respectively. A 3-yr carrot–grain–forage rotation was usually followed in the chosen fields, and minimal tillage was performed in the spring of each year. Field preparation involved chisel plowing at 20 to 23 cm followed by discing to make the field even throughout. Fields were seeded at 33 seeds (30 cm)^–1 using a seven-row pneumatic seeder in three lines spaced 2.5 cm apart (Stanhay Web Ltd., Lincolnshire, UK). Seeds were sown on hills (ridges) of 25 cm height spaced 60 cm apart.

Basal fertilizer (12N–10P–20K) was applied at the rate of 45 kg ha^–1 along with 0.2% B, and a top dressing of ammonium nitrate (34N–0P–0K) at 30 kg ha^–1 was applied 8 wk after emergence. Field management practices were done as per Atlantic Provinces Agricultural Services Coordinating Committee's recommendations (Advisory Committee on Vegetable Crops, 1997). No supplemental irrigation was given.

Treatments consisted of rehilling at different crop growth stages. Rehilling was done manually to 40 to 50 cm high to cover the crown using a "draw hoe" in a seeded line. Rehilling took place 20 to 25 d after emergence (DAE) (early vegetative stage), 45 to 50 DAE (initial bulking stage) and 60 to 70 DAE (final bulking stage). The rehilling treatments were as follows: T1, control, no rehilling; T2, rehilling at 20 to 25 DAE; T3, rehilling at 45 to 50 DAE; T4, rehilling at 60 to 70 DAE; T5, rehilling at 20 to 25 and 45 to 50 DAE; T6, rehilling at 20 to 25 and 60 to 70 DAE, T7, rehilling at 40 to 50 and 60 to 70 DAE; and T8, rehilling at 20 to 25, 45 to 50, and 60 to 70 DAE.

Chlorophyll Index
Greenshoulder and IG were quantified and presented as chlorophyll index (CI) that was measured after harvest using a chlorophyll meter (CM 1000, Spectrum Technologies, Plainfield, IL). The chlorophyll meter measures the ambient and reflected light at wavelengths of 700 and 840 nm to estimate the quantity of chlorophyll in the root tissues. Chlorophyll a absorbs 700 nm light, and as a result, the reflection of that wavelength from the tissue is reduced compared with the reflected 840 nm light. Light with a wavelength of 840 nm is unaffected by leaf chlorophyll content and serves as an indication of how much light is reflected from the root tissues. The ambient and reflected lights are used to calculate and display a CI value. Chlorophyll index was measured on the roots outdoors, with the sun always at the user's back and the line between the sample and the light sensors maintained approximately parallel to the sun's rays. Following CI measurement on the root shoulder, the roots were split longitudinally and CI was measured again in the internal tissues. The sample was held at a distance of 30 cm from the chlorophyll meter. The background of the root tissue was chosen to be dark to avoid capture of any reflected light from the background. A brightness level of 2 in the range of 0 to 9 was maintained to make sure the available ambient light was sufficient to take the reading. A built-in light meter indicates light intensity (brightness, or BRT) and a brightness value of 1 or greater is an approximation that at least 250 to 300 µmol m^–2 s^–1 of PAR were available. Four CI readings were taken on each carrot root crown, and a total of 10 carrots replication^–1 were measured. Chlorophyll index readings at or below 43 showed no greening in the carrot root tissue.

Leaf Area Index
Measurements of LAI were made at harvest on a bright sunny day between 1000 and 1100 h using a LAI-2000 plant canopy analyzer (LI-COR, Lincoln, NE). The LAI-2000 plant canopy analyzer has been used to estimate leaf area indexes in several crop species (Malone et al., 2002; Tewolde et al., 2005). The LAI-2000 calculates LAI from radiation measurements made with a "fish-eye" optical sensor. One measurement was made above the canopy followed by five measurements below the canopy to determine canopy light interception at five different angles from which LAI is computed using a model of radiative transfer in vegetative canopies (Hicks and Lascano, 1995). Five readings, below the crop canopy with the fish-eye field-of-view assure that LAI calculations are based on a large sample of the foliage canopy. Carrot samples were harvested at the same spot where the LAI was measured.

Percentage Length of Internal Greening
The length of the IG of the longitudinal sections of individual carrot roots and the actual length of the individual roots were determined using a Computer Image Analysis System (CIAS) (CI-400, CID, Camas, WA). The percentage length of the IG (PLIG) was determined using the formula

where LIG = length of the IG of longitudinal section of carrot roots and L = actual length of individual roots.

Average length of the roots per plot was measured using CIAS. The total number of roots per meter of a row was recorded to account for unequal germination in the field, if any.

Pot Culture Experiment
The Effect of Hypocotyl Etiolation through Mulching on Greenshoulder and Internal Greening
This experiment was conducted under outdoor conditions at the Nova Scotia Agricultural College in Truro, NS (45°37' N, 63°27' W). Four slicer cultivars, Breman, Bergen (both Bejo Seeds, Geneva, NY), Krakow, and Magno (both Rijk Zwaan USA, Salinas, CA) were chosen based on their relative sensitivity to GS and IG (Ravishankar, 2004). The treatments were as follows: etiolated (mulched) and nonetiolated (no mulch) on Breman (high GS, low IG, and low PLIG), Bergen (high IG and low GS), Krakow (low GS, low IG, and high PLIG) and Magno (high IG and high PLIG) (Ravishankar, 2004). Each of the treatments had four replications. Pro-mix (Premier Horticulture, Riviere-du-Loup., QC, Canada) was used as mulch to etiolate hypocotyls. Carrots were seeded in a line at a rate of 33 seeds (30 cm)^–1 at a spacing of approximately 1 cm apart in 30-cm (diam.) by 50-cm (depth) pots containing field soil. Pro-Mix at 2.5 cm thickness on a transparent glass plate prevented light transmission completely. Based on this protocol, mulch was applied to 2.5-cm thickness after seeding to a depth of 0.2 cm. Mulch was maintained through out the growing season. Green shoulder, IG, and PLIG were measured on the harvested carrots.

Statistical Analysis
The experiment to evaluate the effects of light and hilling methods followed a completely randomized design. The plot size for rehilling experiment was 3 by 5 m. The treatments were randomized using a randomizer program (Urbaniak and Plous, 2000). Data collected were tested for normality using an Anderson–Darling test at = 0.05. The assumption of equal variance was also tested (Montgomery, 2001). Two-way ANOVA was done on hilling and field season using the ANOVA procedure of SAS (SAS Institute, 1999). Mean separation was done using Duncan's multiple range test of SAS.

The experiment to evaluate mulch application on greening followed a two-factor (four varieties x mulching) factorial design with four replications. This was accomplished using the general linear model procedure of SAS. Mean separation was done using Duncan's multiple range test of SAS. Regression analyses between chlorophyll concentration and PAR was performed using Minitab 13.31 (Minitab, State College, PA) and R² values were obtained. The regression model that showed the best fit was chosen to compare the relationship.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Effect of Light on Hypocotyl Greening
The time-based standardization experiment revealed that hypocotyls synthesized chlorophyll after 90 h of exposure to a light intensity of 190 µmol m^–2 s^–1 in a growth chamber maintained at 10°C (data not shown). Chlorophyll a, b, and total chlorophyll concentrations increased linearly as the light intensity increased (Fig. 1). Chlorophyll accumulation was triggered even at the lowest light intensity of 75 µmol m^–2 s^–1. The highest amounts of chlorophyll a, b, and total chlorophyll accumulation occurred at 320 µmol m^–2 s^–1 (Fig. 1).

View larger version (15K): [in this window] [in a new window]   Figure 1. Chlorophyll a, b, and total chlorophyll concentrations in carrot (Daucus carota var. sativus) hypocotyls as influenced by increasing photosynthetically active radiation (PAR).

View larger version (37K): [in this window] [in a new window]   Figure 2. (a) Leaf area index (LAI) and (b) root number per meter of a row, as altered by rehilling during field seasons 2002 and 2003.

During 2002, rehilling even once, at any stage of growth, reduced GS significantly compared with the control (Fig. 3). This was similar to rehilling twice at any stage of growth, and the reduction was 21% compared with the control. In 2003, however, three times rehilling significantly reduced GS compared with the control. This was similar to rehilling twice at any stage of growth, and the reduction was 10% compared with the control (Fig. 3).

View larger version (30K): [in this window] [in a new window]   Figure 3. (a) Greenshoulder (GS), (b) internal greening (IG), and (c) percentage length of internal greening (PLIG), as altered by rehilling during field seasons 2002 and 2003. Greenshoulder and IG were measured as chlorophyll indices (CI). Chlorophyll index readings below 43 had no greening in carrot root tissues.

Rehilling twice at 20 to 25 and 45 to 50 DAE completely eliminated PLIG during 2002. During 2003, however, hilling three times at 20 to 25, 45 to 50, and 60 to 70 DAE had the lowest PLIG compared with the control (Fig. 3).

The Effects of Mulching on Greenshoulder and Internal Greening
Analysis of variance showed a significant interaction between treatment and variety for GS and IG (Table 2). The response of the seedlings to mulching appeared to be genotype specific. Magno showed a significant difference in GS between mulched and nonmulched conditions, while in the other three cultivars, GS did not differ significantly between the treatments (Fig. 4). Breman had the highest GS under both mulched and nonmulched conditions. Although mulching reduced IG, the reduction in IG was not sufficient enough to be statistically significant in any of the varieties tested. Mulching reduced IG in Breman and Magno to 4 and 6%, respectively (Fig. 4). When cultivars were compared there were no significant differences in PLIG.

View larger version (24K): [in this window] [in a new window]   Figure 4. (a) Greenshoulder (GS), (b) internal greening (IG), and (c) percentage length of internal greening (PLIG), as influenced by hypocotyl etiolation. Greenshoulder and IG were measured as chlorophyll indices (CI). Chlorophyll index readings below 43 had no greening in carrot (Daucus carota var. sativus) root tissues.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Effect of Etiolation by Rehilling on Greenshoulder and Internal Greening
Depending on the season, rehilling once at the early vegetative stage (20–25 DAE) or at the active bulking stage (60–70 DAE) controlled GS (Fig. 3). With respect to IG, rehilling once, either at 20 to 25 or three times at 20 to 25, 45 to 50, and 60 to 70 DAE, depending on the season, reduced its occurrence. This suggests that greening can be controlled by reducing the light intensity reaching the crown through rehilling if performed at an early vegetative stage or by the bulking stage. Currently, field management of carrots does not involve rehilling, and GS and IG in field-grown carrots may possibly be due to exposure of the crowns to light. It is also possible that soil and wind erosion can lead to greening through crown exposure to light. However, available soil moisture can influence attainment of critical LAI for plant growth (Egli, 1988). Rehilling three times at different stages increased LAI and root number possibly by increasing infiltration, moisture retention, and better aeration around the root zone. The highest LAI reduced incidence of GS and IG, presumably by reducing the amount of light reaching the root crowns. Seeding at an optimal rate resulted in an optimal LAI that minimized GS and IG development in carrot (Ravishankar, 2004). This is one of the technologies that are currently being followed in carrot cultivation.

Rehilling could have also prevented soil cracks, reducing exposure of carrot roots to light as demonstrated by Ali (1993) for potatoes. Although rehilling can reduce greening in carrots, it may also cause root pruning and cause damage to the edible roots. However, in our study, there was no reduction in yield or quality observed due to rehilling (data not shown). Greenshoulder and IG differed between 2002 and 2003 field seasons. Such a variation, however, cannot be attributed to variations in plant population since the fields were precision seeded. While root numbers between field seasons also differed, it is difficult to say whether such a variation in root number would have resulted in differences in GS and IG between field seasons. Although the number of germinated seedlings determines root number, not all germinated seedlings will survive or develop roots since several factors will determine the success of the seedlings, including drought and diseases, altering the survival of the plants.

Effect of Etiolation of Hypocotyls by Mulching on Greenshoulder and Internal Greening
Carrot hypocotyls are sensitive to light and develop greening following exposure to even a low light intensity (Ravishankar, 2004). Greenshoulder and IG occurrence differs among the genotypes (Ravishankar, 2004). Some cultivars exhibit only GS, and some are sensitive to IG. The cultivars used in this experiment showed differential response to GS and IG. Although etiolation significantly reduced GS in Magno, there was no reduction in GS in Breman, Bergen, or Krakow. While the reason for the varietal specificity is not known, it is possible that the cultivars may have differential levels of cytokinins that are essential for chlorophyll formation. Studies by Mok (1994) have shown that chloroplast differentiation can be altered by endogenous cytokinin levels. Even though PLIG did not differ significantly among the cultivars, etiolation reduced PLIG significantly, compared with nonetiolated plants. Since etiolation reduced GS only in Magno, the etiolation response may be genotype specific and, perhaps, gene controlled (Ravishankar, 2004). In general, the CI values remained low compared with the field-grown carrots as the seedlings were young and harvested early. Pro-Mix at a thickness of 2.5 cm on a glass plate completely blocked the transmission of light when the light was passed through the bottom. Thus, the lack of response of certain cultivars could not be claimed to be due to any inconsistencies in treatment imposition. The lack of significant differences in GS and IG between etiolated and nonetiolated hypocotyls in several varieties may be due to the remote possibility of dislocation of mulching materials during watering since mulch height was carefully adjusted following each watering. Although mulching has been shown to reduce GS, it is not currently being followed in carrot production. To prevent GS and IG, from early exposure of hypocotyls, etiolation by mulching at least in the initial stage of plant development would be beneficial.

While rehilling and mulching could reduce GS and IG, it is not known whether the potential benefits due to rehilling and mulching could compensate the costs associated with these practices.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The studies suggest that GS and IG may be triggered at the hypocotyl stage and that light is the triggering factor. Increasing light intensity linearly increased chlorophyll a and b concentrations. Photosynthetically active radiation, as low as 75 µmol m^–2 s^–1, was sufficient to trigger greening in carrot hypocotyls. Rehilling differentially reduced GS and IG. Etiolating hypocotyls through mulching could reduce GS and PLIG in certain varieties.

	ACKNOWLEDGMENTS

 
This paper is a portion of M.Sc thesis submitted to Dalhousie University, Canada, by Ravishankar Palanisamy.

The authors thank for the funding provided for this study by Nova Scotia Department of Agriculture and Fisheries, Technology Development 2000 program, Oxford Frozen Foods Ltd., and Bragg Lumber Co. Ltd.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication August 10, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

• Ali, M.A. 1993. Effects of cultural practices on reducing field infestation of potato tuber moth (Phthorimaea operculella) and greening of tubers in the Sudan. J. Agric. Sci. 121:187–192.
• Advisory Committee on Vegetable Crops. 1997. Vegetable crops production guide for the Atlantic Provinces: Carrots and parsnips. Publ. no. 1400. Available at http://nsac.ns.ca/lib/apascc/acv/production/99index.htm (verified 30 Apr. 2007) Nova Scotia Dep. of Agric., NS, Canada.
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• Hicks, S.K., and R.J. Lascano. 1995. Estimation of leaf area index for cotton canopies using the LI-COR LAI-2000 plant canopy analyzer. Agron. J. 87:458–464.[Abstract/Free Full Text]
• Malone, S., D.A.J. Herbert, and D.L. Holshouser. 2002. Evaluation of the LAI-2000 plant canopy analyzer to estimate leaf area in manually defoliated soybean. Agron. J. 94:1012–1019.[Abstract/Free Full Text]
• Mok, M.C. 1994. Cytokinins and plant development: An overview. p. 129–137. In D.W.S. Mok and M.C. Mok (ed.) Cytokinins: Chemistry, activity, and function. CRC Press, Boca Raton, FL.
• Montgomery, D.C. 2001. The design and analysis of experiments. John Wiley & Sons, New York.
• Ravishankar, P. 2004. Genotypic variations and physiology of greenshoulder (GS) and internal greening (IG) in processing carrots (Daucus carota L. var. sativus). Master's thesis, Dalhousie Univ., Halifax, NS.
• SAS Institute. 1999. Proprietary software, version 8. SAS Inst., Cary, NC.
• Tewolde, H., K.R. Sistani, D.E. Rowe, A. Adeli, and T. Tsegaye. 2005. Estimating cotton leaf area index nondestructively with a light sensor. Agron. J. 97:1158–1163.[Abstract/Free Full Text]
• Urbaniak, G.C., and S. Plous. 2000. Research randomizer. Available at http://randomizer.org (verified 30 Jan. 2007). Social Psychology Network, Dep. of Psychology, Wesleyan Univ., Middletown, CT.
• Van der Zaag, P. 1987. Developments in potato production techniques in Asia: Workshop on recent and future developments of the potato in the world. Acta Hort. 213:79–90.

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