See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/253342350 Scaling up Production of Traditional Green Leafy Vegetables in Kenya: Perspectives on Water and Nitrogen Management Article · January 2007 CITATIONS READS 0 277 6 authors, including: Chris Ojiewo Gaya Agong International Crops Research Institute for Semi Arid Tropics Jaramogi Oginga Odinga University of Science and Technology (form… 69 PUBLICATIONS   240 CITATIONS    42 PUBLICATIONS   383 CITATIONS    SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Vegetable Breeding and Seed Systems for Poverty Reduction in Africa (vBSS) View project Tropical Legumes II Project View project All content following this page was uploaded by Chris Ojiewo on 21 May 2014. The user has requested enhancement of the downloaded file. Dynamic Soil, Dynamic Plant ©2007 Global Science Books Scaling up Production of Traditional Green Leafy Vegetables in Kenya: Perspectives on Water and Nitrogen Management Peter Wafula Masinde1* • Christopher Ochieng Ojiewo2 • Kenji Murakami2 • Stephen Gaya Agong3 1 Jomo Kenyatta University of Agriculture and Technology P. O Box 62000-00200, Nairobi, Kenya 2 Faculty of Agriculture, Okayama University, 1-1-1 Tsushima Naka, Okayama 700-8530, Japan 3 Maseno University P.O. Box Private Bag, 40105, Maseno, Kenya Corresponding author: * masinde_peter@yahoo.co.uk ABSTRACT Traditional green leafy vegetables are promising alternative vegetable crops consumed in Kenya and other African countries. The crops whose consumption is on the increase include vegetable amaranth (Amaranthus spp.), African nightshades (Solanum spp.) and spiderplant (Cleome/Gynandropsis gynandra). They are popular in the Kenyan retail markets and key supermarket chains as surveys have shown. This offers the urban dwellers access to these vegetables on one hand, while offering a reliable market for growers on the other hand. They are also potential export crops as the consumption continues to widen in most parts of Asia too. For a long time these crops have not been integrated into mainstream agriculture. Consequently, they have received little attention in terms of research and development, resulting in many gaps in information. Production continues to be on small-scales, with the farmers being the major custodians of the genetic materials and production technologies. With the current upsurge of interest in traditional vegetables, there is need to raise production to meet the increasing demand. Some agronomic studies aiming to develop optimal cultivation practices for improved yield and nutritive quality of these crops have been reported. More research work on these crops is necessary to facilitate increased production. In this paper, research into the water and nitrogen use in traditional leafy vegetables is reviewed. The paper aims to show the current status of research, major gaps in information in an effort to scale up production of these crops to meet the increasing demand. _____________________________________________________________________________________________________________ Keywords: Amaranthus spp., Cleome gynandra, nitrogen use, plant growth, Solanum spp. CONTENTS INTRODUCTION...................................................................................................................................................................................... 105 WATER MANAGEMENT DURING PRODUCTION OF TRADITIONAL GREEN LEAFY VEGETABLES....................................... 106 NITROGEN MANAGEMENT DURING PRODUCTION OF TRADITIONAL GREEN LEAFY VEGETABLES................................ 107 Plant responses to nitrogen stress .......................................................................................................................................................... 107 Use of nitrogen critical dilution curves in nitrogen management .......................................................................................................... 108 Nitrate accumulation in traditional leafy vegetables.............................................................................................................................. 108 CONCLUSION.......................................................................................................................................................................................... 109 ACKNOWLEDGEMENTS ....................................................................................................................................................................... 109 REFERENCES........................................................................................................................................................................................... 109 _____________________________________________________________________________________________________________ INTRODUCTION tinely reported in the Ministry of Agriculture yearly reports. Consequently the crops are highly priced, both the leafy Past surveys have shown that traditional vegetables in vegetable and the seed for planting. Kenya are produced mainly in “kitchen gardens” (Chweya The crops whose consumption is on the increase include and Eyzaguirre 1999). In these surveys, it was noted that vegetable amaranth (Amaranthus spp.), African nightshades large-scale production of traditional vegetables was ham- (Solanum spp.) and spiderplant (Cleome/Gynandropsis gy- pered by lack of promotion and markets. Okeno et al. nandra). They are popular in the Kenyan retail markets as (2003) argue that the introduction of “exotic vegetables” all surveys have shown. Besides, they are also consumed in has contributed to the confinement of the traditional vege- various African countries (Chweya and Eyzaguirre 1999). tables into small backyard gardens. We have done similar These crops have nutritional value comparable or even surveys recently in 2003 and 2004 (Agong and Masinde higher than that of the well established “exotic vegetables” 2006) and it is clear that production has remained largely on (Grubben and Denton 2004). Increased utilization of these kitchen garden scales. However, it is apparent that the con- vegetables will go a long way in alleviating malnutrition in sumption has continued to increase. There have been suc- communities whose staple food is maize and have few other cessful campaigns and promotional activities aimed at en- sources of vitamins and minerals. Welch and Graham couraging the consumption of traditional green vegetables (2004) suggest that developing micronutrient-enriched sta- mainly by non-governmental organisations like FORMAT, ple plant food through traditional plant breeding or mole- KENRIK and SACRED-Africa. Consequently, production cular techniques could be one way to counter micronutrient and consumption of these vegetables is spreading out of the malnutrition. Use of traditional leafy vegetables may be an traditional rural areas to urban and peri-urban areas. The in- easier and cheaper way to intervene in micronutrient mal- creased demand has resulted in shortages, which are rou- nutrition among the vulnerable populations in Kenya. It is Received: 26 March, 2007. Accepted: 25 August, 2007. Mini-Review Dynamic Soil, Dynamic Plant 1(2), 105-111 ©2007 Global Science Books important to note that these vegetables have already entered plant water status. In vegetable amaranth (Amaranthus spp.), the supermarkets, and are sold alongside the well estab- Liu and Stützel (2002b) found that transpiration and stoma- lished “exotic vegetables” like cabbage (Brassica oleracea tal conductance declined when plants were subjected to soil var. capitata) and kales (Brassica oleracea var. acephala). moisture below 22-53%, and 32-60% of available soil water, This offers the urban dwellers access to these vegetables on respectively. The stomatal conductance ranged from 0.8 one hand, while offering a reliable market for growers on mol m-2 s-1 at the time of water deficit imposition to close to the other hand. Scaling up production of these vegetables to zero at the time when all the available plant water had been meet the demand has to be addressed urgently. This paper exhausted from the soil. In both spiderplant and African reviews the current status of research and major gaps in nightshades, transpiration declines when the soil moisture information for vegetable amaranth, African nightshade and falls below 46-77% of the available soil water while the leaf spiderplant, especially in the aspects of water and nitrogen relative water content declined to 54-60% as compared to application. above 70% in control plants (Masinde et al. 2005a, 2006). Eggplants grown under short term, long term and severe WATER MANAGEMENT DURING PRODUCTION OF moisture stress (irrigating to field capacity at 14, 28-days TRADITIONAL GREEN LEAFY VEGETABLES interval and no irrigation, respectively) had total dry bio- mass of 54-63%, 33-40% and 14-25% of the control, res- Previously, it was thought that traditional vegetables were pectively (Sarker et al. 2005). In these plants, photosyn- tolerant to water stress, considering that they grew unatten- thesis rate was highest in control plants in the range of 6-24 ded to in the wild. This resulted in a general feeling that µmol m-2 s-1 at 32 days after the start of the treatments as traditional vegetables are low input crops, which are more compared to values of 0.83-0.99 µmolm-2s-1 under severe tolerant to abiotic and biotic stresses as compared to exotic stress at 42 days after the start of the treatments (Sarker et vegetables (Okeno et al. 2003; Adebooye and Opabode al. 2005). Withholding water for 18 days reduced beans 2004). Studies have shown that the response of traditional (Phaseolus vulgaris L.) dry weight by about 55% compared green leafy vegetables depends on the species. Vegetable with control plants (Ferrat-Lazcano and Lovatt 1999). amaranth is thought to be tolerant to water stress (Schippers Monti et al. (2006) found that in sugarbeet (Beta vulgaris L. 2000; Palad and Chang 2003). Liu and Stützel (2002a) var. Monodoro), plants grown under adequate water supply found that vegetable amaranth was characterised by a high had 34-52% higher total dry weight than plants exposed to capacity of osmotic adjustment in the range of 1.08-1.24 transient and permanent drought stress. In addition, they MPa that could sustain turgor maintenance and hence dry found that photosynthesis rate was 120% higher in control matter production during drought stress. They also showed plants than in drought stressed plants. In these crops, the that under drought stress, the reduction of transpiration of final impact of water deficit on the yield will depend on dry vegetable amaranth was mainly due to reduction of stomatal matter partitioning to the fruit in eggplant, seed in beans conductance, rather than reduction of leaf expansion, and and root in sugarbeet. However, with the leaf as edible part that the responses depended on the genotypes (Liu and Stüt- for traditional leafy vegetables, water deficit has a direct zel 2002b). Despite showing significant osmotic and stoma- impact on the yield. Generally, for spiderplant and African tal adjustments as mechanisms, drought stress causes reduc- nightshades, decline in leaf growth and yields begin once tions in vegetative growth of amaranthus. Ayodele (2000) the soil moisture declined below 60% of the available plant found that imposing water stress at the vegetative stage soil water (Masinde et al. 2005a, 2006). reduced amaranthus leaf area by 18-20%. He also showed In scaling up production of traditional leafy vegetables, that water stress generally reduced plant height, number of it is crucial to address the question of water management. leaves and root length irrespective of the stage at which Irrigation should be part and parcel of the increased pro- water stress was imposed. Spiderplant and African night- duction. Water-saving irrigation strategies such as deficit shades are highly sensitive to drought (Chweya and Mnzava irrigation (DI) and partial root drying (PRD) (Kang and 1997; Edmonds and Chweya 1997). Leaf expansion and Zhang 2004) may be of use in vegetable crops. In both stem elongation for both crops declined significantly when cases, plants undergo partial stomatal closure in response to the soil moisture fell below 40-60% of available soil water root signals probably the hormone abscisic acid (ABA) and they showed limited osmotic adjustment in the range of from drying soil (Sobeih et al. 2004; Liu et al. 2006) and 0.10-0.33 MPa (Masinde et al. 2005a, 2006). In addition, this may maintain high plant water status. Root signals well watered plants had 3-5 fold higher leaf area and up to include ABA and inorganic ions like nitrates, K+ and Ca2+ 45% more dry weight than plants exposed to water deficit. (Davies et al. 2002). Deficit irrigation and partial root These crops therefore respond to drought mainly by drastic drying have been tried in various crops. In pepper (Capsi- reductions in leaf area and vegetative growth. Plants grow- cum annuum L.) Dorji et al. (2005) have shown that leaf ing under drought stress may accumulate proline as one of water potential was lower in plants grown under DI and the compatible solutes during osmotic adjustment (Chaves PRD than in control plants, with values of -1.1 MPa, -0.9 and Oliveira 2004). In soybean (Glycine max L. cv. ‘Willi- MPa and -0.7 MPa, respectively at 130 days after sowing. ams’), plants growing in a soil allowed to dry to 70% field They found significant differences in fresh fruit yield with capacity accumulated up to 9- fold more proline in shoots values of 3778.6, 3054.5 and 2468.0 g/plant for the control, and up to 30-fold more proline in roots as compared to PRD and DI respectively, but no significant differences plants maintained at near 100% field capacity (Porcel and were observed in the dry fruit weight per plant. Leaf water Ruiz-Lozano 2004). Sarker et al. (2005) subjected egg- potential did not differ significantly between well watered plants (Solanum melongena L.) to four moisture stress treat- (WW) and PRD tomato (Lycopersicon esculentum Mill.) ments: irrigation to pot capacity after every 7 days (control), plants even though the PRD plants received half the water 14 days (short term stress), 28 days (long term stress) and of WW plants (Sobeih et al. 2004). The stomatal conduc- no irrigation (severe stress). They found that at 56 days tance of these PRD plants was 64% that of WW plants. after the start of the treatments, proline levels in leaves were Trials with potato (Solanum tuberosum L. cv. ‘Folva’) 74-115 times greater in the stress treatments as compared to showed that leaf water potential between WW, DI and PRD the control. David et al. (1984) have shown that spiderplant remained similar at -0.53 MPa in early stages of treatments can accumulate up to 9-fold more proline when plants are (Liu et al. 2006). At later stages, WW and DI plants had grown under water deficit. They however found that this similar leaf water potential of -0.73 MPa as compared to a accumulation did not change net photosynthesis or leaf con- lower value of -0.85 MPa in PRD plants. Sobeih et al. ductance leading to the conclusion that proline had no role (2004) suggested that exploiting genotype variation in gene- in inducing drought tolerance. ration of and response to chemical signals in order to main- Soil water deficit has negative effects on the physiolo- tain leaf growth under soil drying maybe a better crop-im- gical functions of the plant such as transpiration, stomatal provement strategy in leafy crops. There is need for a com- conductance and phostosynthesis as well as reducing the prehensive evaluation of the spiderplant, African nightshade 106 Scaling up production of traditional green leafy vegetables in Kenya. Masinde et al. and amaranthus genotypes to quantify their ability to gene- Plant responses to nitrogen stress rate root signals under drying soil. This information is crit- ical for the development of appropriate water saving irriga- It is known that nitrogen deficiency exerts its effects on tion strategies that will enable affordable production of tra- plant growth through reduced leaf area index and hence low ditional leafy vegetables without relying on rainfall. light interception and low dry matter production (Jones 1992; Grindlay 1997). The leaf nitrogen content correlates NITROGEN MANAGEMENT DURING PRODUCTION well with the leaf chlorophyll content, hence a low leaf N OF TRADITIONAL GREEN LEAFY VEGETABLES content as occurs during N deficiency leads to reduced pho- tosynthesis resulting in lower biomass accumulation (Sin- Our recent surveys (Agong and Masinde 2006) have shown clair and Horie 1989; Zhao et al. 2005). Potato (Solanum that most farmers of traditional leafy vegetables do not use tuberosum L.) plants supplied with 1500 mg N/plant at 7-10 mineral fertilizers, opting to apply manures instead. How- day interval (non-limiting N) had 3 times higher area of leaf ever, in peri-urban areas where production was more market numbers 8 and 10 as compared to plants supplied with 250 oriented, fertilizers were in use. Scaling up production will mg N/plant (limiting N) at the same interval (Vos and van require application of fertilizers to supply the necessary der Putten 1998). Similarly, Zhao et al. (2005) showed that nutrients, except under organic farming systems. There are in sorghum (Sorghum bicolor (L.) Moench), leaf area was conflicting reports on the nitrogen requirements of the vari- 3-fold higher in plants supplied with half strength Hoag- ous traditional vegetables grown in Kenya. Various fertilizer land’s nutrient solution (100% N, adequate nitrogen) as recommendations for traditional leafy vegetables are avail- compared with those receiving no N in the nutrient solution able in the literature (Table 1). (0% N, nitrogen-deficient). On the other hand, maize in- Being green leafy vegetables, they may be high nitrogen creased leaf area only by 30% in plants supplied with 6.0 g consumers with optimum levels of up to 5 g N/plant repor- N/plant (non-limiting N) as compared with those supplied ted for African nightshades (Murage 1990). At this level, with 0.5-0.84 g N/plant (limiting N) (Vos et al. 2005). plants gave leaf yields of 51 t/ha compared to 20 t/ha in Understanding the strategy adopted by traditional leafy control plants. However, this level of N increased leaf vegetable crops when faced with nitrogen limitation is im- nitrate content 7 times more than in the control plants. Khan portant in their nitrogen management. A strategy that leads et al. (1995) found that applying 1.80 g N/plant in a pot to larger reductions in leaf area under nitrogen stress as the experiment gave 14.4% higher plant dry weight, 13.2%, case in potato will result in larger reductions in leaf yield as 42.3% and 12.0% higher leaf N, P and K content, respec- compared to the case of maize, which experiences less re- tively, than control plants. In a field experiment, Opiyo duction in leaf area. (2004) applied 52-104 kg N/ha (0.23-0.94 g N/plant) to Increasing nitrogen supply to 100-200 kg N/ha (equiva- black nightshade (Solanum nigrum L.) and found leaf yields lent of 3.1-6.3 g N/plant) in tomato (Lycopersicon esculen- of up to 15 t/ha as compared to 9 t/ha in control plants. He tum Mill.) gave optimum leaf area index and increased ab- found no significant effects on the levels of oxalates and sorbed photosynthetically active radiation to 2222-2923 phenolics in the leaves. In amaranth (Amaranthus cruentus mol m-2 as compared to 1860-2272 mol m-2 in control plants L.), application of 3 t/ha (12 g/plant) maize-stover compost (Tei et al. 2002). Nitrogen supply of 25-100 kg N/ha signi- with 30 kg N/ha (0.12 g N/plant) gave plant height, number ficantly increased dry matter production in soybean (Gly- of leaves, leaf area/plant and cumulative fresh shoot yield cine max (L.) Merr) to levels of 705-860 kg/ha as compared of 46 cm, 9.2 leaves, 469.1 cm2 and 24.3-25.2 t/ha, respec- to 630 kg/ha in control plants (Taylor et al. 2005). In sun- tively as compared to respective values of 20.4 cm, 7.1, flower (Helianthus annuus L. var. CATISSOL-01), plants 31.8 cm2 and 12.1-12.5 t/ha in control plants (Akanbi and supplied with 70% of full strength Long Ashton solution Togun 2002). In addition, plants supplied with 3 t/ha com- containing 282 ppm N (high nitrogen) produced nearly post and 30 kg N/ha gave the highest contents of N, P, K, four-fold the dry matter produced by plants supplied with with values of 2.42%, 0.41%, 0.64%, respectively, while the same solution containing 2.82 ppm N (low nitrogen) control plants had N, P, K contents of 0.92%, 0.18%, 0.59%, (Cechin and de Fátima Fumis 2004). As plants restrict their respectively. For spiderplant, Onyango et al. (1999) ob- leaf area probably to maintain a high leaf nitrogen concen- tained yield of 65.2 g edible part/plant when 2.5 t compost tration under nitrogen stress, they may end up with a re- and 50 kg/ha Diammonium phosphate (18N:46P:0K) were duction in specific leaf area (SLA). Meziane and Shipley applied as compared to yields of 1.5 g edible part/plant in (2001) working on 22 herbaceous species have shown that control plants. supplying 6 mM N in Hoagland’s solution under low irradi- Nitrogen recommendations for nightshades ranged from ance (200 μmolm-2s-1 photon flux density) gave a higher 1.8 g N/plant to 5 g N/plant. Recommendations for spider- SLA in the range of 169-473 cm2/g as compared to a range plant and vegetable amaranth were also variable. Schulte of 93-348 cm2/g under high irradiance (1100 μmol m-2 s-1 auf’m Erley et al. (2005) have shown that application of 80- photon flux density) or 120-355 cm2/g under 1 mM N and 120 kg N/ha in grain amaranth gave grain yields of up to low irradiance. However, Cechin and de Fátima Fumis 2767 kg/ha with harvest index of 0.23 as compared to 1986 (2004) found that in sunflower, nitrogen supply had no kg/ha with harvest index of 0.22. Similarly, Pospišil et al. effect on the specific leaf weight, the reciprocal of specific (2006) found that applying 50-100 kg N/ha significantly leaf area. A low SLA suggests thicker leaves as opposed to increased grain amaranth yield to 1434-1525 kg/ha com- thinner leaves which have a high SLA. A reduction in SLA pared to 1042 kg/ha under control treatment but had no sig- under nitrogen deficiency has also been associated with ac- nificant effect on plant height and dry matter. Application of cumulation of starch in leaves (Grindlay 1997) as has been nitrogen has the potential to increase leaf yields and change observed in tomato (Le Bot et al. 1998). It is thought that the nutritive values of traditional leaf vegetable crops. whereas thicker leaves have a greater concentration of the photosynthetic apparatus per unit leaf area, broad thinner leaves can intercept more light (White and Consuelo Mon- Table 1 Recommendations for nitrogen application to selected traditional vegetables. Crop Experiment site Recommendation/ treatments giving highest yield Reference Amaranthus Field 3.0t/ha compost+30 kg N/ha Akanbi and Togun 2002 - 10t/ha compost+94 kg/ha Palad and Chang 2003 African nightshade Field 5 g N/plant Murage 1990 Greenhouse 1.8 g N/plant Khan et al. 1995 Spiderplant Field 2.5 t/ha manure + 500 kg/ha DAP Onyango et al. 1999 - 200 t/ha farm yard manure + 260 kg N/ha Chweya and Mnzava 1997 107 Dynamic Soil, Dynamic Plant 1(2), 105-111 ©2007 Global Science Books tes 2005). Thin large leaves are desirable for traditional crop nutrition husbandry. Use of empirical models like criti- leafy vegetables since leaf size is a quality attribute. Thus cal nitrogen concentration curves can be an important tool genotypes that maintain a relatively high SLA under low in nitrogen management (Greenwood et al. 1990, 1991; nitrogen conditions are high yielding. Lemaire et al. (1992) Lemaire et al. 1992). Seginer et al. (2004) and Tei et al. found that as the aerial biomass increased, the leaf to stem (2002) reported such curves in lettuce (Lactuca sativa var. ratio of lucerne (Medicago sativa L.) declined in a power capitata) and tomato (Lycopersicon esculentum Mill.), res- function irrespective of the nitrogen supply. In this case, as pectively. The curves take a general form of % N=aW-b, the plants increased in size, they were allocating relatively where W refers to plant dry weight and a and b are coeffici- more dry matter to the stem for support than leaves. Tradi- ents which could depend on the plant species. These curves tional leafy vegetables that can allocate relatively more dry show the plant N concentration declining in a power func- matter to leaf blades than the stem with increasing plant tion of the increasing plant dry weight. Use of critical nitro- size may be more suitable since the plants will have a rela- gen curves has been suggested for an accurate diagnosis of tively larger leaf (edible part) fraction. wheat (Triticum aestivum) N nutrition (Justes et al. 1994). Leaf nitrogen concentration is an important physiologi- In this case, nitrogen nutrition level is represented by a cal parameter that indicates the plant nitrogen status. Plants nitrogen nutrition index (NNI), which is obtained as a ratio supplied with nitrogen have higher plant nitrogen concen- of total nitrogen concentration measured in the plant to the tration as has been shown in soybean (Glycine max) (Taylor critical nitrogen concentration corresponding to the amount et al. 2005), wheat (Triticum aestivum L.) (Sinclair et al. of plant dry matter produced. The nitrogen nutrition is 2000), sorghum (Zhao et al. 2005) and various herbaceous optimal when NNI is 1, limiting when it is lower than 1, species (Meziane and Shipley 2001). Lemaire et al. (2005) and in excess when its higher than 1. There is need to estab- showed a linear relationship between shoot nitrogen content lish the critical nitrogen concentration curves for traditional and leaf area in lucerne (Medicago sativa L.) irrespective of leaf vegetables for optimal nitrogen management. Extensive the growing conditions. They argued that the amount of evaluation of nitrogen requirements for the various tradi- nitrogen in the shoot represents the N available for leaf tional vegetables in different agro-ecological zones will be expansion. A clear understanding of the response of tradi- necessary to calibrate the curves. tional leafy vegetable crops to nitrogen supply in terms of leaf area expansion, leaf nitrogen concentration and dry Nitrate accumulation in traditional leafy vegetables matter production is vital in the efforts to develop appropri- ate nitrogen management strategies. As traditional leafy vegetable crops become commercial, field-scale growers may resort to supplying high amounts of Use of nitrogen critical dilution curves in nitrogen fertilizers in order to obtain high leaf yields. This may be management harmful to the consumers since these vegetables are known to accumulate phytochemicals like phenolics, alkaloids, nit- Nitrogen uptake rate is regulated by both soil N availability rates and oxalates, whose concentrations may depend on the and crop growth rate (Gastal and Lemaire 2002). Notably, level of fertilizer use. Khan et al. (1995) showed that the the N concentration in plants declines as they grow even alkaloid solasodine found in Solanum nigrum L. was 118% when the N supply is ample suggesting that the relationship higher in plants supplied with 1.80 g N/plant than in control between N uptake and plant growth is complex (Gastal and plants. Slower growth rate of plant crops under moisture Lemaire 2002). Decline in N concentration with plant stress prevents the dilution effect of nutrient elements growth has been attributed to a dilution effect related to (Alam 1999). This as well as luxury N uptake may result in greater plant dry matter increase than N accumulation rate. accumulation of nitrates in plant tissues (Wright and Davi- This has been demonstrated in canola (Brassica napus L. son 1964). ssp. oleifera var. annua) in which plants supplied with 150 Nitrate accumulation in vegetable crops is a well recog- kg N/ha had 3.97% N at the flower bud stage, but declined nized problem that poses health hazards to consumers. to 1.47% N at maturity stage (Chamorro et al. 2002). Espi- Nitrates once ingested by humans are reduced in the liver to nola et al. (2001) also found that greater dry matter ac- nitrites, which in turn combines with haemoglobin render- cumulation rate than N accumulation rate resulted in dec- ing it unable to bind oxygen, a condition called methemo- line in nitrogen concentration in pickling cucumber (Cucu- globinemia (Taiz and Zeiger 1998). Nitrates can also be mis sativus L.). This dilution is brought about by self sha- converted into nitrosamines that are known to be potent ding in closed canopies and a change in leaf to stem ratio as carcinogens and are associated with various cancers (Hill the plant grows. Shaded leaves have low N content. Leaves 1991; Taiz and Zeiger 1998). Lettuce (Lactuca sativa L.) is with a large protein content are formed early in growth, but known to accumulate high amounts of nitrates depending as growth proceeds, proportionally more structural tissues on the source of nitrogen. Maršic and Osvald (2002) used of low N content are formed, reducing the leaf to stem ratio nitrates as a source of nitrogen in a Resh solution at 13 mM and contributing to the decline in plant N with growth (Gas- and 5 mM and found nitrate contents in the outer parts of tal and Lemaire 2002; Lawlor 2002). lettuce of 2591-3277 mg/kg and 825-1608 mg/kg, respec- Stem nitrogen concentration of spiderplant under well tively. When they used ammonium at similar levels of 13 watered conditions declined from 5% when dry weight was mM and 5 mM, nitrate content of outer parts of lettuce were below 5 g/plant (seedling stage) to 1.5% when dry weight 32.9-38.8 mg/kg and 31.7-46.2 mg/kg, respectively. van der was above 30 g/plant (flowering stage) (Masinde et al. Boon et al. (1990) found nitrate levels in lettuce of 2542- 2005b). Under water deficit, there was a similar decline but 2850 mg/kg when the nitrogen source was 80% ammonium the concentration remained about 2.5% with final plant dry as compared to 5053-5148 mg/kg when nitrate was the only weight at 16 g/plant. Similar declines were consistently de- source of nitrogen. In celery (Apium graveolens), Martig- monstrated for petioles and roots under glasshouse and field non et al. (1994) found nitrate levels of 5321-6130 mg/kg conditions, but for leaf blades the decline was only slight or fresh weight when nitrate fertilizers were used as compared absent. The slower decline in N concentration of plant tis- to levels of 2721-3695 mg/kg fresh weight when nitrate fer- sues observed in plants exposed to water deficit was attrib- tilizers were avoided a week before harvesting. Increasing uted to reduced dry matter production hence low dilution. NO3-N from 30% to 70% in a nutrient solution (8 mM N) Similar results have been reported in strawberry (Fragaria increased the NO -3 concentration in endive (Cichorium en- × ananassa Duch.), where plants irrigated according to a divia L. var. crispum Hegi) from 2400 to 6100 mg/kg fresh climatic water balance model had 2.94% N in leaves, which mass (Santamaria and Elia 1997). Guvenc (2002) compared was significantly lower than 3.04% N in leaves of plants not radish (Raphanus sativus L.) plants grown at 0, 100 and 200 irrigated although these differences were not significant in kg N/plant and found that nitrogen application significantly other experimental years (Krüger et al. 1999). increased nitrate contents in the roots. In a pot experiment, Modelling approaches have the potential to improve Gonzalez-Ponce and Salas (1999) found that black night- 108 Scaling up production of traditional green leafy vegetables in Kenya. Masinde et al. shade (Solanum nigrum) accumulated close to 800 mg NO -3 in Germany and The Netherlands. Maynard et al. (1976) put /plant at 45 days age compared to below 400 mg NO -3 /plant the fatal dose of nitrates in adult humans at 15 to 70 mg for same age tomato (Lycopersicon esculentum Mill. Duke), NO3-N per kg body weight (65-304 mg NO -3 /kg body pepper (Capsicum annuum cv. ‘Dulce Italiano’) and thorn weight). While these levels are high and unlikely to be apple (Datura ferox L.) even though all had been supplied attained at once by consuming spiderplant blades, infants with 0.9 g N/plant. could be at considerable risk considering that their fatal Plants absorb nitrates from the soil solution (Marschner dose is less than 10% of that for adults (Maynard and Bar- 1986; Taiz and Zeiger 1998). The nitrates are then reduced ker 1972). Moreover, the acceptable daily intake levels are to nitrites and further to ammonia through two reactions much lower, at 0-3.65 mg/kg body weight for NO -3 and 0- catalysed by nitrate and nitrite reductases, respectively. Fin- 0.13 mg/kg body weight for NO -2 (Santamaria and Elia ally, ammonia is converted into amino acids using organic 1997). Thus there is always the risk of exceeding the accep- acids from photosynthesis in enzymatically catalysed reac- table daily intake levels by consuming spiderplant even in tions (Taiz and Zeiger 1998; Lawlor 2002). Nitrate assimi- adults especially if the crop has been exposed to severe lation can occur in both leaves and roots depending on plant stress. species and on the concentration of nitrates available (Law- lor 2002). In chicory (Cichorium intybus L. var. Witloof cv. CONCLUSION ‘Turbo’), Druart et al. (2000) found that nitrate assimilation in young plants occurred mainly in the roots, with the nit- There is a clear need to scale up production of vegetable rate reductase activity peaking at 42 days age. The assimila- amaranth (Amaranthus spp.), African nightshades (Solanum tion then shifted to the leaves as the roots formed cambium spp.) and spiderplant (Cleome/Gynandropsis gynandra) in and underwent thickening. The roots had a peak nitrate Kenya in order to meet the increasing demand. This should content of above 5.0 mg NO -3 /g dry weight (DW) at 42 days entail intensifying the production systems, increasing the age, declining to the lowest of 3 mg NO -3 /g DW at 63 days acreages or both. The success of the scaling up will depend age, while in the same period, the foliar nitrate levels partly on adoption of appropriate production methods. increased to a peak of 6.2 mg NO -3 /g DW. Unfavourable Nitrogen and water management is key to this success. On environmental conditions like low light, low temperature or one hand, nitrogen deficiency reduces leaf yields to uneco- extremely high temperatures which inhibit photosynthesis nomical levels, while on the other hand excessive nitrogen can lead to nitrate accumulation. Roorda van Eysinga and application posses the risk of NO3-N accumulation. Produc- van der Meijs (1985) reported that under conditions of tion of these vegetables in Kenya is mainly rainfed. How- higher light intensity in Netherlands about mid-April, the ever, water deficits, especially where soil moisture fall nitrate content of lettuce without nitrogen application was below about 60% FC can lead to high levels of NO3-N in less than 3000 mg NO -3 /kg fresh mass while plants under the plant tissues. It is therefore important that more research low light conditions in winter accumulated more than 3000 be conducted to develop precise water and nitrogen mg NO -3 /kg fresh mass. Under unfavourable growth condi- management practices in different agro-ecological zones of tions, the plant’s requirement for protein synthesis decrea- Kenya to ensure high yield and quality produce. ses, amino acids accumulate and the demand for nitrates declines prior to any decline in nitrate uptake resulting in ACKNOWLEDGEMENTS nitrate accumulation (Lawlor 2002). Absorption of nitrates reduces in a drying soil (BassiRadi and Caldwell 1992). We are grateful to the collegues in the department of Horticulture, However, drought is known to decrease the activity of nit- Jomo Kenyatta University of Agriculture and Technology for their rate reductase and this could lead to nitrate accumulation useful comments and suggestions during the preparation of this (Maynard et al. 1976). mini-review. Spiderplant and African nightshade are known to accumulate nitrates (Chweya and Mnzava 1997; Edmonds REFERENCES and Chweya 1997). 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