W. Voogt

Biofortification of lettuce (Lactuca sativa L.) with iodine: The effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce grown in water culture

BACKGROUND Iodine is an essential trace element for humans. Two billion individuals have insufficient iodine intake. Biofortification of vegetables with iodine offers an excellent opportunity to increase iodine intake by humans. The main aim was to study the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce, grown in water culture. RESULTS In both a winter and summer trial, dose rates of 0, 13, 39, 65, and 90 or 129 microg iodine L(-1), applied as iodate (IO(3)(-)) or iodide (I(-)), did not affect plant biomass, produce quality or water uptake. Increases in iodine concentration significantly enhanced iodine content in the plant. Iodine contents in plant tissue were up to five times higher with I(-) than with IO(3)(-). Iodine was mainly distributed to the outer leaves. The highest iodide dose rates in both trials resulted in 653 and 764 microg iodine kg(-1) total leaf fresh weight. CONCLUSION Biofortification of lettuce with iodine is easily applicable in a hydroponic growing system, both with I(-) and IO(3)(-). I(-) was more effective than IO(3)(-). Fifty grams of iodine-biofortified lettuce would provide, respectively, 22% and 25% of the recommended daily allowance of iodine for adolescents and adults.

Nutrient management in substrate systems

Speaking about nutrient solutions in soilless cultivation, different solutions can be discerned. Originally, in soilless culture only one nutrient solution was taken into account, being the solution in the containers in which the plants were grown. Such solutions were intensively moved by air bubbling and thus, the composition of the solution in the whole root environment was equal. The root environment was restricted to the container in which the plants were grown and thus, the whole root system of the plant was surrounded by the same nutrient solution. However, this is not the case for hydroponics and substrate systems under practical growing conditions, where great differences occur in time and place within the root environment. The main reason for these differences of salt concentrations between spots within the root environment are the inequality of water supply and water uptake by the crop as discussed in Section 6.3, at the one hand and the lack of movement of the solution within the root environment to equalize them on the other. In Chapter 8 some examples were shown of the inequality of the distribution of nutrients and salts within the root environment of substrate grown plants and the consequences of it on plant development.

Salinity and Water Quality

The impact of salinity on greenhouse grown crops, especially when grown in substrate systems, differs from the impact of salinity on crops grown under field conditions. The most striking difference between greenhouse and field conditions is the overall much higher concentrations of nutrients in greenhouse soils and substrates. This especially holds where high ion levels are knowingly maintained in the soil or substrate solution to control plant growth under poor light conditions or to improve quality of the produce (Sonneveld, 2000). Thus, in greenhouse cultivation nutrients contribute substantially to the osmotic potential of the solution in the root environment. This especially is the case in substrate systems when water of a low salinity status is used and thus, the osmotic potential is more or less solely brought about by nutrients. Furthermore, factors strongly affecting salinity effects on crops, like the climatic conditions in the greenhouse and the addition of water, are artificially controlled and therefore, differ much from those under field conditions. On thing and another induce special requirements on the management of salinity under greenhouse conditions.

Substrates: Chemical Characteristics and Preparation

In this chapter the characteristics of substrates will be discussed with respect to their effects on plant nutrition. Therefore, the chemical composition will be taken into account in the first place, because the mineral elements present in the material can be directly available to plants or can become available to plants dependent on the growing conditions. Besides mineral elements also other chemical compounds can be available in the material, which affect the plant growth negatively as well positively. Furthermore, with the preparation of some substrates mineral fertilizers are added to supply the plants grown in it with sufficient nutrients at the start of the growing period. Such applications with the preparation depend on the objective for which the substrate is prepared. Requirements in this field differ for substrates, crops grown and growing conditions. Important factors with respect to the growing conditions are for example the length of the growing period of the plant – a short propagation or a long production period – the growing system aimed at, the irrigation system and in relation with the last the method of fertilization that will be applied. If for example a substrate is prepared for a growing system in which directly at the start a complete nutrient solution is supplied, the requirement for the addition of mineral nutrients is less in comparison when is started with irrigation of just pure water. Substrates with a high cation adsorption capacity (CEC) will be fertilized differently from substrates with a low CEC. In this chapter mainly characteristics of substrates that affect the uptake of mineral elements by plants will be presented, while physical characteristics not directly affecting the mineral composition of plants are outside the context of this book.

Plant Nutrition in Future Greenhouse Production

In the introduction chapter it was claimed that greenhouse production has no longer arguments as a supply market. The products of the greenhouse industry became in free competition with those from field production from all over the world. In this competition the greenhouse industry has developed into a branch operative to a consumer market and through that self-condemned to bring better and cheaper products on the market than those from the open field. Many greenhouse products have a luxurious image, which even more is the case for flowers than for vegetables. That will be the reason that the critical view as exists on the production methods in agriculture in general, possible is even more critical on those used in greenhouses. Therefore, presentation of such products on a consumer market does not mean only high standards for quality, a great diversity and low prices, but also the production methods play a prominent part. Thus, greenhouse production will survive in future under conditions of sustainable production methods. This means low energy use, an effective use of raw materials and low environmental pollution.

Kasza: design of a closed water system for the greenhouse horticulture

The need for a closed and sustainable water system in greenhouse areas is stimulated by the implementation in the Netherlands of the European Framework Directive. The Dutch national project Kasza: Design of a Closed Water System for the Greenhouse Horticulture will provide information how the water system in a greenhouse horticulture area can be closed. In this paper the conceptual design of two systems to close the water cycle in a greenhouse area is described. The first system with reverse osmosis system can be used in areas where desalination is required in order to be able to use the recycle water for irrigation of all crops. The second system with advanced oxidation using UV and peroxide can be applied in areas with more salt tolerant crops and good (low sodium) water sources for irrigation. Both systems are financially feasible in new greenhouse areas with substantial available recycle water.

Calcium Nutrition and Climatic Conditions

The climatic conditions are one of the most striking differences between the growing conditions of field crops and those of protected crops, especially in the moderate climate zones. The increased temperature and the humidity in greenhouses are the dominating factors responsible for the differences. The radiation and the CO2 level in greenhouses are lower, when not artificially adjusted Bakker (1991). Another striking difference between the cultivation under protected conditions in comparison with cultivation in the open field is the crop production under poor light conditions in moderate climate zones. Cultivation of most crops is impossible under field conditions in these climate zones in the period from late autumn until early spring, because of too low outside temperatures. However, under protected conditions crop production occurs year round in moderate zones, which includes production under winter conditions. Heating, and possible artificial lighting contribute to successful crop productions in winter, but the growing conditions differ strongly from those during summer. The low light intensity in combination with a high humidity and relatively high temperature stimulate the vegetative development of plants, which induces negative effects on the quality of the produce. This results in winter time to crops with a lush growth and high water contents (De Koning, 1994).

Nutrient Solutions for Soilless Cultures

Nutrient solutions intended for plant growth are already used from the middle of the 19th century, when the importance of mineral elements for plant growth was made clear by Justus von Liebig. In advance, the nutrient solutions used to grow plants in so called “water cultures” had a simple composition and consisted of salts like KNO3, Ca(NO3)2, KHPO4, MgSO4, and a little Fe-compound (Hoagland and Arnon, 1950), thus, containing all the major elements and some Fe. The relative success with these solutions will be due to the not knowingly supplied micro nutrients from the impurities of the chemicals and fertilizers used to compose the solutions. It can be supposed that the impurities contained sufficient micronutrients, to prevent the crops grown from serious nutrient disorders. Knowledge about the necessity of micro nutrients for plant growth was mainly gathered in the first half of the 20th century (Marschner, 1997), when the purification of fertilizers and chemicals were improved. The first systematic description for the preparation of nutrient solutions was given by Hoagland and Arnon (1950) and since then in many publications reference is given to them, when one or another nutrient solution is used to grow plants in soilless cultivation systems.

Crop Response to an Unequal Distribution of Ions in Space and Time

Nutrient and salt ions often are unequally distributed in the root environment of plants and it will be expected that this strongly affect the plant reaction on the uptake of minerals and the osmotic potential. An unequal distribution of salts for example will be found with field grown crops in arid areas where the water supply is carried out by trickle irrigation (Meiri, 1984; Mmolawa and Or, 2000; Prichard et al., 1983). When under these conditions brackish water is used for irrigation, the salt accumulation on the soil surface of the dry areas between the emitters sometimes will be that strong that crystallization of salts occurs in the top layer whereby the surface is coloured white. Despite such tremendous local salt accumulations, crops often develop relatively quite well.

Soil and Substrate Testing to Estimate Nutrient Availability and Salinity Status

In the greenhouse industry methods have been developed for the determination of the nutrient availability and salinity status of soils and substrates. As in other agriculture branches, soil testing has the aim to estimate the availability, including the solubility as well the quantity, of plant nutrients to enable the farmer to get maximum production with minimum fertilizer use. The success of the farmer thereby does not depend only on the precision of the method, but also on the knowledge of the requirements of the crop. Both the utility of the soil testing method and the fertilizer application in relation to the results to get maximum yield will be calibrated in fertilizer experiments. Until lately, farmers based their decision about the amount of fertilizer addition on the costs of the fertilizer and the profits of the expected yield increase. However, in recent years farmers also have to consider the environmental aspects in their decisions. Fertilizer applications should be focussed also on their effects to pollution of soil, water and air. Beside the availability of nutrients, the determinations of characteristics for the salinity status are important and interact with the fertilization programme considered. The definitions given so far are operative for greenhouse crops as well as for crops grown in the field. However, soil testing for greenhouse industry has some specific aspects which will be mentioned beforehand, because they are important in relation to the methods used. The aspects in view for greenhouses are following.