Grafting for the use of root systems as biological filters to prevent contaminants’ penetration into vegetable plants
A major part of the Mediterranean region is characterized by water scarcity, with long dry summers and short wet winters. To satisfy the demand for food and to combat desertification in this region, marginal water sources, such as treated domestic sewage (effluent) and saline water, are being increasingly used for irrigation (Ben-Hur, 2004). Moreover, the pressure to avoid disposal of nutrient-rich effluents into water bodies has contributed to the rapid expansion of effluent reuse for irrigation (Halliwell et al., 2001).
The electrical conductivity (EC) of saline water is much higher than that of fresh water, and it may exceed 5 dS/m when the dominant ions are Na and Cl. Similarly in effluents, the EC and pH values, and the concentrations of microelements such as heavy metals and B, and of nutrients and dissolved organic matter are, in general, significantly higher than in fresh water. Long-term use of these types of water for irrigation could increase the accumulation and concentrations of microelements and saline elements (Na, Ca, Mg, and Cl) in the soil (Ben-Hur, 2004; Feigin et al., 1991). Relatively high concentrations of Na+, Cl- and microelements in the soil solution could be toxic to plants and to humans. Absorption of these elements by the plants could affect their growth and yield, and increase the possibility of contaminants entering the food supply chain.
Consumers are becoming increasingly concerned about because of the possible adverse effects on environmental quality and human health. This is particularly true for , which are often regarded as a safe and nutritious food source. Edelstein et al. (2005) suggested that grafted plants (Fig. 1) could be used to prevent .
Microelements in plant tissues
The effects of plant grafting on microelement concentrations in the fruit of melon plants under field conditions were studied in field plots with clay soil. The field plots were irrigated with secondary effluent for 4 years, and melon (non-grafted plant) and (grafted plant) were grown in these plots. The concentrations of various microelements in the fruits of the grafted and non-grafted melon plants are presented in Fig. 2. In general, the concentrations of B, Zn, Sr, Mn, Cu, Ti, Cr, Ni, and Cd were significantly lower in the fruits of grafted vs. non-grafted plants.
To determine the mechanisms responsible for the lower microelement concentrations in the fruits of the grafted plants, detailed experiments were conducted in the greenhouse. Grafted and non-grafted melon plants were irrigated with fresh water (EC = 1.8 dS/m), saline water (EC = 4.6 dS/m) or secondary effluent enriched with B at up to 10 mg/L (Edelstein et al., 2005, 2007). B can be absorbed by the root cell symplast or loaded into the xylem by means of two main transport mechanisms: passive diffusion through the lipid bilayer, and passage through proteinaceous channels in the cell membrane (Dannel et al., 2002; Dordas et al., 2000). Edelstein et al. (2005) suggested that the Cucurbita rootstock excludes some B and that this, in turn, decreases the B concentration in the grafted plants.
To determine the differences in selectivity of the root systems of melon () and pumpkin (TZ-148) to B absorption, their seedlings were planted in pots in the greenhouse, and irrigated with fresh water containing various concentrations of B. Thirty days after planting, and immediately after an irrigation event, stems 3 cm above the surface of the growth medium were cut and the xylem sap exudates collected. B concentration was determined in each collected sap sample. The B concentrations in the melon sap exudates were higher than those in the pumpkin sap exudates (Fig. 3). Thus it was postulated that the pumpkin root system was more selective and absorbed less B than of the melon roots. The B-exclusion hypothesis is supported by other studies: Dannel et al. (1998, 2002) suggested that at low B concentrations, B uptake may be active, but at high concentrations, there is evidence of . Dordas et al. (2000) indicated that B enters plant cells partly by passive diffusion through the lipid bilayer of
The plasma membrane and partly through proteinaceous channels. Dordas and Brown (2001) examined B transport in squash plants, and suggested that both of these mechanisms were possible.
Saline elements in the plant tissues
The effects of grafting watermelon (‘Tri-X 313’) onto the commercial Cucurbita maxima × Cucurbita moschata rootstock TZ-148 on growth and yields of plants irrigated with saline water (EC 4.5 dS/m) in disease-free soil in experimental field plots in an arid zone in southern Israel are shown in Fig. 4. Vegetative growth, fruit yield and fruit sizes of the grafted plants were higher than those of the non-grafted plants (Fig. 4). The differences in yield parameters were probably due to the higher salt tolerance of the grafted vs. non-grafted plants or to higher excretion or exclusion of saline ions by the root system of the grafted plants.
Fernandez-Garcia et al. (2003) showed that under saline conditions (60 mM NaCl), Cl- and Na+ uptake by grafted tomato plants is significantly lower than that by non-grafted plants, indicating that the former exhibit higher selectivity toward saline absorption than the latter. Likewise, Romero et al. (1997) found that the effects of salinity on two varieties of melon grafted onto three hybrids of squash were less severe than those on non-grafted melons, suggesting that the grafted plants develop various mechanisms to prevent the physiological damage caused by excessive accumulation of Cl- and Na+ in the leaves. The suggested mechanisms included exclusion of Cl- and/or reduction of its absorption by the roots, and replacement or substitution of total Na+ with total K+ in the foliage.
The concentrations of Ca, Na, Mg, and Cl- in the leaves, stem, and fruit tissues of a non-grafted melon (cv. Arava) plant and melon grafted onto pumpkin rootstock (TZ-148) grown in field plots in the experimental station in Akko are presented in Table 1. These plants were irrigated with secondary effluent. The concentrations of all saline elements except Mg in the stem and leaves were higher in the non-grafted vs. grafted plants (Table 1). The largest difference between the non-grafted and grafted plants was in their Na concentration, which was one order of magnitude lower in the grafted plant tissues than in the non-grafted ones.
Two mechanisms might explain the decrease in shoot Na concentration in plants with pumpkin rootstocks: (i) Na exclusion by the pumpkin roots, and (ii) Na retention and accumulation within the pumpkin rootstock. Quantitative analysis performed by Edelstein et al. (2010) indicated that the pumpkin roots excluded ~74% of available Na, while there was nearly no Na exclusion by melon roots. Na retention by the pumpkin rootstocks decreased its amount in the shoot by an average 46.9% compared to uniform Na distribution throughout the plant. In contrast, no retention of Na was found in plants grafted on melons.
Intensive agriculture has increased the use of toxic chemicals on cultivated lands. In addition, to satisfy the demand for food in arid and semiarid regions, the use of marginal water sources, such as treated domestic sewage (effluent) and saline water, for irrigation is on the rise. These can enhance soil and water contamination, and the possibility of toxic microelements and saline elements entering into the food supply chain via plants. From laboratory, greenhouse and field experiments, it can be concluded that grafting of vegetable plants can be used as a technique to prevent the entry of toxic microelements and saline elements into the food chain.
Fig. 1: Melon plant grafted onto pumpkin rootstock in the nursery (left) and in the open field (right).
LEFT: Fig 2. : Microelement concentrations in fruits from grafted and non-grafted melon plants irrigated with secondary effluent water.Vertical bars represent ± SE (unpublished data).
LEFT: Fig. 4. : Growth performance, fruit yield and mean fruit weight of non-grafted (‘Tri-X 313’; NG) and grafted (onto TZ-148; G) watermelon irrigated with saline water (EC = 4.5 dS/m) (after Cohen et al., 2007). RIGHT: Table 1: Average concentrations of saline elements (g/kg, DW) in different organs of non-grafted and grafted plants irrigated with effluent water± SE (unpublished data).
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M. Edelstein1 and M. Ben-Hur2
1Department of Vegetable Crops, Agricultural Research Organization,
Newe Ya’ar Research Center, P .O. Box 1021, Ramat Yishay 30095, Israel
2Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization,
Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel
(Published in ISRAEL AGRICULTURE, 2011)