ÁMBITOS DE APLICACIÓN DE LA VIRTUALIZACIÓN

Furthermore, alleviation of environmental stress including draught and cold conditions, as well as acid rain or metal stress may also account for performance enhancing effects observed in plants after rare earth treatment.

Thus, significant growth enhancement was apparent when lanthanum was applied to barley un- der water limiting conditions (Maheswaran et al.,2001), (Meehan et al.,2001). While application of lanthanum to well-watered plants did not show any significant effects on the water use efficiency (WUE), significant increases in WUE were observed when water supply was deficient. Highest values were obtained at an application rate of 10 kg lanthanum per ha with increases of 21 %. At 5 kg/ha, lanthanum significantly promoted the tiller production by 33 %. Stomatal resistance (SR) also generally increased consistent with lanthanum application which indicates that transpiration was regulated and water conserved while photosynthesis was maintained. These physiological measurements, including WUE and SR observed in plants treated with lanthanum under drought conditions, implicated that lanthanum application enables the plants to maintain yield levels even under environmental stress. Furthermore, plants treated with lanthanum presented lower leaf wa-

11.4 Effects of Rare Earth Elements on Plants

ter potential as well as higher osmotic adjustment whereas the relative water contents remained unchanged. It is known that high relative water contents when found with low leaf water potential may enhance crop yield (Maheswaran et al.,2001).

Field studies performed under relatively dry conditions confirmed previous results in barley with yield increases amounting up to 19 % (Reddy et al., 2001). Further experiments in which lanthanum was applied to both soil and leaves of a drought intolerant variety of wheat at rates of 500 g/ha also presented increased yield of 11 % compared to untreated control plants. Although in both wheat and barley a constant pressure was measured, differences were observed when com- paring the water and osmotic potential of both plant species. Both parameters decreased in wheat, while an increase was noted for barley plants. However, the relative water content was affected in barley as well as in wheat which indicates that cell hydration was not out of order. Other investi- gations on the effect of rare earths on drought tolerance showed that in accordance with significant corn yield increases of 16.8 % the lowest water potential in corn leaves was measured when 1 kg seeds were mixed with 3 g rare earth nitrates (Wen et al., 1992). Furthermore, compared to the control, the electrical conductivity of wheat leaves treated with rare earth nitrates at 500µg/l was shown to be reduced by 29.6 %. As to this reduction, it was assumed that rare earths might in- crease the ability of cell electrolyte leakage hence reducing damages to the cell membrane usually caused by drought (Yang and Zhang,1986).

Rare earths might also induce their effects on water use efficiency by increasing the proline content in plants. Yu and Liu(1992) noticed higher proline concentrations in sugarcane after the plants were treated with rare earth elements. Additionally, decreased free-water contents as well as increased amounts of tied water were observed leading to a decreased free water/tied water ratio, which furthermore improves drought tolerance in sugarcane. Increased amounts of proline may therefore help plants to conserve water even during drought periods as proline exhibits a strong ability for hydration. To the contrary,Sun et al.(1998) observed that the proline content decreased due to higher temperature in Jun Date, a Ziziphus species (Z. Jujuba Mill or Chinese Date). It was supposed that the protein was damaged. Yet resistance to high temperatures was enhanced in Jun Date following rare earth application.

It has been reported earlier that growth enhancement and increase in dry weight of plants af- ter low concentrations of rare earths were applied, are pronounced under extreme environmental conditions (Guo et al., 1988). Thus, in wheat plants grown under -8 ◦C, rare earths reduced the amount of electrolyte effusion and enhanced the concentration of proline in leaves and seedlings. This indicates that besides increasing drought resistance, rare earths could also enhance the ability of resisting cold in wheat seedlings. Increased proline contents were also demonstrated at low temperatures in Jun date (Sun et al.,1998) thus accounting for increased cold resistance. Further- more, low temperature induced electrolyte leakage was also reported to be decreased in the roots of crotalaria (Crotalaria lium L.) after lanthanum and ytterbium application thus maintaining the integrity of the plasma membrane (Shen and Yan,2002). In accordance withShen and Yan(2002),

Tian(1990) also ascribed increased resistance to low temperature adversities in plants observed after rare earth supply to their membrane stabilizing abilities which in turn prevents electrolyte losses.

In addition to increased resistance in plants to unfavorable conditions as cold and draught due to rare earth supplementation reported previously, rare earth application could also increase the resistance to acid rain (Yan et al.,1998), (Yan et al.,1999).

In a similar manner, alleviation of metal stress as reported byZhou et al.(1998) andZhou et al.

11 APPLICATIONS OFRARE EARTH ELEMENTS TOAGRICULTURAL PLANTS

According toKataoka et al.(2002) different rare earth elements, with erbium and ytterbium being most effective, were able to stimulated the malate efflux in wheat roots. This is considered as the primary mechanism for aluminium tolerance in wheat since it is assumed that these organic anions chelate the aluminium cations in the rhizosphere to form nontoxic complexes. By activating malate efflux, rare earths may support plants to overcome aluminium toxicity. Trivalent cations have often been shown to be toxic to plants and especially aluminium toxicity has been found to be a major factor limiting crop production on acid soils (Foy et al.,1978), (Kochian,1995). Even micromolar concentrations of aluminium are capable of rapidly inhibiting root growth if the root apex is directly exposed to aluminium ions. Toxic effects of the rare earth element lanthanum on the root elongation were described in corn, mungbean and wheat (Diatloff et al.,1995b), (Diatloff et al.,1995c). However, just as toxic effects exhibited by aluminium, inhibitory effects of erbium ions on root growth could be overcome by increasing concentrations of malate which probably forms nontoxic complexes with erbium (Kataoka et al.,2002). On this basis, it might be assumed that indirect effects such as reducing aluminium toxicity accounts for growth enhancing effects of rare earths on plants.

Additionally, it has been suggested that the antioxidant potential (Chapter 5) of rare earths might be responsible for their supporting effects on stress resistance in plants. Several studies reported enhanced antioxidant abilities of rare earths (Pang et al.,2002). Being oxidized to Ce4+, Ce3+ may reduce O−₂ to H2O2; moreover, Ce4+ could oxidize O−₂ to O2 while being reduced to

Ce3+ (Wang et al.,1997a).