Growth Effects

Growth Stimulation

Vanadium was considered to be a micronutrient for the green alga Scenedesmus obliquus Kützing during experiments in which impure iron salts were being used to assess the iron requirement of the species (12). It was difficult to confirm a similar requirement in higher plants (13). First, it is difficult to eliminate vanadium entirely from nutrient cultures (13). Also, although vanadate is a well-known inhibitor of plasma membrane proton-pumping ATPases, trace concentrations have been reported to benefit plant growth. In an experiment on sand-grown corn (Zea mays L.), a supply of vanadium increased grain yield, probably because leaf area was increased but also possibly due to physiological effects (14). Supply of vanadium to tomato (Lycopersicon esculentum Mill.) at 0.25 mg L^-1 of nutrient solution gave greater plant height, more leaves, more flowers, and greater plant mass than supplying no vanadium (15).

Hewitt, working with data from Welch and Huffman (16), calculated that the concentrations of vanadium in tomato plant cells are less than 1% of the concentration of vanadium in vanadiumdeficient Chlorella cells, suggesting that vanadium is not an essential element for the growth of higher plants (13). In the paper on which Hewitt's calculations were based, lettuce (Lactuca sativa L.) and tomato plants were grown to maturity in nutrient solutions containing less than 0.04 mg V L^-1 and with tissue concentrations of <2 to 18 mg V kg^-1 dry weight (16). Plant growth in this low concentration of vanadium was comparable to that in nutrient solutions containing 50mg V L^-1, with tissue concentrations of 117 to 419 mg kg^-1 dry weight, whereas it might have been expected that the low concentration of vanadium should have had a beneficial effect on growth. However, iron was supplied as the citrate salt, and in work on Chlorella pyrenoidosa, vanadium stimulated growth when iron was supplied as FeCl₃ but had only negligible effect when iron was supplied as citrate or iron EDTA (17). Therefore, part of its requirement as an essential element in algae, at least, is as a replacement for unavailable iron, and supply of iron in a readily available form removes this requirement. If vanadium is a beneficial element for higher plants it may be so only when iron or other metals are limiting.

Toxicity

If some doubt exists about the role of vanadium as a beneficial element, there is no doubt that at high rates of supply (10 to 20 mg L^-1) it is harmful to plants (12). Sorghum (Sorghum bicolor Moench.) seedlings supplied with vanadium as ammonium metavanadate at 1, 10, or 100 mg L^-1 in nutrient solutions showed no toxic effects in the 1 mg L^-1 solution, but showed a noticeable reddening of the lower stems, and later the leaf tips, in the 10 mg L^-1 or higher solution (7). In an experiment on bush beans planted 15 months after application of 5.6 kg VOSO₄ H₂O ha^-1 on the surface and harvested 3 months later, growth of shoots and roots was significantly less than in unfertilized plants (2).

In the experiments in which soybeans were grown in a fluvo-aquic soil or in an Oxisol, plant growth was inhibited when the concentration of vanadium supplied exceeded 30 mg kg^-1 in the fluvo-aquic soil, a rate of supply that gave a shoot concentration of approximately 1 mg V kg^-1 dry matter (10). With a supply of 75 mg V kg^-1 soil, the shoot concentration was approximately 4 mg kg^-1 dry matter, and plant growth was even more depressed than with the lower supply of vanadium (10).

One of the reasons for the harmful effects of vanadium is that it induces iron deficiency. Noticeably decreased concentrations of iron were measured in leaves of a manganese-sensitive bush bean cultivar supplied with vanadate (18). Cereals, strawberries (Fragaria X ananassa Duchesne), and flax (Linum usitatissimum L.) are noted as being very sensitive species (19). Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) are more sensitive than rice (Oryza sativa L.) or soybean (20). In addition to causing chlorosis from iron deficiency, vanadium has been shown to lower the concentration of iron in roots of soybeans (21) and to lower root concentrations of magnesium and potassium in soybean (22,23) and lettuce (23). Vanadium also decreased root and hypocotyl accumulation of molybdenum in white mustard (Sinapis alba L.) (25) and decreased calcium concentrations in leaves of soybean (23,24). Root and hypocotyl concentrations of manganese, copper, and nickel were increased in Sinapis alba (25), and leaf concentration of manganese was increased to toxic levels in bush bean (18). Some evidence indicates that vanadium may increase aluminum concentrations in soybeans (22).

In a field experiment with soybean, seed yields decreased with an increase in vanadium concentration in the soil, or more precisely with an increase in the V:(V⁺P) ratio (26). Seed yield decreased by approximately 20% as the resin-extractable V:(V⁺P) ratio increased to 0.15 mol mol^-1 (26), although a decrease also occurred in relation to vanadium alone (27). The negative relationship between vanadium and phosphorus is not surprising given that the inhibition of ATPases by vanadate is brought about by competitive inhibition of phosphate-binding on the enzymes. If the harmful effects of vanadium become more important with time as anthropogenic sources increase, it would be helpful to be able to alleviate them. The effects of vanadium in the soil can be reduced by adding a chelating agent, such as ?-irradiated chitosan, to the soil (20). Furthermore, it might be expected that since vanadium induces iron deficiency in plants, increased iron supply might alleviate vanadium toxicity, and this effect has been shown to be the case (28).