Metabolism

Vanadium has been shown to enhance chlorophyll formation and iron metabolism of tomato plants and to enhance the Hill reaction of isolated chloroplasts (15). Corn plants that had higher grain yield with a supply of vanadium in sand culture had increased concentrations of chlorophyll a and chlorophyll b (14). Supply of vanadium increased the synthesis of chlorophyll through enhanced synthesis of the porphyrin precursor δ-aminolevulinic acid in the green alga Chlorella pyrenoidosa Chick. (29), although the pH optimum for the enhancement of chlorophyll synthesis by vanadium was slightly different from the pH optimum for enhancement of algal cell growth (30). The substitution of vanadium for iron in green algae highlights the involvement of both ions in chlorophyll synthesis.

No clear evidence is available for the role of vanadium in chlorophyll synthesis in higher plants, but iron deficiency gives rise to lower amounts of chlorophyll per chloroplast (31), and the requirement for iron in chlorophyll synthesis has been narrowed down to a specific step (32) rather than to secondary effects. The requirement for iron is clear, and vanadium may possibly influence chlorophyll synthesis only through an effect on iron metabolism. At one stage it was proposed that green algae may have a pathway of synthesis of d-aminolevulinic acid that is vanadium-dependent but differs from the pathway in higher plants (13); however, such a pathway has not been identified. In recent years, genes coding for the enzymes involved in this synthesis have been identified in higher plants and in algae, so differences in the pathway, if they exist, appear to be at the level of control rather than in the pathway itself. It is possible that vanadium is an essential cofactor for one of the enzymes of chlorophyll biosynthesis in green algae, but in higher plants this role is normally taken on by another metal for which vanadium can substitute.

Vanadate (but not vanadyl) promoted the evolution of oxygen from intact cells of Chlorella fusca at the same concentrations that gave maximum promotion of algal growth (1 to 2 �M) (33). Vanadium was thought to work in the chain of electron transport between photosystems 2 and 1 by virtue of the ability of the vanadium to change reversibly between its tetravalent and pentavalent states (33). Vanadium also increased photosystem 1 activity (but not photosystem 2 activity) in isolated chloroplasts of spinach (Spinacia oleracea L.), with an optimum at approximately 20 �M V (33).

Corn plants that showed enhanced grain yield with supply of vanadium had more nitrogen, phosphorus, potassium, calcium, and magnesium in the leaves, although high concentrations of vanadium decreased the concentrations of these elements (14). Vanadium was shown to increase foliar concentrations of calcium and iron in lettuce, although in these plants, yield was actually depressed by the vanadium supplied (23).

The presence of vanadium certainly affects the metabolism of plants. Addition of vanadium at 1mg L^-1 to solution reduced nicotine concentrations in tobacco (Nicotiana tabacum L.) by 25% (34). In lupin (Lupinus polyphyllus Lindl.), a negative correlation between alkaloid and vanadium concentrations in the leaves has been observed (35).

Given the inhibitory effects of vanadate on plasma membrane ATPases, it is not surprising that vanadium should affect metabolism. Changes in concentrations of other ions in plants supplied with vanadium could in part be due to the effects on proton-pumping APTases, although uptake of phosphate into isolated corn root tips was inhibited less than the activity of ATPase in the tips at the same amount of sodium vanadate supplied (36). Nevertheless, heavy exposure of these enzymes to vanadium might be expected to stop plant transport completely. Some evidence indicates that vanadium may also inhibit the absorption of water (37).

Absorption of vanadium appears to be a passive process as it is a linear function of external vanadium concentration and is not affected by putting excised roots into anaerobic conditions (38). Absorption is highly pH-dependent, being fastest at pH 4 and dropping to a very slow rate by pH 10, although being relatively constant between pH 5 and 8 (38). This effect of pH on absorption appears to be due to the ionic form in which vanadium is present, with VO₂⁺ predominating at pH 4, HVO₃ predominating between pH 4 and 5, VO₃^- predominating between pH 5 and 8, and HVO₄2^- predominating at pH 9 to 10 (38). The VO₂⁺ form that predominates in acid soil is taken up by plants far more readily than the other forms that predominate in neutral and alkaline soils (11).

Absorption of vanadium appears to occur at the expense of calcium uptake, there being a linear decrease in calcium accumulation into sorghum cultivars with log concentration of vanadate supplied (39). This result is probably due to an effect on calcium channels that more than compensates for the inhibition by vanadate of the H⁺-translocating ATPase responsible for calcium flux. The presence of calcium is required for absorption of vanadium, and this effect, together with the fact that vanadium concentrates in the roots at up to twice the concentration in the external medium, indicates that the passive absorption cannot be purely by diffusion. A concentration gradient from outside to inside the root could be maintained by the vanadium changing form inside the root, with up to 10% of VO₃^- taken up being reduced to VO₂⁺ (40), or it could be chelated (38).

Indeed, various complexes of vanadium have been detected in plants. At low rates of vanadium supply, plants form low-molecular-weight complexes thought to be vanadyl amino compounds, and at high rates of supply, plants form high molecular weight complexes, probably vanadyl cellulose compounds (41). It seems that following absorption, vanadium is partially immobilized on the root cell walls. It then develops soluble complexes outside the plasmalemma and finally is absorbed into the vacuoles within the cells (41). Concentrations in roots are usually higher than in leaves. Calcium seems to accumulate in roots along with vanadium. In soybeans supplied with vanadium, both elements were concentrated in the roots, and very high concentrations of calcium have been detected in the roots of vanadium-accumulating species. Perhaps, calcium may work to detoxify the vanadium (7,24). It is possible that the vanadium occurs as insoluble calcium vanadate (1). This action may be only a partially successful detoxification as it has been suggested that the accumulation of calcium might give rise to the imbalance in other cations associated with vanadium toxicity(24).

There does not appear to be much inhibition of absorption of vanadium by molybdate, borate, chloride, selenate, chromate, or nitrate (38). However, in Sinapis alba nickel, manganese, and copper inhibited the accumulation of vanadium in roots and hypocotyls, whereas molybdate decreased its accumulation in the hypocotyls and enhanced its accumulation in the roots (25).