Lead for plants: photosynthesis regulator

Lead (= Plumbum) (Pb)

For the plant – a regulator of photosynthesis

The content of lead in plants is usually low: about 0.001–0.002% (of ash weight), or 5 mg/kg. The upper threshold for lead concentration is not established. As plants grow, lead is redistributed among their organs: roots → stems → fruits and seeds.

Lead absorbed by the aboveground mass of plants evidently cannot quickly move to the roots.

In small amounts, lead is necessary for the plant organism. Its deficiency occurs when the content in the aboveground part is 2–6 µg/kg of dry matter.

Lead ions in small concentrations (<200 mg/kg of substrate) can have a positive effect on the content of chlorophyll and the intensity of photosynthesis in the leaves of barley and oats.

High concentrations of lead (> 800 mg/kg of substrate) lead to significant inhibition of the rate of photosynthesis and a decrease in chlorophyll content.

Excess lead inhibits respiration, photosynthesis, and reduces the uptake of zinc, calcium, phosphorus, and sulfur. As a result, the yield of plants decreases, and the quality of the produced products sharply deteriorates.

The maximum allowable concentration (MAC) of lead for soils in Ukraine and Russia is 30 mg/kg. When the lead content in the soil is 100–500 mg/kg, curling of old leaves is observed. Grains are less resistant to its excess, while legumes are resistant.

The least resistant to lead contamination plants are species of maple, onion, and common hedge. A concentration of lead above 10 mg/kg of dry mass is toxic to most cultivated plants.

At the same time, it is reported that even in the presence of a sufficiently high concentration of lead salts – 1000 mg/kg – barley and oats plants were able to transition to generative development and subsequently form full-fledged seeds, although, evidently, in smaller quantities.

The lead content in plants growing along highways is almost 10 times higher than its usual content in plants, with lead absorption occurring from the air rather than from the soil. In dry weather, lead accumulates on the surface of plants, and after heavy rains, a significant portion (up to half) is washed away.

High lead content in the air often causes summer leaf fall in trees (but in doing so, trees intensively purify the air by concentrating lead).
The flowers of poppy under the influence of increased doses of lead and zinc acquire double form.

Lead-accumulating plants include common hazel Corylus avellana L., Betulaceae and European spruce Picea abies (L.) H.Karst., Pinaceae.

    Indicator plants for increased lead content include:
    • European starflower Trientalis europaea L., Primulaceae;
    • common dandelion Taraxacum officinale Webb, Asteraceae;
    • ninebark Physocarpus opulifolia (L.) Maxim., Rosaceae;
    • Siberian elm Ulmus pumila L., Ulmaceae;
    • flat-leaved birch Betula platyphylla Sukaczev., Betulaceae.

Fungi, mosses, and lichens accumulate it intensively.

The record holder among organisms for resistance to lead compounds is yeast.

    Medicinal plants containing lead:
    • species of wormwood Artemisia L., Asteraceae (herb);
    • Regel’s mullein Phlomis regellii M. Pop., Lamiaceae (herb);
    • spreading orache Atriplex patula L., Chenopodiaceae (herb);
    • spiraea hypericifolia L., Rosaceae (bark, shoots, leaves);
    • moneywort Lonicera nummularifolia Jaub. et Spach., Caprifoliaceae (leaves, flowers);
    • diffuse knapweed Centaurea diffusa Lam., Asteraceae (inflorescences);
    • Turkestan hawthorn Crataegus turkestanica Pojark., Rosaceae (flowers, fruits);
    • Tian Shan cherry Cerasus tianschanica Pojark., Rosaceae (fruits).