INFLUENCE OF ZINC NUTRITION IN RELATION TO PLANT METABOLISM : BIOCHEMICAL AND PHYSIOLOGICAL CHANGES IN MUNGBEAN (VIGNA RADIATA CV. K-851) SEEDLINGS WITH PARTICULAR REFERENCE TO CELL WALL PROTEINS

Abstract

Heavy metals, their deficiences on one hand and increased environmental burdens on the other in the soils all over the world are alarming scientists. Crop plants grown in the soils of vast Indo-Gangetic alluvial fertile plain of Ganga-Jamuna doab, particularly in green belt of western Uttar Pradesh which possess large number of sugarcane industries, commonly exhib ited deficiency, tolerance and toxicity symptoms of zinc. We have surveyed the alluvial soils of Indo-Gangetic plain in and around Roorkee (District Haridwar) town for its Zn status and studied the impact of zinc in relation to various biochemical and physiological aspects of hydroponically grown 6 day old mungbean (Vigna radiata cv. K-851) seedlings. All 32 soils sampled in Roorkee-Muzaffarnagar region are highly alkaline with an aver age pH of 7.8. Only 28% soils are deficient in total zinc content. However, more than 90% of soils contained less than 0.5 ppm DTPA-extractable (phytoavailable) Zn, which is widely considered to be the critical deficiency concentration of Zn for plants grown in clacareous soils. Soils appear to be deficient in available Zn content due to high pH values under the influence of salt Na2C03. On the basis of morphological, anatomical and biochemical parameters we have found that the optimum zinc requirement for vigna radiata cv. K.-851 is 0.4 ppm. It confirms the highly zinc sensitive nature of this crop, which shows toxicity beyond 15 ppm and loose tol erance at 100 ppm zinc concentration. Root was found to be most sensitive organ under zinc stress. Shoot/root length ratio was found to be minimum under deficiency and maximum un der toxicity of zinc. However, due to high accumulations of ions and photoassimilates, no con siderable change was recorded in dry matter of mungbean roots under zinc toxicity. Tertiary roots at low level and adventitious roots from hypocotyl at toxic (100 ppm) level of Zn were found to be developed as an alternate of checking in further increase in number of secondary roots and degenerated tap root respectively and contributes morphologically towards the tol- <i) erance to the plants. Deposition of blackish substance in intercellular spaces and changes in stele anatomy of the stem under zinc toxicity might be an anatomical contribution towards the tolerance to the plants. The distribution of zinc in different subcellular fractions of mungbean roots indicate the cytoplasmic sequestration of zinc and an active participation of cell wall in zinc tolerance. Increase in cell wall hydrolases activity under low level of zinc may explain the loosen ing of cell wall during the cell elongation and growth of the plant. Increase in peroxidase activity under zinc toxicity is likely to promote crosslinking of cell wall molecules and this along with decreased hydrolases may be responsible for rigidity of cell wall hi zinc stressed plants. Changes in cell wall and cytoplasmic proteins of root cells were assessed under zinc stress. Both cytoplasmic and cell wall proteins increased quantitatively beyond 15 ppm zinc. Proteins profile of cell wall was not altered, however alteration in proteins profile of cyto plasm was noticed in response to zinc. Certain polypeptides like 56, 46 and 35 kDa were found to be disappear at low level and reappear at toxic level of zinc. However, 27 kDa polypeptide disappeared completely beyond 10 ppm zinc. Thus, zinc may play a role in pro tein synthesis at the translation level and in the fonnation and the suppression of some peptides at the transcription level. Low molecular weight polypeptides seemed to be greatly affected by Zn nutrition. A new 18 kDa polypeptide was found to be appear at and beyond 25 ppm Zn, aminoacid analysis of which indicates that it might be a metallothionem bike polypeptide (phytochelatin) containing cysteine-rich domains. The polypeptides of approximately 30 kDa were tentatively addressed as peroxidases. However, for the other zinc-induced polypeptides, no such suggestion of functions is yet possible. This is needed to be further investigated in order to judge their possible role under zinc (heavy metal) stress.