INFLUENCE OF NICKEL STRESS ON GROWTH, MORPHO- PHYSIOLOGICAL AND ANATOMICAL ATTRIBUTES OF SUNFLOWER (HELIANTHUS ANNUUS L.)

Abstract

The phytotoxic effects of Ni-stress on seed germination, growth, physiological and anatomical attributes were appraised in this study. The available sunflower germplasm was exposed to varying levels of Ni (0, 10, 20, 30, 40, 50 and 60 mg L -1 ). The lowest level of nickel (10 mg L -1 ) showed some improvement in seed germination and reduced the time to achieve 50% germination; it was found to be associated with improvement in Mg contents, activity of hydrolytic enzymes and concentration of reducing and non- reducing sugars, and free amino acid over time. However, higher levels of nickel had a significant inhibitory effect on seed germination that was directly correlated with reduction in K and activity of hydrolytic enzymes leading to alterations in concentration of crucial bio-molecules involved in control of seed germination. Based on the data generated a Ni-tolerant (Hysun-33) and a -sensitive (SF-187) hybrids were selected for further studies. In vegetative stage experiments, seeds of two selected sunflower hybrids were exposed to different levels of Ni (0, 10, 20, 30 and 40 mg L -1 ). Although low levels of Ni had a non- significant effects, high Ni-levels significantly reduced vegetative growth and yield has result of reduction in photosynthetic rate (A), stomatal conductance (g s ), water use efficiency (WUE) and Ci/Ca ratio. However, transpiration rate (E) increased with increase in external nickel. Leaf water and turgor potentials, and relative water contents (RWC) decreased while leaf turgor potential and membrane permeability (electrolyte leakage) increased in both sunflower hybrids. The concentration of soluble proteins increased at lower level of Ni application (10 and 20 mg L -1 ) but decreased with further increase in concentration of external Ni in all harvest intervals. However, the reverse was true for the concentration of free amino acids and soluble sugars. Proline concentration increased consistently with increasing concentration of external Ni. The concentration of nutrients such as Ca, N, P, K, Cu, Zn, Mg, Mn and Fe generally decreased in root, stem, leaves as well as achenes except for achene P and Mg which were not much affected under Ni-stress. The changes in all growth, biochemical, and physiological parameters appeared to be highly correlated with build-up of high Ni levels in root, stem and leaves of both sunflower hybrids over time. The anatomical features of root, stem and leaves were also significantly altered under Ni stress. Root exodermal thickness increased at the lowest level of Ni application (10 mg L - 1 ) but decreased consistently with further increase in Ni concentration. In contrast, root exodermal cell area, increased consistently with increase in external Ni concentration. Shoot and leaf exodermal and second layer thickness and area also increased consistently with increase in external Ni levels. Although, root, stem and leaf cortical, vascular region, metaxylem and phloem thickness and area were not affected at the lowest level (10 mg L - 1 ) of Ni, it decreased consistently with increase in external Ni as well as plant maturity. The reverse was true for leaf adaxial and abaxial stomatal density. In comparison, their thickness decreased during early stages but increased with increasing plant maturity. Leaf abaxial epidermal thickness and area, and leaf lamina thickness increased but spongy mesophyll thickness and area decreased with increase in external Ni as well as time. On the basis of data presented in this study, Hysun-33 was found to be more tolerant as compared to that of SF-187.