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.