Environmental Engineering Reference
In-Depth Information
March 2005 and Na in leaves during summer 2005 (r
June
= −0.66, n = 24; r
July
= −0.65,
n = 30; r
Aug.
= −0.66, n = 30), which demonstrated the importance of soil chemical
composition to plants just after winter.
As the external salt concentration rises, the concentration of Na
+
increases and
that of K
+
decreases in the roots and leaves. It appears that Na
+
out-compete K
+
for
specific binding sites on the K
+
transport proteins, causing K
+
deficiency
[3]
.
Besides, Na
+
in the soil can replace not only K
+
, but also Ca
2+
, Mg
2+
, NH
4
+
and other
cations on the soil exchange complex. This could lead to nutrient deficiencies
[2, 18]
causing injury.
A wide dispersion of K concentrations and supply levels were stated in lime
leaves during 2005 and 2007 (Fig.
3
). The smallest K result (0.15%) was even up
to 6.33 times lower compared with the minimum K concentration in lime leaves in
the park. At the same time the highest concentrations of K (max. 3.40%) were also
stated in lime leaves with necrosis, which mean that sufficient supply with K could
not prevent the appearance of leaf necrosis. In Riga, K concentration in healthy
lime leaves without necrosis ranged from 0.76% to 2.42%, which was in good
agreement with results generalized by
[19]
. For normal plant growth, K concentra-
tion in plant tissues should be 0.5-1.2%. The decrease in K content in leaves during
the vegetation season could be explained by K reutilization in plant
[2]
, as well as
an increase of Na content (r
Na,K aug.2005
= −0.76, n = 30; r
Na,K aug.2007
= −0.43; n = 29).
As a result, the decrease of K/Na ratio from June to August was stated. The K/Na
ratio for healthy lime leaves without necrosis in Riga's street greenery ranged from
2.92 to 237 (in park: 24.12-270.83), but for severe necrosis (>30% leaf necrosis)
ranged even from 0.15 to 72.00. In general, the stated K/Na ratios diapasons in Riga
were substantially wider compared with results reported by
[9]
in Opola for
Tilia
cordata
L. (1.76-15.65). It can be concluded that sufficient K/Na ratio in lime
leaves did not have a determinant role, if the stated leaf necrosis were caused by
Cl
−
toxicity, or if the Na concentration in leaves reached toxic level.
The leaf chemical results revealed significant higher levels of Na and Cl
−
in
August 2005 (p > 0.05) compared with results from August 2007, whereas the level
of K was significant higher during the vegetation season of 2007. This fact could
be the main reason for improved ecological status of street trees and decreased leaf
necrosis in 2007 compared to 2005 (Table
2
). The current research revealed signifi-
cant close correlation between the intensity of the leaf necrosis and the concentra-
tion of Na and Cl
−
in leaves (maximum in August 2005, r
Na
= 0.92; r
Cl
= 0.85; n =
30), as well as negative correlation with concentrations of K in leaves (maximum
in August 2007, r = −0.50).
The leaf necroses were observed in cases when the Cl
−
content in leaves reached
at least 6,600 mg/kg of dry weight. These concentrations were lower in comparison
with other investigations on leaf injury caused by Cl
−
toxicity
[9, 12, 20]
. These
differences might be explained in the scope of factors such as the different tree spe-
cies studied, methods used, a misbalanced supply of mineral nutrients, insufficient
supply of water during the vegetation period, and too high concentrations of
de-icing salt in the soil solution causing 'physiological drought' which promotes
leaf necrosis development, as well as specific ion toxicity.
