be similar to that of other plant growth regulators such as auxins,
gibberellins and cytokinins that affect plant metabolism in a
positive manner. This may explain the positive results of the
organo-mineral fertilizer on proline and chlorophyll contents
under saline soil conditions that were then positively reflected in
the growth of tomato plants. Humic acid leads to higher rates of
uptake of elemental K (Table 5), thus leads to a corresponding
increase in chlorophyll fluorescence which can serve as an
indicator of the stress induced by alterations in the balance of
endogenous hormones (Marschner, 1995). Proline accumulation
under stress conditions may either be caused by induction or
activation of enzymes of proline biosynthesis or a decreased
proline oxidation to glutamate, decreased utilization of proline in
protein synthesis, and enhanced protein turnover (Delauney and
Verna, 1993). The increased content of proline has been shown to
alleviate salinity-induced oxidative stress by scavenging some of
harmful reactive oxygen species (ROS). Therefore, being a
hydroxyl and singlet oxygen scavenger, proline has efficiently
reduced the threat of ROS in the salts-excess tomato leaves under
salinity stress (Rady, 2011). However, the interesting thing that
emerged in the present study is that the indirect treatment of
plants with the organo-mineral fertilizer (as soil amendment)
enhanced the level of proline (Table 3) under salt-stress condition.
Therefore, maximum values were recorded in the plants
grown in the saline soil amended with the highest levels of the
organo-mineral fertilizer (20 and 25 g kg−1 soil). The acceleration
of increased pool of proline resulted in an increase in the
capacity of tolerance to salinity in the present study.