GROWTH BIOREGULATORY ROLE OF ROOT-APPLIED THIOUREA: CHANGES IN GROWTH, TOXICITY SYMPTOMS AND PHOTOSYNTHETIC PIGMENTS OF MAIZE

While seed or foliar application of thiourea (TU) has the potential to modulate plant growth, studies reporting such bioregulatory role(s) of root-applied TU are limited. Here we report the growth regulation by root-applied TU in nine maize (Zea mays L.) varieties grown using sand culture. Thirty day old plants were treated with 0‒1.25 mM TU levels dissolved in nutrient solution, and the data recorded 20 days later. The parameters studied were symptoms of TU toxicity, growth and photosynthetic pigment contents. Results revealed that 0.25 mM TU promoted the growth of all varieties; 0.50 mM did so in some, while 0.75‒1.25 mM TU was inhibitory to all varieties. Higher TU levels (1.00‒1.25 mM) produced more intensive toxicity symptoms such as yellowing of leaves, and browning and constriction of roots but with significant varietal differences; Pak-Afgoi showed the lowest while EV-20 had the highest incidence of symptoms. Lowest TU level (0.25 mM) increased the photosynthetic pigments in all the varieties, while higher levels (1.00‒1.25) reduced chlorophyll (Chl) b and carotenoids (Car) contents and increased Chl:Car ratio more legibly in the sensitive varieties. There were no correlations of toxicity symptoms with growth and leaf pigments contents at 0.25 mM TU but these correlations were negative for shoot weight, number and area of leaves and Chl b, Car and Chl:Car ratio of leaves at 1.25 mM (toxic) level of TU. Likewise, root dry weight of maize varieties was negatively correlated with root constriction and root browning. Results suggest that higher levels of TU had pronounced influence on the root functions leading to reduced dry weight. From the current results we believe that TU has a definitive growth bioregulatory activity as it improved photosynthetic pigments contents at lower level (0.25 mM) but deteriorated them at higher levels (0.75‒1.25 mM). Substantial varietal differences suggested that TU effects are genetically-related in maize. Possible physiological basis of the current observations are discussed.