Spatial distribution of arsenic in different leaf tissues and its effect on structure and development of stomata and trichomes in mung bean, Vigna radiata (L.) Wilczek

• Enhanced arsenic (As) in soil affects development of stomata and trichomes in Vigna radiata.
• Stomatal abnormalities are due to alteration and delay in mitotic divisions.
• Wall thickening is affected so that stomata remain permanently closed.
• Arsenic affects arrangement and distribution of microtubules in guard cells.
• In As-treated leaves, trichomes display structural disruption and seem to be involved in sequestration and detoxification of As.

Arsenic (As) is a toxic metalloid contaminating the environment. The aim of this research was to find out the effect of enhanced level of As in soil on developmental ultrastructure of guard cells of stomata and glandular trichomes on leaves of Vigna radiata (mung bean). Stomata were adversely affected. Stomatal index showed a concomitant increase with rise in As level in soil. Stomatal size was reduced and at higher concentrations of As stomatal aperture was permanently closed. Also, frequency of arrested, fused and abnormal stomata increased. Guard cells in plants exposed to As displayed much vacuolation and poor cytoplasmic differentiation. At high concentration (30 mg As kg−1 soil), loss of structural organization and disintegration of cytoplasm were observed. Cytoskeleton of guard cells is sensitive to As and stomatal structure and development suffered due to microtubular disruption and organelle disorganization. Trichome density decreased with increase in concentration of As in soil on both upper and lower leaf surfaces. In As-treated leaves, trichomes were smaller in size and displayed structural disruption. Energy dispersive X-ray microanalysis revealed that the spatial distribution of As in leaves was such that the highest concentration occurred in epidermal tissues. No As was detected in mesophyll tissue. However, presence of As was recorded in the epidermal cells and the trichomes. Arsenic storage in the trichome could be a detoxification mechanism. The findings from the present study imply that more attention needs to be paid to the role of trichomes in understanding uptake and accumulation of As in plants.