225 GEN. APPL. PLANT PHYSIOLOGY, 2005, 31(3-4), 225-234 MORPHOGENETIC EFFECTS OF MERCURY IN LAGENARIA SICERARIA (MOL) STANDL AND THEIR PARTIAL REVERSAL BY EXOGENOUS AUXIN Aisha Saleem Khan *1 and Najma Yaqub Chaudhry 2 1 Punjab University, Botany Department, Pakistan, 2 Punjab University, Botany De- partment, Lahore Received 27 December 2005 Summary. The effects of mercuric chloride (HgCl 2 ), indole-3-acetic acid (IAA) and their combination on the development of different cell types in the stem of Lagenaria siceraria (Mol) Standl. were studied. Mercury treat- ment resulted in a reduced stem diameter when compared to untreated con- trol plants. This is attributed to a decrease in the diameter of fibre cells, sieve tube members, large xylem vessels and phloem cells as well as to the inter- ference with cambium development. IAA treatment caused enhanced growth of cambial region, sieve tube members and xylem vessels both in transverse and longitudinal planes. Application of HgCl 2 in combination with IAA caused less reduction in growth parameters, thus suggesting that the inhibi- tory effect of mercury can be restored to some extent due to the application of IAA. Keywords: Auxin; Cucurbitaceae; mercury; secondary growth; sieve tube members; xylem vessels. Abbreviations: IVC - inner vascular cylinder; OVC - outer vascular cylin- der. INTRODUCTION Normally in the environment plants are exposed to a range of abiotic stresses like osmotic, salinity, temperature and heavy metals toxicity, which affect their growth and other physiological processes (Levitt, 1980). Heavy metals cause irreversible damage to a number of vital metabolic constituents in plants (Tomar et al., 2000). Corresponding author, e-mail: aishasaleemkhan@hotmail.com, aishaskhan2003@yahoo.com