Effects of Leaf Establishment on Shoot Growth and Hydraulic Characteristics of Excoecaria agallocha in Sathurukkondan, Batticaloa

Abstract

Excoecaria agallocha (Family: Euphorbiaceae, Tamil: Thilla, Sinhala: Thela or Thelakiriya) is a semi deciduous true mangrove found in both inland (opposite site to Batticaloa lagoon) and shoreline areas in Sathurukkondan, Batticaloa, Sri Lanka. Leaf shedding in Excoecaria agallocha is a common phenomenon that occurs during September – November and leaves rejuvenate in January onwards. Preliminary ecophysiological experiments were conducted using adult plants coppices in an inland of Sathurukkondan mangrove site from December 2005 to March 2006. Above ground hydraulic and growth parameters were investigated at leaf establishment stages of early unfolded (December – January, 2005) and fully unfolded (March, 2006), whether any leaf establishments related to hydraulic and growth parameters of E. agallocha or not, was studied. Further, hydraulic and assimilate resistances were increased at the above ground levels by long term manipulations of xylem - phloem notching and phloem girdling in December, 2005 and it responses were investigated in March, 2006. Leaf establishments and xylem – phloem manipulations of notching and girdling has shown impact on the growth parameters of E. agallocha in terms of stem over bark diameter, total leaf area, total leaf dry mass, total stem dry mass and specific leaf area (SLA). Hydraulic parameters of maximum absolute hydraulic conductivity (kmax), leaf specific hydraulic conductivity (kl, kmax / leaf area distal) and specific hydraulic conductivity (ks, kmax / sapwood area) were significantly increased concurrently with leaf establishments and xylem - phloem notched E. agallocha. Surprisingly, xylem - phloem notched branches shown higher ks compared with branches measured at fully unfolded stage and this could be suggested as a survival strategy (drought avoider) of E. agallocha, while alleviating ks under stem level stress condition. kl increased with leaf establishments and in manipulated xylem – phloem notched stems. In accordance, a positive relationship was established between kl and vessel diameters. Vessel diameters measured at distal ends of stems were progressively and relatively widened and shown significant differences in early unfolded, fully unfolded and notched E. agallocha. This suggests that progressive increased hydraulic characteristics were an impact of vessel diameter of stem. Further, E. agallocha need not necessarily depend on the leaves for conservative water use like other forms, as this species falls under semi-deciduous with a phenomenon of shedding leaves.