Influence of seed size on seedling growth of Albizia procera under different soil water levels

Albizia procera Benth, is an early successional leguminous tree species tha t occurs naturally in dry tropical forests in India. The growth response of seedlings of A. procera from seeds of different sizes was studied under fo ur soil water levels. Seeds were surface sterilized, germinated and grown i n a glasshouse for 3 weeks under optimal water supply, and were subsequentl y maintained at four soil water levels for 4 months. Soil matric potentials for 1, 1/2, 1/3 and 1/4 water holding capacity (WHC) were -0.01, -0.1, -0. 61 and -1.20 MPa. respectively. Plant height, stem diameter and leaf area w ere measured fortnightly. Seedlings were harvested at the end of the experi ment. Biomass, relative growth rate (RGR), stem weight ratio (SWR), root:sh oot ratio (R:S ratio) and net assimilation rate (NAR) were determined. Seed lings from large seeds had a higher biomass and leaf area and were more tol erant of long-term extreme water stress compared to those from small seeds which were more tolerant of moderate levels of water stress. Seedlings from small seeds did not survive beyond 75 d at 1/4 WHC. Increase in the R:S ra tio along with a decrease in SWR reflected a shift in biomass allocation fr om above- to below-ground parts as soil water level decreased. SLA declined sharply with increasing water stress, which in turn led to a decrease in R GR. NAR increased with water stress. The study revealed the ability of the species to acclimate to a wide range of moisture stress by morphogenetic (i ncreased R:S ratio and decreased SWR) and physiological (increased NAR) pla stic responses. (C) 2000 Annals of Botany Company.