Responses to chilling of recalcitrant seeds of Ekebergia capensis from different provenances

Abstract

Recalcitrant seeds are shed at relatively high water contents and are metabolically active, initiating germination without exogenous provision of water. For whole seeds, specialised storage under hydrated conditions at the lowest possible temperatures not having adverse consequences, is required. This study aimed to establish the effect of chilling on the recalcitrant seeds of a single species, Ekebergia capensis, from three different populations occurring in different climates. These were collected from temperate Port Elizabeth (PE; Eastern Cape), sub-tropical St Lucia (KwaZulu-Natal [KZN]) and Tanzania (tropical southern Africa). Viability, axis ultrastructure, solute leakage and protein synthesis were assessed as indicators of response to chilling. These parameters were measured at intervals during storage under hydrated conditions. The seeds from PE retained 80% viability after 12 weeks in storage at 1° and 3 °C and 100% when stored at 6 °C. Those from St Lucia were dead after 38 d in storage at 3 °C, but had retained 100% viability at 6° and 16 °C for the same period. In contrast, all the seeds from Tanzania were dead after 9 d when stored at 3 °C and retained only 10 and 35% viability when stored for 9 d at 6° and 16 °C, respectively. A reduced rate of protein synthesis and increased solute leakage in axes during storage implied a breakdown in metabolic activity. Upon establishing that the PE seeds tolerated slow drying, it is suggested these seeds might be considered to be minimally recalcitrant. Studies of variation in nuclear ribosomal ITS1 sequences revealed the presence of three well-to-strongly-supported monophyletic clades corresponding to the geographical areas from which the seeds were sampled (PE, KZN and Tanzania). Observations indicated that axes from the seeds originating in St Lucia and Tanzania lost ultrastructural integrity when stored at 3 °C. Axes from the PE seeds did not show any deterioration in ultrastructure – for relatively prolonged storage periods. It is clear from the present study, that E. capensis seeds of temperate (PE) origin do tolerate chilling temperatures well, whereas those from sub-tropical St Lucia or the tropical Tanzanian provenance, do not. It is probable that what has been tacitly assumed to be a single species, may not be so.