EFFECTS OF CALCIUM ON GERMINATION AND FURTHER ZOOSPORE RELEASE FROM ZOOSPORE CYSTS OF PHYTOPHTHORA-PARASITICA

Cysts of Phytophthora parasitica, induced by vortex treatment of zoosp ore suspensions, germinated poorly when held in Eppendorf tubes but ge rminated well when transferred to glass slides within the first few mi nutes of encystment. The responsiveness to a surface, or to other trea tments where tested, was lost after 12-15 min in the Eppendorf tubes. Slides pretreated with silicone (hydrophobic) were as effective as unt reated slides in promoting germination. Dilution of cyst suspensions w ith distilled water or with the liquid into which zoospores were relea sed caused a consistent decrease in germination. Dilution with CaCl2 ( 50 mM) or with supernatant from centrifuged cysts gave germination equ ivalent to control (undiluted) cysts. Lower concentrations of CaCl2 we re less effective, and there was no response to calcium by older (20 m in) cysts. Dilution with zoospore lysate (caused by filtering zoospore suspensions through a Nucleopore filter) or with water passed through glass microfibre filters gave significant stimulation of cyst germina tion, above the undiluted control levels. Cysts that did not germinate within 2 h in response to treatments could release a further zoospore (diplanetism) by 8 h but calcium suppressed this in a concentration-d ependent manner. The findings support a previous model that encysting zoospores release a substance (probably calcium) that must be reabsorb ed for germination, and that this reabsorption is facilitated when cys ts are on a surface. However, other factors provide the germination tr igger, because calcium added to diluted cyst suspensions only restored germination to the initial (undiluted) level. The loss of germination responsiveness within a few minutes of encystment is suggested to inv olve a loss of signal. transduction, consistent with the reported micr otubular rearrangements in young cysts. Release of a further zoospore from non-germinated cysts is a default mechanism that requires no spec ific bigger, but it is suppressible by external calcium, with potentia l for control of P. parasitica in recirculating irrigation systems in glasshouses.