Photoperiodic and temperature responses in the growth and tetrasporogenesis of Gigartina acicularis (Rhodophyta) from Ireland

Gigartina acicularis is an intertidal, perennial red alga which reaches its northern distributional limit in the north-eastern Atlantic on the Irish coast. It has only rarely been found with reproductive structures in the British Isles. Plants isolated vegetatively from field-collected plants near the northern distributional limit in Ireland formed tetrasporangia, the tetraspores of which gave rise to plants which formed gametangia and carposporophytes at 16°C, {Mathematical expression}. Sporelings grown from the carpospores of these plants formed tetrasporangia at all daylengths tested (16-8 h) at 13-20°C; but there was a quantitative photoperiodic response in the numbers of plants forming tetrasporangia, and in the numbers of sori formed, at 13-16°C. Only one in 20 plants became fertile at 16°C, {Mathematical expression} and {Mathematical expression}, but 16 in 20 plants reproduced at 16°C, {Mathematical expression}. At 20°C, {Mathematical expression} and {Mathematical expression}, all plants formed tetrasporangia, and formation was most rapid the long-day regime. No tetrasporangia were formed at 9-10°C, regardless of daylength. Apical elongation of these plants also appeared to show a quantitative photoperiodic response at 16°C, 1 h light breaks in a 16 h night giving more or less a long-day response. This effect was apparent about 4 weeks before the formation of the first tetrasporangia. There would therefore appear to be quantitative photoperiodic control of tetrasporogenesis in the Irish strain of G. acicularis. Temperature and photoperiodic control of reproduction in this species only partially accounts for the observed rarity of reproductive structures in populations of this species near the northern limit of its distribution.