PHOTOREVERSIBLE CONTROL OF HETEROPHYLLY IN HIPPURIS VULGARIS L.

Abstract

summaryThe freshwater macrophyte Hippuris vulgaris exhibits heterophylly. During July and August aerial‐type leaves occur even on immersed shoots down to a depth of 15 m in two clear‐water limestone lochs in Scotland. Complementary field and laboratory data indicate that a high intensity photoperiod and temperatures above 10°C are prerequisites for aerial‐type leaf production on submerged shoots. In these conditions, regular injections of additional sucrose, the natural photosynthate, at 05 mol kg−1 fresh weight will not induce the aerial‐type leaf. There is relatively little attenuation of blue light with depth in these lakes and additional blue light just after the photoperiod does not induce the aerial‐type leaf. There is no evidence for an ontogenetic or circadian photoperiodic requirement for the production of aerial‐type leaves, or that heterophylly is mediated by ethylene.However, a low ratio of red (R 660 run) to far red (FR 730 run) light throughout, or very dimly for a brief period just after, the photoperiod treatment regularly induces aerial leaf formation on submerged shoots of any length. A similar low R/FR ratio is found naturally in these lakes in summer at depths where submerged aerial leaves occur. Selective attenuation of far red by lakewaters raises the R/FR ratio steeply with increasing depth. At R/FR ratios above a critical range, which is similar in field and laboratory, aerial leaf formation is inhibited and only submerged‐type leaves are formed. Simply by changing the R/FR ratio, either during or briefly after the photoperiod, transitions between submerged and aerial‐type leaves are repeatedly reversible, implying direct phytochrome control.