PHOTOREVERSIBLE CONTROL OF HETEROPHYLLY IN HIPPURIS VULGARIS L.

Abstract

summary The 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.

Endogenous Abscisic Acid Content Correlates with Photon Fluence Rate and Induced Leaf Morphology in Hippuris vulgaris

“…Finally, an earlier study (4) reported evidence that leaf type in H. vulgaris could be controlled as a phytochrome response. An attempt was made here to confirm some of those results using postphotoperiodic irradiations of various R/FR ratios.…”

Effects of light and nutrient availability on the growth, allocation, carbon/nitrogen balance, phenolic chemistry, and resistance to herbivory of two freshwater macrophytes

“…Depth and light availability are inversely related in the field, with both light quality and quantity changing with depth. The quality of light can influence heterophylly (Bodkin et al 1980) but, in our study, the quality of light was unaltered as shaded and unshaded plants were kept at the same depth: only light quantity was manipulated. Unshaded N. advena had a greater proportion of floating leaves than did shaded plants, due to the former having fewer submersed leaves and a trend towards more floating leaves (P=0.09).…”

Adaptations for an amphibious life: changes in leaf morphology, growth rate, carbon and nitrogen investment, and reproduction during adjustment to emersion by the freshwater macrophyte Littorella uniflora

“…The changing ratio of red to far-red light, which stimulates the development of aerial-type leaves in emergent species inhabiting deep, clear water (e.g. Hippuris vulgaris in 2 m ; Bodkin et al, 1980), as they grow towards the surface, seems less likely to be a good indicator of emersion for L. uniflora. The relatively small absorption of far-red in the few centimetres of water which cover the plants before emersion is often offset by absorption of red wavelengths by organic material, other vegetation and micro-organisms (Spence, Bartley & Child, 1987 ;Smith, 1994).…”

“…In L. uniflora there is no ontogenetic sequence of leaf forms irrespective of whether the plant is growing submersed or on land, and the observed morphological plasticity must therefore have been triggered by some external and\or internal stimuli. Experimental work with other species suggests that leaf morphology can respond to photoperiod, temperature, light regime, CO # and O # concentration, moisture conditions and mechanical pressure, and that the response might be mediated by plant growth substances (see, for example, Bradshaw, 1965 ;Sculthorpe, 1967 ;Bristow, 1969 ;Anderson, 1978 ;Bodkin et al, 1980 ;Deschamp & Cooke, 1983 ;Goliber & Feldman, 1989 ;Maberly & Spence, 1989 ;He, Morgan & Drew, 1996 ;Hellwege, Dietz & Hartung, 1996 ;Trewavas & Mulho! , 1997).…”