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Regulation of Foliar Plasticity in Conifers: Developmental and Environmental Factors

Foliar plasticity is widespread among woody plants, and historically most observations have been made in angiosperms. This review concentrates on examples from gymnosperms, particularly the Pinaceae, since there have been a number of recent studies on coniferous forest species. Foliar plasticity can be defined as variation in the morphology and physiological function of foliage produced over time and space within a single individual. For example, leaves produced in the sun have a higher leaf mass area (LMA) than leaves produced in the shade, and differ in their photosynthetic capacities. Sun and shade leaves are examples of heterophylly, or variation in leaf morphology in response to environmental variation in the immediate space surrounding the apical meristem producing the leaf primordia. The LMA of coniferous foliage also tends to increase with age, independently of the external environment, and this variation, called heteroblasty, appears to be a result of maturation of the apical meristem, which occurs over time. The regulation of variation due to heterophylly and heteroblasty appears to be very different. Heterophylly in response to light appears to vary linearly with available light. More massive sun foliage produced by a given apical meristem may be a response to available Photosynthetically Active Radiation (PAR) and its effects on net photosynthesis, or may be a photomorphogenetic response to the ratio of red to far red light. In contrast, heteroblastic variation can occur independently of available light, and may be the result of the developmental age of the apical meristem. It appears to vary curvilinearly with age, with the most rapid change occurring during the earliest life stages of the plant. Heteroblastic variation appears to be less plastic than heterophyllic, since grafted scions from mature or juvenile donors retain LMA characteristics of donor whether grafted onto juvenile or mature rootstocks.