Physiological Aspects of Parasitism in Mistletoes (Arceuthobium and Phoradendron). II. The Photosynthetic Capacity of Mistletoe.

In the preceding paper (7) it was noted that the true mistletoe, Phoradendron, derives very little if any carbohydrate from its host, while the dwarf mistletoe, Arceuthobium, draws heavily upon the photosynthate of its host. These findings imply that Phoradendron is capable of furnishing its own energy requirements but say little regarding the extent of dependence of Arceuthobium upon its host for carbohydrates. The aerial shoots of Arceuthobium frequently exhibit an off-green color which Peirce (9) claimed is due to an abundant supply of chlorophyll that is often masked by dark pigments in the epidermal cells. Heinricher (6) described green-colored bodies in the cells of the endophytic system, however, this observation was not confirmed by Peirce (9) or Kuijt (8). A capacity of Arceuthobium to provide much of its energy requirements was implied by findings of Weir (16). He conducted a test on 8to 12-year old lodgepole pines in which 6 infected and 6 uninfected trees were repeatedly defoliated. During the second year the 6 infected trees continued to live while the uninfected trees had died. He suggested that under such conditions the mistletoe may contribute to the nutrition of its host but he added that mutual subsistence on sto.red materials in the infected region of host stems may explain the results. On the other hand, Buckland and Marples (2) noted that defoliation of western hemlock by the hemlock looper had a much more detrimental effect on dwarf mistletoe infected trees than on healthy trees. When Rediske and Shea (11) exposed aerial shoots of Arceuthobium americanum Nutt. infecting Pinus murrayana Grev. and Balf. seedlings to C1402, they noted a marked fixation of C14 in the shoots and movement of label into the host plant. This evidence was used to support their claim that the decline of the host plant normally associated with Arceuthobium infection is not a direct result of parasitism by the mistletoe. Freeland (5) analyzed leaves of Phoradendron flavescens (Pursh.) Nutt. for chlorophyll and found that both chlorophyll a and chlorophyll b were present. Apparent photosynthesis was measured and a net rate of CO2 fixation was determined. The mass of photosynthetic tissue produced by some species of Phoradendron has prompted the speculation that a symbiotic relationship might exist between the true mistletoe and its host. Wagener (15) noted that junipers heavily infected with Phoradendron juniperinum Engelm. will often have the entire upper portion of their crown replaced by mistletoe foliage. After an extremely cold winter, which killed the mistletoe, severe crown die-back was observed. He concluded that when the functioning mistletoe foliage was killed the very small amount of host foliage remaining was unable to support the crown and it died. Phoradendron infections are frequently found to terminate branches of its host (3, 15), that portion of the host branch distal to the site of infection is suppressed and killed with the proximal portion subsequently "sustained" by the mistletoe. Similar conclusions were drawn from defoliation and girdling experiments involving the European mistletoe Viscum albumt (14). Infections of Viscum were observed to prolong the life of denuded branches and even whole trees but such a situation could never be maintained for an extended period of time. The concept of symbiosis was not supported by findings of Seledzhanu and Galan-Fabian (12) who fed C1402 to foliage of Viscum infecting black poplar. They observed fixation of C02 by mistletoe leaves during all seasons of the year. Movement of C14 into adjacent mistletoe shoots was noted within 2 hours as well as into the endophytic system and to a small extent into the host tissue. The authors concluded that the transfer of organic materials between mistletoe and its host occurs only to a very limited extent. The purpose of the investigations reported here was to determine to what extent mistletoes can contribute to their own carbon nutrition and possibly to that of their hosts. This has been attempted primarily through C1402 feeding experiments, in which the photosynthetic products of mistletoe shoots were traced by radioautography and analyzed chemically to determine their similarity to carbon fixation products in other tissues. 1 Received for publication March 30, 1964. 2 This work was supported in part by National Science Foundation Grant G-13070. 3 Present address: Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana.