halimedatetraacetate requires small organic carbon compounds and 

 is therefore coupled to photosynthesis. Halimedatrial and 

 epihalimedatrial appear to be enzymatic products of 

 halimedatetraacetate. Thus, the trials may be enzymatically 

 formed at night and pumped into the nutritionally valuable new 

 tips. At sunrise, photosynthetic activity begins and new 

 halimedatetraacetate can be produced. Since halimedatrial is a 

 stronger herbivore deterrent than halimedatetraacetate (Paul and 

 Van Alstyne, 1987), this is consistent with the hypothesis that 

 the valuable and more morphologically defended older segments are 

 protected by a less potent compound that can be converted to the 

 stronger forms by enzymatic action. 



Interactions between anti-herbivore defenses and plant food value 



Newly produced segments of Halimeda are soft, unpigmented, 

 and rich in food value as evidenced by their high concentrations 

 of nitrogen and organic contents (Fig. 5). The morphological 

 defense of calcification (Littler and Littler, 1980; Steneck and 

 Watling, 1982) cannot play a role during the first night since 

 the process of calcification in Halimeda is dependent upon 

 photosynthesis to raise internal pH and cause precipitation of 

 CaCC>3 in the semi-isolated compartments that occur between 

 utricles (the swollen terminal portions of the filaments that 

 coalesce to form the thallus of Halimeda ) (Borowitzka, 1977). 

 Thus, if soft new growth is to be defended during the night and 

 early portions of the first day, then concentration of chemical 

 feeding deterrents within the new sections may be necessary. 

 This is the pattern shown in Table 1 and Figs. 6 and 7. The 

 halimedatrials are abundant in the youngest tissues and 

 significantly decrease in abundance with increasing age of the 

 segment (Fig. 6). Halimedatetraacetate is absent in the newest 

 tips, rapidly increases in abundance at dawn, and is low in 

 abundance in old segments (Fig. 6). These data are consistent 

 with the hypotheses that the plant is reducing its chemical 

 defense of tissues as they become better defended morphologically 

 and of less nutritional value (Fig. 5) to herbivores. These 

 patterns are summarized qualitatively in Fig. 10. 



The timing of pigmentation in newly forming segments may 

 also be an example of how diel patterns in the allocation of 

 scarce resources may have evolved to minimize losses to 

 herbivores. In the field, newly forming segments of Halimeda do 

 not become pigmented until just before sunrise. Thus, the 

 valuable, nitrogen-containing molecules necessary for 

 photosynthesis are not placed in the new, and more herbivore- 

 susceptible, segments until just before the lighted period of the 

 day when they can begin producing income for the plant. A 

 similar pattern occurs in some terrestrial understory plants 

 since many do not pigment their leaves until they are fully 

 formed and structurally capable of rapid photosynthesis. It has 



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