Control of Tundra Plant Allocation Patterns and Growth 161 



Glucose 



New Tissue or Compound 



B+Rq B 



FIGURE 5-8. Processes of growth and growth respiration. When new tis- 

 sue is created, glucose is metabolized, some glucose appearing in the form of the 

 new biochemical compounds (B), and some being lost (Re) as it supplies energy 

 for the synthesis of these compounds. For example, to produce 1 g of lignin (BJ, 

 2. 15 g of glucose is used fIB + Kc) and Re involves 1.15 g of glucose. Other com- 

 pounds use varying amounts of glucose per gram, as listed in Table 5-5. Re is 

 calculated for each compound by subtracting the gram of product from the total 

 gram of glucose used. The total Re for creating new tissue is found by summing 

 the Re's for synthesizing each compound, weighting each by the percentage com- 

 position. 



Tundra graminoids have a relatively low lignin content and a relatively 

 high polysaccharide and sugar content and hence may be able to produce 

 more biomass per unit carbon fixed than can temperate counterparts. 

 More complete data on chemical composition of tundra graminoids are 

 needed to verify this hypothesis. 



Translocation is a vital process requiring energy, particularly for the 

 loading and unloading of phloem elements, and energy cost is propor- 

 tional to the translocation rate; available estimates (Penning de Vries et 

 al. 1974) suggest a value of 0.05 for the proportionality constant. Be- 

 cause both loading and unloading costs are involved, 1 g of glucose will 

 be required to translocate 10 g from source to sink. Because of the high 

 belowground-to-aboveground biomass ratio of tundra graminoids, more 

 of the maintenance and growth respiration occurs in nonphotosynthetic 

 tissues than would be the case in their temperate counterparts. Hence, 

 translocation respiration may be relatively more important in tundra 

 than in temperate graminoids. 



The carbon cost involved in growth is partially regained during se- 

 nescence when some of the biomass components are broken down and 

 retranslocated to storage areas to support future growth. Remobilization 

 is not complete, and different classes of compounds are remobilized to 

 different extents (Table 5-5). 



Sucrose and the various storage polysaccharides require a minimal 

 energy investment for synthesis and subsequent breakdown. In contrast, 

 Hpid requires a major energy investment, half of which is lost during 

 reconversion to glucose. Although complete conversion of lipid to glu- 

 cose is unlikely in any system, it is clear that lipid is a costly compound to 

 synthesize and would be utilized as an energy source primarily where 

 space or weight is limited, as in a seed. Plants growing in permafrost soils 

 have larger amounts of membrane lipid but smaller amounts of storage 



