Carbon dioxide assimilation in some aerial plant organs and tissues

Summary CO₂ fixation characteristics of a number of mature (but not senescing) tissues and organs (the outer layers of green pod and the seed testa of Vicia faba L.; the outer layers of green pod and seeds of Trigonella foenum-graecum L.; the outer layers of the green fruit of Lycopersicon esculentum Mill.) were studied and compared with their respective C₃ leaf characteristics. On a chlorophyll basis phosphoenolpyruvate carboxylase, malic enzyme (NADP) and malate dehydrogenase (NAD and NADP) acitivites were much higher in the non-leaf tissues (except for V. faba seed testa) than the leaf tissues. Generally, on a protein basis the differences were less significant. All tissues possessed ribulose-1.5-diphosphate carboxylase activity though there was great variation in activities both on a protein and chlorophyll basis. Protein: chlorophyll ratios varied greatly from tissue to tissue being lowest in the leaf tissue (11.5–14.0) and highest in V. faba seed testa (805.5). Chlorophyll a:b ratios were all between 2 and 3. ¹⁴CO₂ uptake in the dark by L. esculentum fruit slices was about 1/3 that in the light and the major, initially labelled product was malate both in the light and dark. Neither typical C₄-photosynthesis or crassulacean acid metabolism were exhibited by the non-leaf tissues and it was considered that the increased levels of certain enzyme activities were present to refix and recycle respired CO₂.Abbreviations PEP phosphoenolpyruvate - RuDP ribulose -1,5-, diphosphate - MDH malate dehydrogenase - CAM Crassulacean acid metabolism - OAA oxaloacetic acid