No abstract

The ratio of deuterium to hydrogen (expressed as δD) in hydrogen released as water during the combustion of dried plant material was examined. The δD value (metabolic hydrogen) determined on plant materials grown under controlled conditions is correlated with pathways of photosynthetic carbon metabolism. C3 plants show mean δD values of-132‰ for shoots and -117‰ for roots; C4 plants show mean δD values of -91‰ for shoots and-77‰ for roots and CAM plants a δD value of-75‰ for roots and shoots. The difference between the δD value of shoot material from C3 and C4 plants was confirmed in species growing under a range of glasshouse conditions. This difference in δD value between C3 and C4 species does not appear to be due to differences in the δD value (tissue water) in the plants as a result of physical fractionation of hydrogen isotopes during transpiration. In C3 and C4 plants the hydrogen isotope discrimination is in the same direction as the carbon isotope discrimination and factors contributing to the difference in δD values are discussed. In CAM plants grown in the laboratory or collected from the field δD values range from-75‰ to +50‰ and are correlated with δ(13)C values. When deprived of water, the δD value (metabolic hydrogen) in both soluble and insoluble material in leaves of Kalanchoe daigremontiana Hamet et Perr., becomes less negative. These changes may reflect the deuterium enrichment of tissue water during transpiration, or in field conditions, may reflect the different δD value of available water in areas of increasing aridity. Whatever the origin of the variable δD value in CAM plants, this parameter may be a useful index of the water relations of these plants under natural conditions.

From the hygrohalophyte Borszczowia aralocaspica Bunge (Chenopodiaceae), a new leaf type with 1‐layered chlorenchyma is described as “borszczovoid” and compared with other leaf types in subfamily Salsoloideae. The chlorenchyma is suspected to represent a unique C4 type. Evidence is cited from anatomical studies and documented by micrographs and Carbon isotope determinations (ä13C values). The 1‐layered photosynthetic tissue combines all essential anatomical characters of a 2‐layered chlorenchyma of regular C4 plants and is in intimate contact with concentrically arranged peripheral bundles. The ä13C values are − 13.03 ‰ from young stems and − 13.78 ‰ from leaves. The results are discussed in the anatomical, physiological and taxonomic framework. In addition, from distantly‐related Suaeda species of section Conosperma the conospermoid leaf type is re‐described. It is characterized by typical palisade and Kranz layers and differs from the C4 suaedoid type by an external water‐storaging hypodermis and an arrangement of Kranz cells reminiscent of the atriplicoid type from subfamily Chenopodioideae. From eight other species of Chenopodiaceae ä13C values are given for the first time.

No abstract

The occurrence of Crassulacean acid metabolism (CAM), as judged from δC values, was investigated in epiphytes and some related plant species at a series of sites covering the approximate altitudinal range of epiphytes in Papua New Guinea. Comprehensive collections were made at each site and the occurrence of water storage tissue and blade thickness was also determined. Some 26% of epiphytic orchids from a lowland rainforest (2-300 m.a.s.l) showed δC values typical of obligate CAM and possessed leaves thicker than 1 mm. A second group of orchids, mostly with succulent leaves, possessed intermediate δC values between -23 and -26% and accounted for 25% of the total species number. Some species of this group may exhibit weak CAM or be facultative CAM plants. The remainder of the lowland rainforest species appeared to be C plants with δC values between -28 and -35%. and generally possessed thin leaves. Obligate CAM species of orchids from a lower montane rainforest (1175 m.a.s.l) comprised 26% of the species total and mostly possessed thick leaves. The remainder of the species were generally thin-leaved with δC values between -26 and -35%. largely indicative of C photosynthesis. Orchids with intermediate δC values were not found in the lower montane rainforest. Obligate CAM appeared to be lacking in highland epiphytes from an upper montane rainforest and subalpine rainforest (2600-3600 m.a.s.l). However the fern, Microsorium cromwellii had a δC value of -21.28%. suggesting some measure of CAM activity. Other highland ferns and orchids showed more negative °C values, up to-33%., typical of C photosynthesis. The highland epiphytic orchids possessed a greater mean leaf thickness than their lowland C counterparts due to the frequent occurrence of water storage tissue located on the adaxial side of the leaf. It is suggested that low daytime temperatures in the highland microhabitats is a major factor in explaining the absence of CAM. The increased frequency of water storage tissue in highland epiphytes may be an adaptation to periodic water stress events in the dry season and/or an adaptation to increased levels of UV light in the tropicalpine environment.