On the ecophysiology of the Clusiaceae in Trinidad: expression of CAM in Clusia minor L. during the transition from wet to dry season and characterization of three endemic species

Abstract

SUMMARYA study was made of photosynthesis and expression of crassulacean acid metabolism (CAM) in naturally exposed and shaded populations of Clusia minor L. during the transition from wet to dry season in Trinidad (mid-February to mid- April, 1990). At the start of the dry season, plants from exposed and shaded habitats showed a capacity for CAM either through the fixation of external or internal (respiratory) CO.^. Exposed plants showed continuous uptake of CO^ over 24 h although dark fixation accounted for only a small proportion of CO^ fixed over the day. The expression of CAM was considerably enhanced as the dry season progressed with substantial increases in the overnight accumulation of titratable acidity, particularly in leaves of exposed plants. This was accompanied by a reduction in day-time photosynthesis and an increase m dark fixation, with shaded plants showing only night-time fixation of CO,^. The magnitude of CAM in C. minor was substantial with a maximum AH" of 1410 mol m"'' measured in leaves from exposed branches. Both malic and citric acids were accumulated overnight. The highest citric.malic acid ratios were found in young leaves from exposed plants with 250 mol m~' * malic and 125 mol m'ĉ itric acid accumulated near the time of maximum CAM activity. Photosynthetic efficiency, measured as light responses of O^ evolution, also varied on a daily basis dependent on the incident photosynthetic photon flux density (PPFD). Apparent quantum yield and photosynthetic capacity showed marked reductions depending on the degree of exposure, suggesting that photoinhibitory responses are important under natural conditions. An analysis of three members of the Clusiaceae endemic to Trmidad showed that each had the capacity to induce CAM activity, despite being found in a narrow range of habitats which have higher rainfall than those of C. minor. However, despite the variable expression of CAM activity, carbon isotope composition suggested that when integrated throughout the year, carbon accumulation is predominantly mediated via the C^ pathway in all the species studied.

Carbon-Isotope Composition of Biochemical Fractions and the Regulation of Carbon Balance in Leaves of the C3-Crassulacean Acid Metabolism Intermediate Clusia minor L. Growing in Trinidad

“…Despite the considerable capacity for CAM in C. minor (Borland et al, 1992) and the observation that C4 carboxylation can account for up to 30% of C 0 2 fixed during the day in the dry season (Borland et al, 1993), 613Cs of leaf organic material reveal that during the annual cycle of leaf growth most carbon is fixed via the C3 pathway (Borland et al, 1992). However, given the large diel changes in the size of organic acid and carbohydrate pools in CAM plants, measurements of 613C of different biochemical fractions at dawn and dusk could provide a means of reconciling hourly changes in instantaneous A with seasonal changes in 613C.…”

“…In contrast to many CAM plants, in Clusia sp. soluble sugars form the major reserve carbohydrate pool, and both malic and citric acids may be accumulated at night (Popp et al, 1988;Borland et al, 1992;Franco et al, 1992). Moreover, it has recently been suggested that partitioning of fixed carbon between various biochemical fractions during the day controls the rapid switches between C3 and CAM in Clusia uvitana (Winter et al, 1992;Zotz and Winter, Plant Physiol.…”

“…Estimates of carbon-isotope discrimination expressed during CAM, derived from the trafficking among inorganic carbon, organic acids, and carbohydrate pools overnight, ranged from 0.9 to 3.1%0. The 613C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C3 carboxylation.Clusia minor L. is a tropical dicotyledonous tree that shows extreme flexibility in regulating the relative amounts of C 0 2 fixed during the day and night in response to changes in the environment (Franco et al, 1990Borland et al, 1992Borland et al, , 1993Winter et al, 1992;Zotz and Winter, 1993). Under natural conditions, short-term changes in on-line, instantaneous A have illustrated how shifts in C3 and C4 carboxylation occur on a daily basis in C. minor, reflecting changes in environmental parameters (Borland et al, 1993).…”

Discrimination Processes and Shifts in Carboxylation during the Phases of Crassulacean Acid Metabolism

“…Measurements were taken on March 1,1995, during the dry season on a stand of C. minor growing 500 m from the Simla Research station (10" 41'N, 61" 17'W; grid reference PS869 823) in the Arima Valley, where annual rainfall is approximately 2.5 m per year. The population of C. minor (previously used by Borland et al, 1992) grew terrestrially on a rocky limestone outcrop and was surrounded by deciduous seasonal forest (for full description, see Broadmeadow et al, 1992). Here plants were growing fully exposed 6 to 8 m high, and access was gained to the most exposed branches overhanging a cliff-face by means of a bamboo platform.…”

Carbon isotope composition and water-use efficiency in plants with crassulacean acid metabolism

“…The genus harbours an exceptional diversity of life forms, which include terrestrial trees and shrubs, lianas, hemi-epiphytes and epiphytes (Zotz and Winter 1994a, b;Lüttge 1996). Clusia is singular in that not only do many species have the ability to grow as hemi-epiphytes, epiphytes or as terrestrial plants but the genus contains C 3 species and species that exhibit weak, inducible or obligate CAM (Franco et al 1990;Winter et al 1990Winter et al , 1992Borland et al 1992;Zotz and Winter 1993;Lüttge 1999), including the only known dicotyledonous CAM plants that exhibit a classic tree-like arborescent habit.…”