Field and laboratory studies of growth and photosynthesis of Acacia auriculiformis (A. Cunn. ex Benth.) in the Northern Territory of Australia
Creators: name: Cole, Stephen
Publisher: name: Charles Darwin University
Year: 1994

description: Field studies of CO₂ assimilation, phyllode elemental analysis, stomatal densities, chlorophyll contents and growth rates, were carried out on three to four-year old trees of Acacia auriculiformis A. Cunn. ex Benth., growing in a seed production stand on Melville Island, Northern Territory, Australia. The trees were from three provenance regions: Papua New Guinea (PNG - Bensbach River and Morehead River provenances); Queensland (Qld); and the Northern Territory (NT). The PNG provenances were the largest trees and had accumulated the largest biomass. Significant differences in Anet between provenance regions were observed diurnally, within a season, and between seasons. The Northern Territory provenance region generally exhibited the largest Anet and the least variation diurnally and between seasons; the PNG provenances the lowest Anet, and the Qld provenance region varied most in Anet between seasons. Rankings of provenance regions by seasonal Amax were opposite to rankings based on growth rates. Seasonal and annual Anet of the provenance regions could be accurately modelled using the measures of Anet at 0900 and 1500 hours, thus making time consuming diurnal studies unnecessary for determining annual carbon assimilation rates. Canopy areas of the PNG provenances were seven times greater than those of the NT provenances during the late dry season. It is concluded that the PNG provenances achieve their greater growth rates primarily by maintaining a larger canopy throughout the year. These results highlight the need for caution in extrapolating from measurements of assimilation of single phyllodes to infer comparative growth rates. Laboratory studies of CO₂ assimilation were carried out to determine the effects of phyllode temperature (T1), leaf-to-air vapour pressure difference (LAVPD), interstitial [CO₂] (Ci), and photosynthetic photon flux density (PPFD) upon net CO₂ assimilation rate (Anet) of the various provenance regions. There were no differences between provenances in the response of Anet to LAVPD. Thus all provenance regions showed a ca. 10% decline in Anet as LAVPD increased from 1.0 to 4.5 kPa. This decline in Anet appeared to be independent of stomatal limitations, and suggested a decline in Anet in response to LAVPD per se, perhaps in response to a root signal, and/or alternatively the results may reflect non-uniform stomatal closure. The response of Anet to T1 showed temperature optima for Anet of between 25 and 28 ºC. Large declines in Anet as T1 were increased above or reduced below the optimum (ca. 10% reduction per 3 ˚C change) were found. The response of Anet to PPFD produced apparent quantum yield (ф) values of ca. 0.035, with no significant differences observed between provenance regions. Large differences between provenance regions were found for the light saturation point (LSP), and these differences were concluded to be the result of the larger nitrogen allocation to photosynthesis in the NT provenances. No significant differences between provenance regions were found in response of Anet to Ci. The narrow temperature optimum for photosynthesis in A. auriculiformis places constraints on phyllode A. because in the wet-dry tropics the T1 in most seasons and at most times of day was at least 5 ˚C above the optimum. Differences in Anet between provenances appear to be at least partly attributable to nitrogen allocation to photosynthesis, although the diurnal decline in Anet in response to T1 and LAVPD, and the seasonal decline in Amax in response to changes in T1, LAVPD, soil moisture status and appear far more important in determining differences in Amax between provenance regions.