Field and laboratory studies of growth and photosynthesis of Acacia auriculiformis (A. Cunn. ex Benth.) in the Northern Territory of Australia

Abstract

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 A_net between provenance regions were observed diurnally, within a season, and between seasons. The Northern Territory provenance region generally exhibited the largest A_net and the least variation diurnally and between seasons; the PNG provenances the lowest A_net, and the Qld provenance region varied most in A_net between seasons. Rankings of provenance regions by seasonal A_max were opposite to rankings based on growth rates. Seasonal and annual A_net of the provenance regions could be accurately modelled using the measures of A_net 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] (C_i), and photosynthetic photon flux density (PPFD) upon net CO₂ assimilation rate (A_net) of the various provenance regions. There were no differences between provenances in the response of A_net to LAVPD. Thus all provenance regions showed a ca. 10% decline in A_net as LAVPD increased from 1.0 to 4.5 kPa. This decline in A_net appeared to be independent of stomatal limitations, and suggested a decline in A_net 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 A_net to T₁ showed temperature optima for A_net of between 25 and 28 ºC. Large declines in A_net as T₁ were increased above or reduced below the optimum (ca. 10% reduction per 3 ˚C change) were found. The response of A_net 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 A_net to C_i. 

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 A_net between provenances appear to be at least partly attributable to nitrogen allocation to photosynthesis, although the diurnal decline in A_net in response to T₁ and LAVPD, and the seasonal decline in A_max in response to changes in T₁, LAVPD, soil moisture status and appear far more important in determining differences in A_max between provenance regions.

Date of Award	May 1994
Original language	English
Supervisor	Derek Eamus (Supervisor)