AbstractThe growth and physiological characteristics of Acacia auriculiformis, A. mangium and their interspecific hybrids and the intraspecific hybrids of Acacia auriculiformis were investigated in the field at two sites in the wet-dry tropical region of Australia and Thailand. The study in Australia involved 1- to 2-year-old saplings of the intraspecific and interspecific hybrids and the parental species. In Thailand, mature trees of A. auriculiformis, A. mangium and the natural A. mangium x A. auriculiformis hybrid were used to examine the influence of light and seasonal drought on canopy structure, composition and photosynthetic performance.
Saplings of A. auriculiformis, A. mangium and the A. auriculiformis x A. mangium hybrids showed significant differences in growth (4.8-7.9 m at 25 months) and canopy size, and physiological adaptation to seasonal drought. Canopy phyllode area appeared to be a good indicator for growth in these acacias. Furthermore, larger canopy size was supported by lower-cost phyllodes and greater resource-use efficiency. The results help to identify some of the important physiological parameters underlying growth performance in these tropical acacias.
Among the three species, A. auriculiformis and the interspecific hybrids had the highest phyllode photosynthetic activity in the dry season and A. auriculiformis also had the most rapid photosynthetic recovery after early rains in the wet season. The A. auriculiformis (NT) x A. mangium hybrid was the best of the two interspecific hybrid genotypes with respect to drought tolerance. The excellent performance of this hybrid genotype and A. auriculiformis seemed to be related to their ability to maintain high photosynthesis at low soil water availability. The evidence suggests that this may possibly be genetically determined at the maternal level. Surprisingly, A. mangium performed poorly in both situations despite a rapid increase in soil water availability at the first drought-breaking rains. Their low photosynthetic activity could be attributed primarily to stomatal limitation at the leaf-air interface. However, phyllode chlorophyll content also declined dramatically in all three species in the dry season indicating that non-stomatal factors were also important regulators of photosynthesis during drought. Interestingly, chlorophyll content, but not photosynthesis, recovered completely in the early wet season. The results indicate that different factors may be involved in the regulation of photosynthesis in these acacias at different seasons.
Heterosis is evident in the interspecific and interprovenance hybrids examined in this study. On average, the interspecific hybrids were superior in growth performance to the intraspecific hybrids and were equal to the best intraspecific hybrid genotypes in drought tolerance. Mature trees of A. auriculiformis, A. mangium and their interspecific hybrids in Thailand had dramatically different canopy structure and form. A. auriculiformis had diamond-shaped, A. mangium dome-shaped and the natural interspecific hybrid columnar-shaped canopy. Over half of the canopy area in A. mangium was presented at the top 3 m, twice the value determined for A. auriculiformis and the hybrid. Seasonal drought had different effects on the acacias. The canopy of A. auriculiformis and the hybrid was reduced to half of their size with most of the loss occurring in the top 6 m of the canopy. In contrast, the canopy size of A. mangium decreased less than 15%. Consequently, light availability (penetration) increased within the canopy in the dry season. Phyllode nitrogen also increased but photosynthesis, SLA and chlorophyll content decreased. However, sun phyllodes still had greater photosynthetic activity, Rd and phyllode construction cost than shade phyllodes in both seasons. Canopy phyllode area was the predominant factor for canopy gain (photosynthesis) and loss (Rd and construction cost) for both seasons. Nevertheless, the hybrid canopy, which had the smallest canopy phyllode area and lower photosynthesis, was much more efficient in the utilization of resources compared to the other two acacia canopies.
The results show that the acacia species and genotypes examined in this study had adopted distinctive strategies for growth, light capture and drought tolerance and recovery in a seasonally wet-dry tropical environment. Some of the physiological traits involved may have a genetic basis.
|Date of Award||Jul 2001|