Abstract:
Presently, there are no procedures for selecting superior genotypes at seedling stage. We do not know which
morphological or physiological characteristics can be used to predict superior growth in trees. Field testing of
genotypes requires a substantial amount of time and money before a genotype shows significant promise in the
field. For this reason, morphological and physiological parameters that correlate with growth rate were sought as
early indicators of field performance. Six seed provenances of Acacia tortilis (Forsk.) Hayne collected from
areas of varying aridity where compared in biomass productivity and gas exchange traits. After 3.5 months of
growth, biomass ranged from 1-2 g. Significant provenance variation was observed in total biomass productivity,
root dry weight, leaf area, net photosynthetic rates per unit leaf area, stomatal conductance (P<0.001) and leaf
transpiration rate (P<0.05). More xeric provenances exhibited lower biomass productivity compared to mesic
ones. They also showed lower photosynthetic rates, stomatal conductance’s and low photosynthetic capacity.
Larger leaf areas, high stomatal conductances and photosynthetic rates appeared to be positively correlated with
total biomass productivity since faster growing provenances had a greater leaf area, higher stomatal conductance
and photosynthetic rates. Taken together, the results suggest that differences in leaf area, stomatal conductance
and photosynthetic rates (photosynthetic capacity) among provenances may be responsible for the variation in
biomass productivity in Acacia tortilis provenances. The probable premise and sequence of physiological events
responsible for the variability depends on photosynthetic rate, total leaf area and leaf longevity.
