Effects of leaf and tree age on chlorophyll absorbance in diploid black wattle (Acacia mearnsii)

Abstract

The invasiveness of black wattle (Acacia mearnsii) in South Africa has created a need to investigate ways to reduce seed production. The Institute for Commercial Forestry Research is investigating polyploidy production with the view to inducing reduced fertility. Accurate, reliable and affordable identification of ploidy level is necessary. An effective technique for ploidy identification quantifying total chlorophyll content using absorption spectra has been developed, but its accuracy could be compromised by a number of factors of which this investigation assesses leaf and tree age. Young leaf material (new flush) and older leaf material (old flush) were collected from 20 genetically unrelated plantation trees in five age classes (two, four, six, eight and nine years). Seedling reference leaf material, comprising essentially new leaf flush, were collected from 10 genetically unrelated eight-month-old seedlings. Chlorophyll was extracted in 90% acetone from five leaf samples of each new and old leaf type and each tree age. Absorbance spectra were obtained by passing light of wavelength 400–700 nm through the solution, with absorbance resolution of 1 nm intervals. Absorbance spectra peaked at 433, 456 and 663 nm in each case. Significant differences in the value of peak absorbance were recorded among leaf samples, according to leaf type and tree age, respectively. Mean chlorophyll absorbance values of corresponding peak wavelengths for all leaf types and age groups were mostly significantly different from one another, and all values were significantly different from the mean values for seedling reference samples. All values of new versus old flush at a particular age also differed significantly (p < 0.05). These results demonstrate that tree age and leaf type affect chlorophyll content significantly (p < 0.05), and should be considered when chlorophyll absorbance is used to determine ploidy.