The 2012 ASHS Annual Conference

One Tonnage (T) and one Simmonds (S) avocado (Persea americana Mill.) tree and four TxS crosses were evaluated for differences in chlorophyll content and maximal quantum yield of photosystem II in sun and shade-type leaves. Total chlorophyll content by area (Chl a+b_ar) ranged from 984 mg·m^-2 in TxS240 to 4320 mg·m^-2 in Simmonds. Chlorophyll a/b ratio (Chl a/b) ranged from 9.8 to 5.5 in TxS238 and TxS243, respectively. Tonnage and Simmonds had similar Chl a/b with a wide range in values found among the avocado trees tested. Shade leaves contained more Chl a, Chl b and Chl a+b_wt than sun leaves. Differences in Chl a/b were insignificant or greater in shade adapted leaves for all trees except TxS238; this did not follow the expected sun/shade pattern. A low chlorophyll a/b ratio indicates more light harvesting proteins and higher stacking of thylakoids. Chl a+b_ar indicates Simmonds, Tonnage and to a lesser extent TxS238 had dense packing of chloroplasts in both sun and shade adapted leaves. Shade leaves had more efficient Fv/Fm values than those adapted to sun for all varieties except TxS240. Tonnage had the largest range of total chlorophyll content between shade and sun adapted leaves and likely has the largest genetic variation in its ability to acclimate to changing light intensities. The range in efficiency of photosystem 11 found between the avocado trees tested indicates a potential for improvements through selective breeding. More research is needed to evaluate the entire USDA avocado germplasm collection for traits associated with photosynthetic efficiency and to determine their heritability.