Morphological and photosynthetic responses differ among eight apple scion-rootstock combinations

Abstract

‘Red Fuji’ apple (Malus × domestica Borkh.) scions were grafted onto eight size-controlling rootstock combinations (dwarfing, semi-dwarfing, and vigorous types). During the third year after grafting, morphological and physiological parameters, such as growth, hydraulic conductance, non-structural carbohydrates, gas exchange and chlorophyll fluorescence, as well as leaf anatomy, were examined and instantaneous whole-canopy photosynthetic assimilation was modelled. Compared with Baleng, rootstocks M.9 and B.9 resulted in reduced growth vigour and photosynthetic capacity. Significantly lower soil-to-leaf hydraulic conductance, stomatal density and palisade/spongy parenchyma ratios were also observed in ‘Red Fuji’ grafted on M.9 and B.9. We propose that the down-regulated hydraulic conductance contributed to the decline in photosynthetic rate, which in turn caused a reduction in instantaneous whole-canopy photosynthetic assimilation. These results provide evidence that the limited photosynthetic capacity and leaf area are major causes of the reduced canopy photosynthetic assimilation observed in M.9 and B.9 grafted apple trees. The reduced photosynthetic assimilation may partly result from a sustained lower photochemical efficiency. However, dwarfing and semi-dwarfing rootstocks grafted plants contained more starch, which may be a significant factor underlying the observed advanced precocity.