The effect of NAA and BA on carbon dioxide assimilation by shoot leaves of spur-type 'Delicious' and 'Empire' apple trees

The effects of NAA, BA, or Accel on CO2 assimilation of shoot leaves of mature bearing Redchief 'Delicious' and 'Empire' apple (Malus xdomestica Borkh) trees were evaluated over two seasons. BA at 50 mg.L-1 did not significantly affect any of the gas-exchange parameters measured. NAA (15 mg.L-1) consistently suppressed CO2 assimilation rate (from approximate to 10% to 24% below that of the control). This suppression was NAA-concentration dependent, continued for >15 days after treatment, and was completely overcome in 'Empire', but only partially or not at all in 'Delicious' when BA was combined with NAA. These results are discussed in relation to fruit thinning and NAA-induced inhibition of fruit growth in spur-type 'Delicious'. Chemical names used: 2-(1-napthyl) acetic acid (NAA); N-(phenyl)-1H-purine-6-amine (BA); BA + gibberellin A (GA)(4+7) (Accel).