Revisiting the role of carbohydrate reserves in fruit set and early-season growth of apple
Breen, K and Tustin, S and Palmer, J and Boldingh, H and Close, D, Revisiting the role of carbohydrate reserves in fruit set and early-season growth of apple, Scientia Horticulturae, 261 Article 109034. ISSN 0304-4238 (2020) [Refereed Article]
In apple (Malus x domestica Borkh.), carbohydrate reserves are considered essential for maintaining tree functions during dormancy and for supporting spring growth. The contribution of carbohydrate reserves to fruit set and early-season tree development is unclear. In ‘Royal Gala’/‘M9’ trees, we used artificial spur extinction (ASE) to remove buds in late dormancy and reduce sink numbers. Final fruit numbers were set using hand thinning after final fruit drop. These trees were compared with trees where bud numbers where unmodified and fruit numbers were only modified using post fruit drop hand thinning (PDHT). On both these treatments, either early or natural defoliation was used to manipulate carbohydrate reserve replenishment and alter carbohydrate reserve concentration the following spring. This treatment combination allowed investigation of the interaction of early-season sink demand and altered carbohydrate reserve supply on fruit set. Early defoliation reduced total non-structural carbohydrate concentration in roots the following winter by 44 % and concentration of starch in spurs by 60 % compared with natural defoliation. However, neither defoliation treatment affected fruit set the following spring, implying that at these concentrations, stored carbohydrates in these tissues played little role in fruit set. This conclusion was supported by increase in total non-structural carbohydrate concentration in roots and spurs during the period 2-4 weeks after bloom, when limitations in carbohydrate resources are most likely to affect fruit set. ASE reduced the number of developing floral sinks in spring by 50 % and increased within-bud fruit set compared with PDHT trees, without affecting concentration of total non-structural and soluble carbohydrates in root or spur tissues sampled before final fruit set. In ASE, improved irradiance of spurs may increase fruit set through increased availability of photosynthates to developing fruit. Therefore, supply of newly synthesised carbohydrates probably plays a greater role in determining fruit set than remobilised, carbohydrates. The increase in soluble carbohydrates in roots during fruit set, challenged that concept that storage sinks only accumulate carbohydrates during periods of low carbohydrate demand from other sinks.