Effect of shading and water stress on light interception, physiology and yield of apple trees

Net shading was explored as a corrective tool for mitigating the effect of a single year drought event on apple (‘Imperial Gala’) yield in Bologna (Italy). In 2013, trees were grown under three nets with different shading levels (red-50%, white-50%, and black-20%) and without nets. Those treatments received three irrigation doses from 60 days after full bloom until harvest: 260 (control), 115 (moderate water stress, WS) and 50 (severe WS) mm of water. Control trees had midday stem water potential (Ψₛₜₑₘ) values around -1.0 MPa when they were shaded, but values were more negative when they were grown without nets. Ψₛₜₑₘ ranged between -1.0 and -1.5 MPa under moderate WS and was about -1.5 MPa for severe WS although this value was reached sooner in trees grown without nets. Leaf photosynthesis decreased with more negative Ψₛₜₑₘ values but was not affected by shading. Yield was very low (3–4 kg per tree) for trees grown without nets with no differences between irrigation treatments. Yield was also very low for shaded trees grown under severe WS (5–7 kg per tree). Under control and moderate WS, shaded trees had higher yields (9–13 kg per tree) than trees grown without nets, but no differences were found between shading. The benefits of net shading on yield were explained by several additive factors: i) improved water status, ii) delay in fruit maturity giving more time to the fruit to grow, and iii) reduction of photo-inhibition. These results may encourage fruit growers to install nets in their orchards when water is limited. Under severe water stress conditions net shading was not effective and low yields are expected. No effect of net colour was observed but its effect may be observed in a longer term. Further research is necessary to determine the sustainability of shading over multiple years.