Crop load and time of defoliation in 'Imperial gala' apple trees: Effects on leaf photosynthesis, fruit growth, and yield

Abstract

This report focus on the effect of crop load and time of defoliation on photosynthesis (A), chlorophyll content (Chl), fruit size and yield. The experiment was conducted at Clarksville Horticultural Experimental Station (Michigan State Univ.) on 9-year-old ‘Imperial Gala’ apple trees on M-9 (n = 36). During the summer (20 June) after June drop, yield was adjusted according to different number of fruit per trunk cross sectional area (TCSA) to obtain three crop load levels: high (13.5 fruit/TCSA), medium (7.1 fruit/TCSA), and low (2.1 fruit/TCSA). Each crop load level was defoliated (25%) three times during the season (8 July, 25 and 19 Aug.) leaving three thinned trees for each level of crop load un-defoliated as control treatment. Spur canopy daily assimilation (A), stomatal conductance (gs), internal CO2 concentration (Ci), fruit growth and shoot length were measured at weekly intervals after crop load adjustment until harvest, and 1 week before harvest (14 Sept.), Chl and nitrogen leaf content was also determined. A in high cropping trees was not affected by defoliation at any time, however in medium and low cropping trees there was an increase from 5% to 20% at each time after 3 days from defoliation. A was higher during the season (from 10 to 18 %) in high cropping trees than in medium and low cropping trees. The total Chl and nitrogen was lower in low cropping trees than in medium and high cropping trees and not affected by defoliation, as well as total yield, fruit diameter and shoot length. Results reveal compensation capacity of spur leaves subtending the fruits in medium and low cropping trees. This indicates that leaf photosynthesis compensation to leaf damage occurs without reduction in fruit diameter or total yield per tree.