Irrigation and crop load interactions in relation to pear yield and fruit-size distribution

The interaction between irrigation and crop load in their effect on fruit size distribution was investigated in a 'Spadona' (Pyrus communis) pear orchard located in a semi-arid zone. Five crop coefficient treatments were applied during the main fruit expansion phase: 0.25 K-C, 0.46 K-C, 0.60 K-C, 0.80 K-C, 1.00 K-C where the crop coefficient (K-C) is the applied irrigation level divided by the USDA "Class A" pan evaporation rate. An additional irrigation treatment (0.80 K-C-ED) studied the effect of early water deficit. Crop load was adjusted to 200 to 800 fruits per tree in the 900 trees per ha orchard, by hand thinning. Cumulative irrigation levels up to harvest were 271, 351, 465, 572, 502 and 688 mm, in the 0.25 K-C, 0.40 K-C, 0.60 K-C, 0.80 K-C, 0.80 K-C-ED and 1.00 K-C treatments, respectively. Crop yield increased with irrigation rate and crop load, with no interactions between the two. Total yields of fruits larger than 55, 60 and 65 mm increased with increasing irrigation level up to crop coefficient of 0.80, whereas the relative yields of fruits larger than 55, 60 and 65 mm increased with increasing crop coefficient up to 0.60, 1.00 and 1.00, respectively. Relative yield of large fruits decreased with increasing crop level. The fruit weight was highly correlated with the midday stem water potential, although the maximum water potential in the present study was much lower than those reported elsewhere for non-stress conditions. This suggests that there was some limitation in the water-absorption capacity of the root system, or inefficient irrigation practice in the present study. The early water deficit here was too severe and the initial fruit diameter in the 0.80 K-C-ED treatment was significantly lower than that in the 0.80 K-C treatment. Nevertheless, the fruit growth rate in the 0.80 K-C-ED treatment was higher than that in the 0.80 K-C, treatment, indicating that potential fruit growth rate was increased by early water deficit.