Modeling Relationship between Japanese Pear Fruit Growth and Solar Radiation

Abstract

A dynamic model describing relation between fruit growth in Japanese pear (Pyrus pyrifolia Nakai) and solar radiation was developed. The model is based on observations on fruit volume under controlled solar radiation in an orchard.
Japanese pear cv. ‘Kosui’ were grown under seven solar radiation levels controlled with some kinds of cheesecloth from fruit thinning to harvest in 1989. Fruit volume were measured every week or every two weeks. The harvesting date decided from ground color was not influenced by solar radiation level. But fruit growth since 33 days after flowering depended not only on solar radiation but also on fruit volume of 33 days after flowering (V_t); Fruit growth for any growth stage was proportional to V_t^p(p=0.580) regardless of solar radiation levels in any growth stage. That was proportional to S_n^b(b=0.639) also in any growth stage, where S_n is solar radiation for the growth stage. From the facts we deduced a model as follows:
FVGR=k_n⋅S_d^b⋅V_t^p
where FVGR (fruit volume growth rate) is daily fruit volume growth, S_d is daily solar radiation and k_n is parameter which was calculated using the equation and observed values (FVGR, S_d, V_t). The k_n indicating the influence of solar radiation on FVGR changed in every growth stage. The following equation can be used to estimate the fruit volume at x days after flowering (V_x).
V_x=V_t+Σ^x-1_d=33FVGR_d
In order to test the validity of the model and parameters (k_n, p, b), Japanese pear cv. ‘Kosui’ were shaded during four different growth stages with one kind of cheesecloth in 1990. They were justified by comparisons between the observed growth curves and those estimated from the model, parameters and observed daily solar radiation.