‘Daleks’ and ‘Whirligig’ used to uncover the fate of intercepted light by an apple tree canopy

Abstract

Canopy light interception and distribution has a major influence on CO2 fixation and the growth and productivity of fruit trees. Spatial and temporal variations in microclimate alter the gas exchange from the leaves and other organs (fruit, branch, etc.). The variable nature of the light environment within the tree crown, and other factors such as leaf age and various stresses (water, diseases, pests) can often impair our ability to extrapolate whole-canopy net carbon exchange (NCE) and transpiration from a small sample of single-leaf measurements. This paper compares two methods for estimating whole-canopy gas exchange. An open-top, through-flow whole-canopy cuvette (Wünsche & Palmer, 1997, HortScience, 32:653-658) was used to enclose the above-ground portion of a 4 year-old apple tree (Malus domestica Borkh. cv. `Braeburn¿ on M.9 rootstock). NCE and transpiration from the whole canopy was measured over a one week period in mid-season. Measurements of light interception were made on a neighbouring apple tree of the same age and of a similar growth habit using a `whole-plant¿ radiometer known as the Whirligig (McNaughton et al., 1992, Plant Cell Environ., 17:1061-1068). Data from the Whirligig were used to estimate the distribution of PAR radiation over the total leaf area of the tree. This PAR distribution was then combined with leaf stomatal conductance and leaf net photosynthesis response functions to estimate the rates of whole-canopy NCE and transpiration. The Whirligig calculations of transpiration were in good agreement with data from the whole-canopy cuvette, and with independent measurements of sap flow recorded using heat-pulse sensors located in the trunk of the cuvette tree. The Whirligig estimates of NCE were also comparable to values obtained from the whole-canopy curvette. We conclude that both instruments provide an integrated approach for whole-plant productivity studies