MEASUREMENT OF THE INCREASED PAR AND NET ALL-WAVE RADIATION ABSORPTION BY AN APPLE TREE CAUSED BY APPLYING A REFLECTIVE GROUND COVERING

Measurements were made of the effect of a reflective ground covering on the absorption of radiant energy by an apple tree (Malus domestica Borkh. 'Splendour') in an orchard. Total absorbed PAR and net all-wave radiation were measured using the 'Whirligig' radiometer described by McNaughton et al. (1992) (Agric. For. Meteorol., 62: 87-107) during a 4-week period in late summer. The Whirligig rotated about a tree which was 4 m tall and had a total leaf area of 35.5 m(2). During the course of the experiment the ground beneath the tree was left uncovered, or was partially-covered or fully-covered with a reflective, aluminized foil. On a sunny day, when total incident PAR on a horizontal plane was 48 mol m(-2), the canopy absorbed about 500 mol of PAR radiation and about 140 MJ of net all-wave radiation. A linear relationship was established between daily totals of incident PAR radiation and the amount of absorbed radiation when the ground was uncovered. The influence of a reflective ground cover on the tree's absorption of radiant energy was subsequently estimated by comparing measured values of absorbed radiation against those values predicted from this linear relationship. Results show that a full ground cover increased the total absorption of PAR radiation by almost 40%, whilst covering half of the ground with foil increased absorption by about 24%. A similar increase in the absorption of net all-wave radiation was recorded. Meteorological and plant measurements were also made to assess the effect of such changes on transpiration and photosynthesis. Calculations, based on measured stomatal conductance and the photosynthetic light response of sunlit and shaded leaves, showed that a complete reflective covering could increase tree photosynthesis by about 32% and transpiration by about 26%. The extra PAR reflected by the foil also caused a significant increase in PAR radiation coming into the lower parts of the canopy. Results from the literature suggest that this should lead to improved fruit quality of the fruit lower in the canopy.