Comparison between single-leaf and whole-canopy gas exchange measurements in papaya (Carica papaya L.) plants

Abstract

Papaya canopy architecture enables all the leaves to receive sufficient amounts of sunlight to optimize leaf gas exchange due to the spiral phyllotaxis. Thus, there may be a close relationship between photosynthesis and transpiration measurements at single-leaf and whole-canopy levels. We tested this hypothesis by simultaneously measuring single leaf and whole canopy gas exchange of ‘Gran Golden’ papaya during two seasons. Whole-canopy gas exchange was measured on five plants during four days in July (winter) from 08:00 to 1700h and three days in December (summer) from 06:00 to 1700h in 2006. Single-leaf gas exchange measurements were performed using the fully exposed 12th and 13th leaf from the apex on five plants adjacent to those with whole canopy chambers at the same time of the day. Net photosynthetic rate (A) values on a single-leaf or whole-canopy basis in field-grown papaya were similar and highly correlated under clear sky conditions since the open canopy architecture of papaya plants allowed saturating light intensities for photosynthesis to reach all leaves. Conversely, whole-canopy transpiration (Ec), was approximately 50% of single-leaf transpiration (El) likely due to high air temperature and VPDleaf-air and increased leaf fan speed in the LI-6200 leaf chamber (single leaf) compared to the whole canopy chamber. Therefore, water use efficiency measured on a single-leaf basis seems to have reduced applicability in papaya water relations studies.