Carbon Dioxide Enrichment Increases Yield of Valencia Orange

Abstract

The response to elevated CO₂ of 3-year-old fruiting Valencia orange scions (Citrus sinensis (L.) Osbeck) on citrange rootstock (C. sinensis × Poncirus trifoliata (L.) Raf.) was studied over a 12-month period under controlled environmental conditions. CO₂ enrichment to approx. 800 µbar CO₂ which com- menced just prior to anthesis shortened the period of fruitlet abscission. Trees enriched to 800 µbar CO₂ retained 70% more fruit, which at harvest were not significantly smaller in diameter or lower in fresh weight than fruit from control trees grown at approx. 400 µbar CO₂. Fruit from the CO₂ enriched trees also did not differ from the controls in soluble solids content, dry weight, seed number or rind thickness. The progression of fruit coloration was more rapid for the CO₂ enriched trees.

Dry weight of leaves and branches from the scion portion of the trees and the roots and stem of the rootstock portion did not differ between treatments at time of harvest. Leaf areas were also similar. However, specific leaf dry weight was 25% greater for the CO₂ enriched treatment. Changes in dry matter partitioning resulted from the greater fruit yield (58% increase in dry weight) with CO₂ enrichment.

Photosynthetic rates observed at intervals over the experimental period were always lower in the CO₂ enriched treatment compared to controls when measured at the same partial pressure of CO₂. However photosynthetic rates in the CO₂ enriched cabinet were always higher because of the increased level of CO₂. The extent of this difference between the treatments varied with fruit development and increased from 23% higher photosynthetic rates in the CO₂ enriched chamber at the end of flowering to 77% higher rates when fruits were 5 cm in diameter and decreased to 18% higher rates when fruit coloration was well advanced. Flushes of leaves that developed during the experiment also showed similar photo- synthetic responses to CO₂ enrichment and their photosynthetic rates declined as fruit matured.

These results indicate that crop yield by fruit trees will increase as global levels of CO₂ continue to rise, at least in those species that experience source limitation during fruit development.