Light Acclimation in Citrus Leaves. I. Changes in Physical Characteristics, Chlorophyll, and Nitrogen Content

Abstract

Abstract ‘Duncan’ grapefruit (C. paradisi Macf.) and ‘Pineapple’ sweet orange (Citrus sinensis L.) seedlings were grown in full sunlight, 50% and 90% shade; maximum photosynthetic photon flux densities (PPFD) of 2300, 1100 and 200 μmol s−1m−2, respectively. In fully expanded matured (hardened) leaves, leaf thickness, specific leaf weight (SLW), tissue density, and nitrogen content were highest in full sun leaves and lowest in 90% shade leaves. Leaf chlorophyll content was highest in 90% shade leaves. Half of the seedlings which were grown in full sunlight were transferred into 50% shade to simulate normal canopy development; half of the seedlings from 50% and 90% shade were moved into full sunlight to simulate changes that occur after hedging. Specific leaf weight and tissue density changed in the same direction as PPFD. Leaf nitrogen content decreased temporarily when leaves were exposed to new PPFD conditions regardless of the PPFD levels. Total leaf chlorophyll content initially decreased when seedlings were transferred into full sunlight but began to increase after 4–6 weeks. Chlorophyll content increased in seedlings transferred from full sun to 50% shade. Percentage of air space within leaf tissues did not change during acclimation to new PPFD levels. Changes in leaf anatomy, physical characteristics, and chemical components are mechanisms that enable citrus leaves to acclimate to a wide range of changing light environments, even after leaves are fully mature.