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

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.

Light Acclimation in Citrus Leaves. II. CO2 Assimilation and Light, Water, and Nitrogen Use Efficiency

“…The 0 of citrus leaves acclimated to changes in PPFD and was accompanied by changes in maximum rates of C 0 2 assimilation. High rates of A corresponded to increases in leaf tissue density and correlated with highest leaf nitrogen content (26). Differences between 0 of low and high PPFD grapefruit leaves were greater than that of orange leaves.…”

“…Using the mean A data from Fig. 2 and 3 along with mean values of leaf chlorophyll and nitrogen content (26), the correlation between A and leaf chlorophyll content was weak, whereas leaf nitrogen was positively correlated with A. The relationship between A and N apparently was due to changes in Gm rather than Gj as Gm is more strongly correlated to variations in leaf nitrogen content (Table 2).…”

“…Low PPFD leaves increased in leaf density and nitrogen content (26) when transferred into high PPFD conditions. The relatively low rates of A along with leaf N contents comparable to other well-fertilized crops (3,23), results in relatively low A/N.…”

“…'Duncan grapefruit' and C. sinensis L. 'Pineapple orange', were grown either outdoors (high PPFD, maximum PPFD = 2300 jjLmol s -1m -2), under 50% shade (intermediate PPFD) or 90% shade (low PPFD). When seedlings were 9-months-old, mature leaves within the same age group were tagged for study (26). Half the seedlings of both species in the low and intermediate PPFD were transferred to high PPFD to simulate the contrasting light environment that occurs after hedging.…”

“…Data from these 2 groups of seedlings were used to characterize the relative importance of G x and Gm in determining maximum A during growth under widely different PPFD conditions and during acclimation to changing PPFD. In addition, A was tested for correlation with leaf N and chlorophyll content described previously (26), and the amount of C 0 2 fixed per unit leaf total N (A/N) also was tested for correlation with N using linear regression analysis.…”