Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Abstract

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown “Kwai May Pink” and “Salathiel” orchard trees and young potted “Kwai May Pink” plants during summer in subtropical Queensland (27° S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of “Kwai May Pink” were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 μmol m−2 s−1) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 μmol m−2 s−1). Young potted plants of “Kwai May Pink” were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 μmol m−2 s−1) and measured for shoot extension and leaf area development over one flush cycle.

Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 ± 0.5 versus 6.3 ± 0.2 μmol CO2 m−2 s−1), dark respiration (1.7 ± 0.3 versus 0.6 ± 0.2 μmol CO2 m−2 s−1), quantum yield (0.042 ± 0.005 versus 0.027 ± 0.003 mol CO2 mol−1) and light saturation point (1155 versus 959 μmol m−2 s−1) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 ± 12 mm and 497 ± 105 cm2 for each flush, compared with 27 ± 7 mm and 189 ± 88 cm2 in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 ± 1, 143 ± 3, 251 ± 7 and 362 ± 8 cm2, respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 ± 5 mm versus 101 ± 2 mm).

Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.