Journal of Experimental Botany, Vol. 54, No. 381, pp. 365-373,
January 2, 2003
© 2003 Oxford University Press
Increased growth of young citrus trees under reduced radiation load in a semi-arid climate1
Received 26 November 2001; Accepted 6 August 2002
2 Institute of Horticulture, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
3 Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
4 The Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel
5 Department of Environmental Sciences, Weizmann Inst. of Science, Rehovot 76100, Israel
1 Contribution from the Institute of Horticulture, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 208/01.
6 To whom correspondence should be addressed. Fax: +972 3 9604017. E-mail: vwshep{at}agri.gov.il
This study investigated the effects of radiation heat-load reduction by shading on the growth and development of citrus trees in a warm subtropical region. The experiment was conducted from mid-June until late October when daily maximal air temperature averaged 29.3 °C. Two-year-old de-fruited Murcott tangor (Citrus reticulata BlancoxCitrus sinensis (L.) Osb.) trees were grown under 30% or 60% shade tunnels, or 60% flat shade (providing midday shade only), using highly reflective aluminized nets. Non-shaded trees were used as the control. Shading reduced direct more than diffuse radiation. Daily radiation was reduced by 35% for the 30% Tunnel and 60% Flat treatments, and by 55% for the 60% Tunnel. Two days of intensive measurement showed that shading increased average sunlit leaf conductance by 44% and photosynthesis by 29%. Shading did not significantly influence root and stem dry weight growth, but it increased the increment in leaf dry weight during the three month period by an average of 28% relative to the control, while final tree height in the 30% Tunnel treatment exceeded the control by 35%. Shoot to root and shoot mass ratios increased and root mass ratio decreased due to shading because of the increase in leaf dry weight. Shading increased starch concentration in leaves while the shadiest treatment, 60% Tunnel, decreased starch concentration in the roots. Carbon isotope ratio (
13C) of exposed leaves that developed under shading was significantly reduced by 1.9
in the 60% Tunnel, indicating that shading increased CO2 concentrations at the chloroplasts (Cc), as would be expected from increased conductance. Substomatal CO2 concentrations, Ci, computed from leaf net CO2 assimilation rate and conductance values, also indicate that shading increases internal CO2 concentrations. Based on tree dry mass, tree height, and total carbohydrates fractions, the 30% Tunnel and the 60% Flat were the optimal shade treatments.
Key words: 13C, carbohydrate allocation, carbon isotopes, leaf conductance, partitioning, photosynthesis, shade, screen, water relations.
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