© 2003 Heron Publishing—Victoria, Canada
Fruit load and canopy shading affect leaf characteristics and net gas exchange of ‘Spring’ navel orange trees
J. P. Syvertsen (1, 2), C. Goñi (3) and A. Otero (3)
1. University of Florida, CREC, 700 Experiment Station Road, Lake Alfred, FL 33850, USA / 2. Author to whom correspondence should be addressed (jmsn@lal.ufl.edu) / 3. Instituto Nacional de Investigación Agropecuaria, INIA-Salto Grande, P.O. Box 68033, Salto, Uruguay / Received December 11, 2002; accepted March 8, 2003; published online August 1, 2003
Summary
Five-year-old ‘Spring’ navel (Citrus sinensis (L.) Osbeck) orange trees were completely defruited, 50% defruited or left fully laden to study effects of fruit load on
concentrations of nitrogen (N) and carbohydrate, net assimilation of CO2 (Ac) and stomatal conductance (gs) of mature leaves on clear winter days just before fruit harvest. Leaves on defruited trees were larger, had higher starch
concentrations and greater leaf dry mass per area (LDMa) than leaves on fruited trees. Both Ac and gs were more than 40% lower in sunlit leaves on defruited trees than in sunlit leaves on trees with fruit. Leaves immediately
adjacent to fruit were smaller, had lower leaf nitrogen and carbohydrate concentrations, lower LDMa and lower Ac than leaves on non-fruiting branches of the same trees. Removing half the crop increased individual fruit mass, but reduced
fruit color development. Half the trees were shaded with 50% shade cloth for 4 months before harvest to determine the effects
of lower leaf temperature (Tl) and leaf-to-air vapor pressure difference on leaf responses. On relatively warm days when sunlit Tl > 25 °C, shade increased Ac and gs, but had no effect on the ratio of internal to ambient CO2 (Ci/Ca) concentration in leaves, implying that high mesophyll temperatures in sunlit leaves were more important than gs in limiting Ac. Sunlit leaves were more photoinhibited than shaded leaves on cooler days when Tl < 25 °C. Shade decreased total soluble sugar concentrations in leaves, but had no effect on leaf starch concentrations. Shading
had no effects on canopy volume, yield or fruit size, but shaded fruit developed better external color than sun-exposed fruit.
Overall, the presence of a normal fruit crop resulted in lower foliar carbohydrate concentrations and higher Ac compared with defruited trees, except on warm days when Ac was reduced by high leaf temperatures.
Keywords:
carbohydrates, chlorophyll fluorescence, leaf CO2 assimilation, LVPD, photoinhibition, soluble sugars, starch, stomatal conductance.
