Photosynthesis and Diffusion Conductance of the Valencia Orange Fruit under Field Conditions

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Abstract
CO2 uptake and diffusion conductance of Valencia orange fruits (Citrus sinensis L. Osbeck) were measured in the field during the growing season of 1977/78 to ascertain if, as in the leaf, stomata control photosynthesis and transpiration under changing environmental conditions. Measurements were made on 15 year old trees grown in a sandy loam soil and receiving either a dry or a wet treatment. Fruit diffusive conductance was measured with a modified water vapour diffusion conductance meter and gross photosynthesis was measured with a 14CO2 uptake meter. Photosynthetically active radiation (PAR) was measured with a quantum sensor. Fruits exposed to light assimilated CO2 at a rate which was 25–50% of that assimilated by leaves. The uptake was dependent on fruit size, PAR, chlorophyll content, and on diffusive conductance of the fruit epidermis. Epidermal conductance showed a diurnal trend which was similar in shape to that of the leaf except in the late afternoon. Cuticular conductance of the fruit was calculated and ranged between 0.22 and 0.30 mm s−1. It was speculated that the CO2 uptake by the fruit could support the growth of flavedo cell layers when exposed to light. Dry soil caused an increase in the 14CO2 uptake by fruit possibly caused by the increased potential area of the stomatal opening per unit of fruit surface area.