James P. Syvertsen

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 (A_c) and stomatal conductance (g_s) 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 (LDM_a) than leaves on fruited trees. Both A_c and g_s 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 LDM_a and lower A_c 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 (T_l) and leaf-to-air vapor pressure difference on leaf responses. On relatively warm days when sunlit T_l > 25 °C, shade increased A_c and g_s, but had no effect on the ratio of internal to ambient CO₂ (C_i/C_a) concentration in leaves, implying that high mesophyll temperatures in sunlit leaves were more important than g_s in limiting A_c. Sunlit leaves were more photoinhibited than shaded leaves on cooler days when T_l < 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 A_c compared with defruited trees, except on warm days when A_c was reduced by high leaf temperatures.

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 μmol m⁻² s⁻¹ and was sufficient to achieve maximum net CO₂ assimilation rates (ACO₂). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2–6 °C above air temperature and 1–4 °C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (g_s) and ACO₂ were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher g_s, ACO₂ and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to ACO₂ was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO₂ partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to ACO₂ in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating ACO₂ of citrus leaves during radiation and high temperature stress.