During ripening, citrus fruit-peel undergoes 'colour break', a process characterized by the conversion of chloroplast to chromoplast. The process involves the progressive loss of chlorophylls and the gain of carotenoids, changing peel colour from green to orange. In the present work, the in vivo and in vitro effects of supplemented nutrients (sucrose and nitrogen) and phytohormones (gibberellins [GA] and ethylene) on colour change in fruit epicarp of Satsuma mandarin (Citrus unshiu (Mak.) Marc., cv. Okitsu), were studied. The rate of colour break was correlated positively with sucrose content and negatively with nitrogen content. The removal of leaves blocked natural sucrose build-up and nitrogen reduction in the peel. Defoliation also inhibited chlorophyll disappearance and carotenoid accumulation, thereby preventing colour break. In vivo sucrose supplementation promoted sucrose accumulation and advanced colour break. In both in vivo and in vitro experiments, colour change promoted by sucrose was unaffected by ethylene but delayed by GA3. In non-supplemented plants, ethylene accelerated colour break while GA3 had no detectable effects. Ethylene inhibitors effectively counteracted the sucrose effects on colour change. Collectively, these results suggest that the chloroplast to chromoplast conversion in citrus fruit epicarps is stimulated by sucrose accumulation. The sugar regulation appears to operate via ethylene, whereas GA may act as a repressor of the sucrose-ethylene stimulation.

In citrus, the relative contributions of chloride and cations to growth disturbances induced by salinity are a matter of controversy. Chloride salts (15 mol m -3 CaCl 2 , 30 mol m -3 CaCl and 30 mol m -3 KCl) reduced growth and gas exchange parameters, increased leaf damage and abscission and produced anatomical disarrangements and mineral imbalances in seedlings of sensitive Carrizo citrange (Citrus sinensis x Poncirus trifoliata) and tolerant Cleopatra mandarin (Citrus reshni). In both cultivars, Ca 2+ was more beneficial, and K + more detrimental, for growth than sodium. Photosynthesis and growth disturbances were highly correlated (P ≤ 0·001) with leaf Clbuild-up. In the sensitive genotype, Cl -was also significantly correlated with several leaf anatomical disarrangements, such as increase in succulence. In comparison with sodium, both calcium and potassium increased leaf Clcontent (up to 25 and 69%, respectively). Protective calcium effects were not linked to improvement of photosynthesis, reduction of leaf anatomical disarrangements, or prevention of Cl -and Na + increases. It is proposed that the ameliorative effects of calcium on citrus grown under salinity are mostly related to reduction of leaf abscission. Collectively, the data suggest a cause-effect relationship between Cl -build-up and reduced growth, whereas chloride correlations with declines in photosynthesis or increases in succulence appear to be indirect.

No abstract

In short-term water culture experiments with different ~SN labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH 4 at a higher rate than NO~. Maximum NO 3 uptake by the whole plant occurred at 120 mg L 1 NO3_N, whereas NH 4 absorption was saturated at 240 mg L -1 NH4_N. 15NI-1 + l,ii 4 accumulated in roots and to a lesser degree in both leaves and stems. However, ~5NO-3 was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH 4 (15-60 mg L ~ N) to nutrient solutions containing 120 mg L-~ N as 15N labeled nitrate reduced ~5NO3 uptake. Maximum inhibition of ~sNO 3 uptake was about 55% at 2.14 mm NH 4 (30mg L -~ NH4-N ) and it did not increase any further at higher NH 4 proportions.In a long-term experiment, the effects of concentration and source of added N (NO~ or NH4) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH 4 versus NO 3 nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO3-N:NH4-N ratios (100:0, 75:25, 50:50, 25:75 and 0" 100) at 120 mg L -~ total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO 3 or NH 4 as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO3-N/NH4-N ratio. With increasing proportions of N + H 4 in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn N + decreased. Yield in number of fruits per tree was increased significantly by supplying all N as H4, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NOf-N:NH4-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH 4 in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO3"NH ~ ratio.

The absorption and distribution of N was measured monthly throughout a calendar year in 3-year old peach trees (Prunus persica (L) c.v. Maycrest) grafted on Nemaguard rootstock. Plants were grown on siliceous sand in 500-L pots and fertilized with a solution containing 15N enriched KNO 3. During flowering and fruit set (March) approximately 7% of N found in new growth came from the fertilizer and the remainder came from the N stored in the old organs. Maximum N absorption took place during the periods of fruit ripening and maximal vegetative growth (May to August). This nitrogen was relocated from leaves to woody tissues and stored as reserve-N before leaf fall. In the following growth season reserve-N was used for flower development and new shoot growth. The N absorbed during plant dormancy was quite low and remained in the stem bark and roots mainly as soluble-N.