J. L. Guardiola

Photosynthesis down-regulation precedes carbohydrate accumulation under sink limitation in Citrus

Photosynthesis down-regulation due to an imbalance between sources and sinks in Citrus leaves could be mediated by excessive accumulation of carbohydrates. However, there is limited understanding of the physiological role of soluble and insoluble carbohydrates in photosynthesis regulation and the elements triggering the down-regulation process. In this work, the role of non-structural carbohydrates in the regulation of photosynthesis under a broad spectrum of source-sink relationships has been investigated in the Salustiana sweet orange. Soluble sugar and starch accumulation in leaves, induced by girdling experiments, did not induce down-regulation of the photosynthetic rate in the presence of sinks (fruits). The leaf-to-fruit ratio did not modulate photosynthesis but allocation of photoassimilates to the fruits. The lack of strong sink activity led to a decrease in the photosynthetic rate and starch accumulation in leaves. However, photosynthesis down-regulation due to an excess of total soluble sugars or starch was discarded because photosynthesis and stomatal conductance reduction occurred prior to any significant accumulation of these carbohydrates. Gas exchange and fluorescence parameters suggested biochemical limitations to photosynthesis. In addition, the expression of carbon metabolism-related genes was altered within 24 h when strong sinks were removed. Sucrose synthesis and export genes were inhibited, whereas the expression of ADP-glucose pyrophosphorylase was increased to cope with the excess of assimilates. In conclusion, changes in starch and soluble sugar turnover, but not sugar content per se, could provide the signal for photosynthesis regulation. In these conditions, non-stomatal limitations strongly inhibited the photosynthetic rate prior to any significant increase in carbohydrate levels.

Effects of grafting and cytokinin-induced fruit setting on colour and sugar-content traits in glasshouse-grown triploid watermelon.

Summary The effect on fruit quality of grafting and fruit set treatments, was determined in the triploid watermelon (Citrullus lanatus (Thun.) Matsum. and Nakai) cultivar ‘Reina de Corazones’ grown in a heated greenhouse. Grafting the scion onto ‘Shintoza’ (Cucurbita maxima × Cucurbita moschata) rootstock and setting fruit through application of the synthetic cytokinin CPPU [1-(2-chloro-4-pyridyl)-3-phenylurea] retarded the accumulation of sugar and development of the flesh colour of the fruit, compared to fruit from non-grafted, pollinated plants. The effects of grafting and of CPPU application on sugar accumulation were additive. At commercial maturity, the fruit of grafted and/or CPPU-treated plants had lower total soluble solids and sugar concentrations, and lower sucrose to hexose ratios, than fruit from control plants. Fruit from CPPU-treated plants also had a weaker red colour than fruit set by pollination. Despite this loss of quality, fruit from grafted plants, set with CPPU and grown in a heated greenhouse, had a total soluble solids concentration similar to fruit from pollinated, non-grafted plants, grown outdoors.

Effect of gibberellic acid on ripening and peel puffing in ‘Satsuma’ mandarin

Abstract The application of gibberellic acid (GA3) to entire trees of ‘Satsuma’ mandarin (Citrus unshiu Marc.) retarded pigment changes in the fruit and prevented puffiness of the peel. Peak responses for both effects were obtained at the onset of chlorophyll degradation in the peel, before the completion of fruit growth. This application prevented the late peel growth which takes place after the cessation of pulp growth and retarded the loss of juice from the ripe fruit, allowing on-tree storage of the fruit for more than 2 months after commercial ripening. There was no obvious connection between the response to exogenous GA3 and the endogenous gibberellin levels in the peel. The maximal response to GA3 coincided with the maximal contents of endogenous gibberellins, but the application of GA3 retarded the loss of gibberellins in the peel associated with ripening.

The effect of synthetic auxins on fruit growth and anatomical development in ‘Satsuma’ mandarin

Abstract The application of a mixture of the synthetic auxins 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid increased fruit size in ‘Satsuma’ mandarin (Citrus unshiu Marcovitch). Peak response was obtained when the auxin mixture was applied 26 days after flower opening. At that time, the percentage of the fruit cross-sectional area occupied by the peel was maximum, the juice sacs just filled the locular cavity and the rate of expansion of the locular cavities started increasing rapidly. The synthetic auxins had a direct stimulatory effect on fruit growth, and no thinning effect was found at the concentration used (7.5 mg l−1). Differences in weight between the treated and control fruit increased with time until maturity. The growth of both the peel and the pulp segments was stimulated initially to the same extent, but at maturity the treated fruit had a smaller percentage of peel despite its bigger size. Peel puffiness was reduced by the auxin applications.

The influence of explant orientation and contact with the medium on the pathway of shoot regeneration in vitro in epicotyl cuttings of Troyer citrange

The effect of orientation as regards to gravity, and that of contact with the medium of culture, on shoot regeneration at the cut edges of epicotyl explants of Troyer citrange (Citrus sinensis (L.) Osbeck×Poncirus trifoliata (L.) Raf.) have been separated. The shoot regeneration pathway was not affected by the orientation of the explants as regards to gravity, and was determined by explant polarity and the contact with the culture medium. At the apical edge of the explants, the contact with the medium shifted the pathway of shoot regeneration from a direct one to an indirect one, with formation of a callus. This callus formation was cytokinin-dependent, but the change in the pathway of organogenesis was not caused by the increase in cytokinin availability resulting from the contact with the medium. In contact with the media, regeneration at the basal edge of the explants occurred through an indirect pathway after callus formation. No regeneration occurred, at the basal edge, if the contact with the media was prevented. The orientation of the explants as regards to gravity affected shoot formation through the direct pathway of organogenesis. The number of buds differentiated, and that of growing shoots increased when the orientation of the explants departed from the vertical upright position.

Low temperature storage of corms extends the flowering season of saffron (Crocus sativus L.)

Summary Conditions for the cold-storage of saffron crocus (Crocus sativus L.) corms to delay flowering have been characterised. Storage of corms at 2ºC after flower initiation resulted in a time-dependent abortion of those flowers already initiated. The more advanced the stage of flower initiation at the beginning of cold-storage, the faster the rate of flower abortion. Overall, no benefit resulted from cold-storing corms after flower initiation. Corms stored in the cold before flower initiation, formed flowers when incubated after storage at 21º–25ºC. The number and size of flowers formed, and the yield of spice saffron per corm, depended both on the duration and conditions of cold-storage. Storage at freezing temperatures (0º or –1ºC) damaged the corms. Flowering could be induced in corms stored between 0.5º–2ºC. Within this range, temperature had little effect on the subsequent behaviour of corms. Flower number and flower size decreased gradually with increasing duration of cold-storage. This decrease was slower when storage was performed in 1% oxygen than in a normal atmosphere (21% oxygen). Corms lifted after leaf-withering, and stored at 2ºC in 1% oxygen for 70 d, could be forced to flower from early December until the end of January with the same yield of spice saffron as non cold-stored corms.These results complement previous findings on the control of saffron flowering. Taken together, they allow saffron flowering without loss of spice yield from early September to the end of January. Flowering could be further delayed until May by extending the duration of cold-storage, but this delayed flowering resulted in a significant reduction in spice saffron yield.

Bacillus subtilis FZB24® Affects Flower Quantity and Quality of Saffron (Crocus sativus)

The effect of Bacillus subtilis FZB24 on saffron ( Crocus sativus L.) was studied using saffron corms from Spain and the powdered form of B. SUBTILIS FZB24(R). Corms were soaked in water or in B. subtilis FZB24 spore solution for 15 min before sowing. Some corms were further soil drenched with the spore solution 6, 10 or 14 weeks after sowing. Growth and saffron stigma chemical composition were measured. Compared to untreated controls, application of B. subtilis FZB24 significantly increased leaf length, flowers per corm, weight of the first flower stigma, total stigma biomass; microbe addition also significantly decreased the time required for corms to sprout and the number of shoot sprouts. Compared to the controls, picrocrocin, crocetin and safranal compounds were significantly increased when the plants were soil drenched with the spore solution 14 weeks after sowing; in contrast crocin was highest in untreated controls. Results of this study suggest that application of B. subtilis FZB24 may provide some benefit to saffron growers by speeding corm growth (earlier shoot emergence) and increasing stigma biomass yield by 12 %. While some treatment conditions also increased saffron chemical composition, these were generally not the same treatments that simultaneously improved growth yields and thus, more study is required.

The anatomy of the fruit in relation to the propensity of citrus species to split

The anatomy of the fruit has been compared in three prone-to-split mandarin hybrids (Nova, Ellendale and Murcott), several cultivars of clementine mandarin (Fino, Clementina de Nules and Orogrande), and in Owari satsuma mandarin. The fruit of the hybrids is oblate to subglobose and usually presents an open stylar end disrupted by a navel formed by the floral meristem, which is conserved near the apex of the fruit. In the non-prone-to-split clementines as well as in Owari satsuma, the floral axis protrudes inside the style, and the flower meristem is lost through abscission shortly after petal fall. The abaxial side of the carpels fuses with the flower axis forming at the stylar end of the fruit a solid tissue which externally has a small scar at the place of style abscission. Most of the fruits in these cultivars have no navel. In addition, the fruit of clementine is slightly oblate or globose. The relevance of these anatomical characteristics as regards to splitting is supported by the differences between the split and non-split fruits in Nova, the split fruit being more oblate in shape and having a bigger navel than non-split fruit. However, the effect of applied growth regulators on fruit splitting could not be correlated with their effect on the anatomy of the fruit. Additional factors not contemplated in the study also influence fruit splitting.

The use of naphthalene acetic acid as a fruit growth enhancer in Satsuma mandarin: a comparison with the fruit thinning effect

Abstract When applied shortly after the end of the June drop, naphthalene acetic acid (NAA) increases the growth rate of the fruit of Satsuma mandarin (Citrus unshiu Marc.), which results in a bigger fruit size at harvest without any undesirable reduction in yield. In a high-yield regular bearer orchard, the effect of this treatment on the sizing of the fruit is similar to that obtained when NAA is used as a thinning agent at an earlier stage of development of the fruit. The apparently higher response obtained through thinning as determined by the mean weight of the fruit at harvest, is mainly owing to the selective removal of the smallest fruit rather than to a higher stimulation of the growth rate of the remaining fruits. The comparative advantages of fruit thinning vs. the direct enhancement of the fruit growth rate under different conditions are discussed.

Effects of applications of gibberellic acid on late growth, maturation and pigmentation of the Clementine mandarin

Gibberellic acid (GA3) applications were performed periodically from late September to late November on seedless Clementine mandarin (Citrus clementina Hort. ex Tanaka) trees, and their influence on the fruit growth and pigmentation determined. Irrespective of the time of application, GA3 had no effect on the growth of the fruit and its anatomical parts nor on juice content and composition, but early GA3 applications reduced peel thickness at maturation. The pigment changes in the peel were retarded by GA3, and this response was maximal when GA3 was applied during the 10 day interval between the onset of chlorophyll degradation and the onset of carotenoid accumulation.