'Fe-efficiency reactions' are induced in the roots of dicotyledonous plants as a response to Fe deficiency. The role of phloem Fe in the regulation of these reactions was investigated. Iron travels in the phloem of Ricinus communis L. as a complex with an estimated molecular weight of 2400, as determined by gel exclusion chromatography. The complex is predominantly in the ferric form, but because of the presence of reducing compounds in the phloem sap, there must be a fast turnover in situ between ferric and ferrous (k z 1 min-). Iron concentrations in R. communis phloem were determined colorimetrically or after addition of '9Fe to the nutrient solution. The iron content of the phloem in Fe-deficient plants was lower (7 micromolar) than in Fe-sufficient plants (20 micromolar). Administration of Fe-EDTA to leaves of Phaseolus vulgaris L. increased the iron content of the roots within 2 days, and decreased proton extrusion and ferric chelate reduction. The increase in iron content of the roots was about the same as the difference between iron contents of roots grown on two iron levels with a concomitantly different expression of Fe-efficiency reactions. We conclude that the iron content of the leaves is reflected by the iron content of the phloem sap, and that the capacity of the phloem to carry iron to the roots is sufficient to influence the development of Feefficiency reactions. This does not preclude other ways for the shoot to influence these reactions.
The effect of supplemental UV‐A (320–400 nm) radiation on tissue absorption at 355 nm, levels of various antioxidants (ascorbate, glutathione, carotenoids and flavonoids) and of antioxidant scavenging capacity were investigated with leaves and petals of Rosa hybrida, cv. Honesty and with leaves, petals and sepals of Fuchsia hybrida, cv. Dollarprinzessin. Supplemental UV‐A did not result in visible changes in plant morphology of either species. In leaves it induced small increases in levels of chlorophylls a and b, the carotenoids antheraxanthin, lutein and β‐carotene, and high increases in the flavonols quercetin and kaempferol. Petals hardly responded, while the coloured sepals of fuchsia showed an increase in quercetin derivatives. HPLC of unhydrolysed flavonoids showed that individual quercetin derivatives in leaves of both species and kaempferol derivatives in rose leaves increased 2‐fold. Some kaempferol derivatives in fuchsia leaves were more than 2‐fold enhanced or were newly induced by supplemental UV‐A. Increases in l‐ascorbic acid levels in fuchsia leaves, and decreases in rose leaves as result of supplemental UV‐A were observed, but differences appeared statistically not significant, while l‐ascorbate levels remained unchanged in the other tissues investigated. Anthocyanins and reduced glutathione levels were unaffected in all tissues. The combined UV‐A induced increases in concentrations of these antioxidant species, did not lead to significant increases in antioxidant capacity of tissues, measured as Trolox equivalents in 50%‐ethanol extracts. Light absorption at 355 nm of leaf extracts was significantly increased upon UV‐A exposure. Our results indicate that the major protection towards UV‐A exposure, in particular in the leaves, will originate from absorption of irradiation, and not from scavenging reactive oxygen species.
Short-term fumigation of Spinacia oleracca with 380/ig m"'' H2S (250 ppb) resulted in a rapid accumulation of water-soluble SH-compounds in the shoots. After 1 h exposure a substantial inctease in the SH-content was already detectable and maxitnal accumulation, three-to four-fold that in control plants, was observed after 24 h of exposure. Irradiation during H2S exposure only slightly affected the rate and level of SH-accumulation. H2S fumigation did not affect the water-soluble SHcontent of the roots. Glutathione was the sole watersoluble SH-cornpound accutiiulating upon exposure to H2S. It was calculated that during the first hour of exposure to 380/tg m~-^ H2S 39% of the possible absorbed H2S was converted into glutathione. The SH-content of the water-soluble proteins of the shoots was not affected by H2S exposure. When futnigation was stopped, a rapid decrease in glutathione content was observed and after 48 h the content was comparable to that of the control plants. Contrary to H2S, SO2 fumigation did not result in a rapid accumulation of glutathione in spinach shoots. The possible role of glutathione accumulation during HjS fumigation is discussed.
High density planting systems are a prerequisite to economise the use of land and labour costs of orchards. Dwarfing rootstocks controlling the vigour of the scion cultivars form the basis for such orchards (Wertheim and Webster, 2005). In the Netherlands, rootstock research is limited to and focussed on testing rootstocks selected abroad. For the Dutch pear growers the main selection criteria for new rootstocks are: 1) control of tree size; 2) production; 3) fruit size; 4) fruit quality; 5) production efficiency; 6) frost resistance. Additional criteria for Dutch fruit tree nurseries exporting trees to other countries are: 1) compatibility with scion cultivars; 2) suitability for growth in calcareous soils; 3) easy propagation. In all trials rootstock performance is compared to Quince MC, the most commonly used rootstocks for pears in the Netherlands. Recently, a number of Pyrus (Pyrus communis) and Quince (Cydonia oblonga) rootstocks have been tested with 'Conference' and 'Doyenné du Comice' as the scion cultivars. Generally, the production efficiency of the Pyrus rootstocks was much less than for Quince MC. Another disadvantage of the evaluated Pyrus rootstocks was their high sensitivity towards pear decline. Several rootstocks were rejected after examination of the graft union because of suspected compatibility problems. Of the tested Quince rootstocks C 132 shows promise because of its control of tree growth in combination with good fruit size and Eline ® because of its reduction of fruit russeting in 'Conference'. INTRODUCTIONHigh density planting systems are the starting point of modern orchards. Small trees that come into production in the second year after planting are a prerequisite to achieve regular yields of high quality fruits and to economise the use of land and labour costs for pruning and picking. Dwarfing rootstocks, controlling the vigour of the scion cultivars and inducing precocity of cropping, form the basis for such high density orchards.European pears are predominantly grown on rootstocks of Quince (Cydonia oblonga). In the Netherlands, the majority of pears are grown on Quince MC, but Quince MA and Quince Adams are also used. 'Conference' is the most important cultivar. Grown in high density planting systems yearly yields of 60 to 70 tons/ha are feasible. To obtain this production level a good control of shoot growth, flower bud development and fruit set is required.Although Quince MC has been used successfully for many decades in the Netherlands, there are several reasons to look for alternative rootstocks. Until 2001, the growth retardant chlormequat (CCC) was used the reduce shoot growth and stimulate flower bud development. The withdrawal of CCC for use by fruit growers has renewed the interest in rootstocks more dwarfing than Quince MC. Besides dwarfing of the scion cultivar other desired traits of new rootstocks for pears are: 1) precocity of cropping, to ensure early yields starting in the second year after planting; 2) fruit size, for 'Conference' pears the proportion of ...
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