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- Author or Editor: Bruno Quebedeaux x
Abstract
Horticulturists and ASHS Members from both developed and developing countries have had and will continue to have a significant influence on international agricultural development. Horticultural crop production is an extremely important aspect of the total agricultural and rural development in most countries. Horticultural crops, in spite of their importance, have been neglected in many developing countries in favor of agronomic grain crops, livestock, and industrialization. The neglect of fruit, vegetable, and ornamental development is serious in Africa and exists in many Asian and Latin American countries. Several African countries are experiencing a series of droughts that have caused mass starvation and human misery. Horticulturists are concerned with the quality of life and are involved with other scientists and colleagues in implementing freedom-from-hunger strategies in developing countries.
In order to determine whether the changes in the demand for the transported carbohydrates in apple source leaves are associated with specific carbohydrate enzyme changes, we made source–sink manipulation by girdling or defoliation. The girdle was applied to side branches with several fully expanded leaves, and the defoliation was conducted by removing about 90% of source leaves in apple seedlings. 3-year-old apple (Malus domestica Borhk. cv. Gala) seedlings were grown in a 15/9-h light (≈700 μmol photons/m2 per s, 22 °C)/dark (18 °C) in the growth chamber. When the demand for transported carbohydrates from a particular source leaf is limited by girdling, carbohydrates including starch, sorbitol and sucrose accumulated in the source leaves, meanwhile girdling reduced net photosynthetic rates (Pn) dramatically from 12.8 initially to 4.6 μmol CO2/m2 per s over a 7-day period. When the demand for transported carbohydrate in the remaining source leaves was increased by defoliation, all carbohydrate levels decreased while Pn of individual leaves increased from 13.6 initially to a maximum of 19.8 μmol CO2/m2 per s after 7 days. These Pn changes in the carbohydrate depleted and accumulated leaves were due mainly to changes in the photosynthetic capacity as indicated by Pn-Ci curve measurements. The carbohydrate enzyme activities were also dramatically changed during the 7-day experimental period. The activity of aldose-6-phosphate reductase (E.C. 1.1.1.200), an important enzyme in sorbitol biosynthesis, increased significantly from 27.5 to 39.2 μmol/h per g FW in the carbohydrate depleted leaves while it remained unchanged in the girdled leaves, the activity of sucrose-6-phosphate synthase (SPS, E.C. 2.4.1.14), a key enzyme for sucrose biosynthesis, increased from 15.4 to 23.0 μmol/h per g FW in the depleted leaves and declined from 17.4 to 8.2 μmol/h per g FW in the girdled leaves, the activity of fructose 1,6 bisphosphatase (E.C. 3.1.3.11), another key enzyme for sucrose biosynthesis in non-Rosaceae species showed a similar pattern as SPS, ADPglucose-pyrophosphorylase (E.C. 2.7.7.27), a key enzyme for starch biosynthesis, decreased a small amount in the girdled leaves but increased markedly from 42.9 to 56.0 μmol/h per g FW in the depleted leaves. These results indicated the specific roles of the enzymes in the partitioning of carbon between sorbitol, sucrose and starch in apple source leaves.
Low CO2 concentrations ([CO2]) frequently occur in dense crop canopy. To determine plant performance under sub-atmospheric [CO2], young `Gala' apple plants were phytotron-grown at 928 mmole m-2s-1 light intensity. Whole-plant photosynthesis and respiration under [CO2] between 0 and the ambient level (382 to 460 ml 1-1) were measured by monitoring [CO2] of the air entering and coming out of a 38-1 clear plexiglass gas exchange chamber at either 3.4 or 6.2 1 min-1. The chamber seals two plants of up to 77 cm height for long-term experiments. There was a linear relationship between [CO2] and net photosynthesis (Pn), with the R2 being as high as 0.99. The increase of Pn with increased [CO2] was 51% greater for the high air flow than for the low air flow. At the ambient CO2 level Pn at the high flow rate was 49% higher than that at the low flow rate. CO2 compensation points were 57.6 and 58.5 ml 1-1 at the high and low flow rates, respectively. The relationship between [CO2] and dark respiration was linear. Dark respiration decreased by 20% on average as the [CO2] increased from 0 to the ambient level, and it was 11% higher at the high flow rate than at the low flow rate. These results suggest that wind may act to reduce Pn depression in dense crop canopy by both reducing leaf resistance and atmospheric [CO2] gradient outside the boundary layer.
One-year-old `Gala' apple trees which experienced either water stress (WS) or no stress (CK) were exposed to a 60-min pulse of 14CO2. The distributions of newly-fixed 14C-photosynthates and total individual carbohydrates (both labelled and non-labelled) were monitored every 2 or 4 h for a 24-h period. During the 24-h period, half the WS and CK plants received 24-h continuous light and the other half received a 12-h photoperiod (8:00 am to 8:00 PM). WS stimulated the 14C partitioning into sucrose (suc) during the first 2-4 h period while the partitioning into glucose (glu) and fructose (fru) was inhibited in mature leaves. WS significantly inhibited the partitioning of 14C into starch. At the end of the 24-h period, a greater partitioning of 14C into sorbitol (sor) was observed under WS in leaves, stems and roots. WS lowered starch levels in all plant parts and the dark cycle further stimulated starch breakdown. Starch breakdown during the dark cycle resulted in the accumulation of glu and suc but not sor whereas in light sor accumulated with higher sorbitol/starch ratios. Light and energy requirements for sor synthesis and metabolism will be discussed.
Photosynthesis and carbohydrate metabolism in apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] source leaves were monitored during a 7-day period after source-sink manipulations by girdling or partial defoliation treatments. In the girdling treatment, sorbitol, sucrose, glucose, and starch accumulated in leaves, and net photosynthetic rates (Pn) at 350 μL·L-1 CO2 decreased during a 7-day period. Pn measured at 1000 μL·L-1 [CO2] was also decreased but the changes were less. Stomatal conductance and intracellular CO2 concentration decreased markedly in leaves of girdled shoots. When shoots were partially defoliated, starch and glucose concentrations in remaining source leaves declined steadily during the 7-day study period. Sorbitol and sucrose concentrations decreased during the first 2 days after defoliation, then increased the following 5 days. Pn of the remaining leaves measured at ambient and elevated CO2 levels were enhanced markedly. Aldose-6-phosphate reductase activity in source leaves increased markedly from 27.5 to 39.2 μmol·h-1·g-1 fresh weight (FW) after partial defoliation but remained unchanged in leaves after girdling. Selective and maximum sucrose phosphate synthase (SPS) activities increased following partial defoliation and decreased following girdling. ADP-glucose pyrophosphorylase activity remained relatively unchanged in the partial defoliation treatments but increased markedly in the girdled-shoot leaves. These results suggested that girdling-induced photosynthetic inhibition is mainly due to stomatal limitation, however, the photosynthesis enhancement by partial defoliation may be due primarily to acceleration of photosynthetic capacity per se. These studies showed that the metabolism of sorbitol, sucrose and starch, three photosynthetic end products in mature apple leaves, was coordinately regulated in source leaves in response to source-sink manipulations.
Potted `Jonathan' apple trees were subjected to water stress in a greenhouse. Midday leaf water potential (Ψw), osmotic potential (Ψs), and carbohydrates of various plant parts were measured to determine whether active osmotic adjustment occurred and its impact on carbohydrates. Mature leaves had the highest soluble sugar levels (74 mg/ml sap) and the lowest Ψs (-1.85 MPa), followed by young leaves (-1.58), stems (-1.02), and roots (-0.87). Sorbitol (sor) concentrations in young leaves, stems, and roots were 57, 28, and 27% of that in mature leaves. The concentrations of sucrose (suc), glucose, and fructose in young leaves, however, were 27, 35, and 146% higher than that in mature leaves. When Ψw decreased from -1.0 to -3.2 MPa, active adjustments of 0.3 MPa were observed in mature leaves, stems and roots while an adjustment of 1.0 MPa was detected in young leaves. As water stress developed, sor increased and suc and starch decreased in mature leaves and stems. In contrast, the roots had a significant increase in suc but a decrease in sor. Both suc and sor increased in young leaves under water stress. The physiological significance of carbohydrate changes in various tissues under water stress will be discussed.
Studies were conducted to determine whether water stress stimulates the conversion of glucose (glu) to sorbitol (sor) in leaves. 14C-glu or 14C-sor was introduced through the cut stem of detached apple (Malus domestica Borhk.) shoots which previously experienced either water stress or no stress before excision. When shoots were labelled with 14C-glu, the 14C sugars in mature leaves were partitioned as follows: 38% glu, 20% fructose (fru), 24% sucrose (suc), 18% sor in well-watered (CK) shoots and 30% glu, 22% fru, 18% suc, 30% sor in water-stressed (WS) shoots. Water stress enhanced the conversion of 14C-glu to sor but reduced the conversion to suc and starch. When labelled with 14C-sor, the 14C sugars in mature leaves were partitioned as follows: 94% sor, 1% suc, 3% fru, 2% glu in CK shoots and 90% sor, 4% suc, 3% fru, 3% glu in WS shoots. Young expanding leaves and stems had lower 14C-sor, but higher 14C-suc, 14C-glu and 14C-fru levels than mature leaves. Our 14C data support the hypothesis that water stress stimulates the conversion of glu to sor.
Abstract
Parthenocarpy was induced in unfertilized pistillate flowers of cucumber, Cucumis sativus L., by the foliar application at early flowering of 10 to 1000 ppm of 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5,1-a]isoindol-8-one (DPX 1840) a new inhibitor of auxin transport. Treatment of a monoecious cultivar with ethylene or (2-chloroethyl)phosphonic acid (ethephon) increased the pistillate to staminate flower ratio and subsequent application of DPX 1840 increased the no. of fruit which developed parthenocarpically. Ethylene gas was just as effective as ethephon in altering sex expression. The threshold level of ethylene for this response was between 0.06 and 0.16 μl of ethylene per liter of air.