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.
Chlorophyll fluorescence measurements are providing insights into Photosystem II (PSII) quantum efficiency and hence are able to provide a good estimation of carbon assimilation under field conditions. A F2 generation of sibcross seedlings from a cross of `Goldspur' × `Redspur' were selected to identify genetic variations and the relationships among fluorescence parameters, carbon assimilation, and carbon partitioning in apple leaves. Mature leaves from extension shoots were analyzed for chlorophyll fluorescence with a CF-1000 chlorophyll fluorescence measurement system, photosynthetic rate with a LI-6200 portable photosynthesis system, and carbohydrates with a Shimadzu HPLC. Significant variations in leaf chlorophyll fluorescence parameters and photosynthetic rates were found. The ratio of Fv: Fm, an estimation of photochemical efficiency of PSII, decreased from ≈0.90 in June to ≈0.75 in September while the photosynthetic rates decreased from ≈8.5 in June to ≈4.5 μmol·m–2·s–l in September. The relationships between fluorescence parameters, photosynthesis, and carbohydrate partitioning were analyzed and the ratio of sorbitol to sucrose in relation to the efficiency of PSII and NADPH production will be discussed.
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.
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. 18.104.22.168), 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. 22.214.171.124), 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. 126.96.36.199), another key enzyme for sucrose biosynthesis in non-Rosaceae species showed a similar pattern as SPS, ADPglucose-pyrophosphorylase (E.C. 188.8.131.52), 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.
Apple and many other Rosaceae plants translocate sucrose as well as sorbitol. How photosynthates are partitioned between sorbitol and sucrose in the Rosaceae is not understood. This study was designed to examine the effects of elevated air CO2 on partitioning of sorbitol and other soluble sugars in sink and source apple leaves. Young `Gala' apple plants were exposed to the ambient air and 700, 1000, and 1600 μl·liter–1 of CO2 for 8 days under a light intensity of 928 μmol·m–2·s–1 with a 14-h day/10-h night cycle. Sorbitol, sucrose, glucose, and fructose concentration in sink and source leaves were determined by HPLC analysis. In source leaves, sorbitol was significantly increased, while sucrose was decreased as the air CO2 was elevated from 400 to 1600 μl·liter–1. The sorbitol/sucrose ratio varied from 1.31 in air and 2.26 at 1600 μl·liter–1 of CO2. In sink leaves, sorbitol concentration did not vary across the four CO2 levels; however, sucrose was higher at the three super-atmospheric CO2 levels. Our results suggest that increased photosynthesis via elevated CO2 favors photosynthate partitioning into sorbitol rather than sucrose. A mechanism for regulating this partitioning will be discussed.
Parthenocarpy was induced in unfertilized pistillate flowers of cucumbers (Cucumis sativus L.) by the foliar application of N-1-naphthylphthalamic acid (Naptalam). These results substantiate the previous proposed hypothesis that potent auxin transport inhibitors in general are parthenocarpic agents for cucumber.
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.