Changes in water relations induced by CO2 enrichment govern diurnal stem and fruit diameters of Japanese pear

摘要

Previous work showed that CO2 enrichment during fruitlet stages increased the fruit size of Japanese pear (Pyrus sp.) (Soil Sci. Plant Nutr. 45 (1999) 85). The mechanism of acceleration of fruit expansion growth by CO2 enrichment is unknown. We tested fruit expansion under elevated CO, as affected by water supply and photoassimilates supply. Four-year-old Japanese pear (Pyrus pyrifolia Nakai cv. Kosui) trees grafted on Chinese white pear (P. bretschneideri) were exposed to CO2 concentration of 350 +/- 50, 700 +/- 100 and 1400 +/- 200 mul 1(-1) CO2 for 2 days during active expansion fruit growth in controlled environment cabinets. The effect Of CO2 enrichment on fruit diameters in relation to photoassimilate translocation, water relations and fruit growth was examined. The apparent photosynthetic rate (Po) of leaves on spurs and shoots increased linearly with the doubling of the CO2 concentration. CO2 enrichment enhanced the export of photoassimilate from leaves, as evidenced by an increased sorbitol exudation rate in the phloem. Enhanced export was due not to changes in translocation, but to the stimulation of photosynthesis. At 350 mul 1(-1) CO2, fruit diameter expansion showed a diurnal periodicity characterized by slower expansion in the light. This effect was eliminated by CO2 enrichment and the elimination was less pronounced from 700 to 1400 mul 1(-1) than from 350 to 700 mul 1(-1) . At 350 mul 1(-1), CO2 stomatal conductance, high xylem water flow rate and a large decrease of 'xylem and pith' diameter were recorded during the light period. CO2 enrichment decreased stomatal conductance, xylem water flow rate and alleviated shrinkage of 'xylem and pith' diameter. The decrease in stomatal conductance and the alleviation of stem shrinkage were less pronounced from 700 to 1400 mul 1(-1) than from 350 to 700 mul 1(-1). The results provided an indication for two conclusions through which elevated CO2 might affect the plants. High CO2, first, increased photosynthesis and assimilate transport to the fruit is enhanced. More importantly, however, is the effect of high CO2 on water use efficiency and improved water relations Of CO2 concentration of 700 mul 1(-1) and above.