Seasonal Changes in the Photosynthetic Rate in Apple Trees

Abstract

Seasonal changes in photosynthesis of apple trees (Malus domestica Borkh.) were monitored to examine the effect of source-sink interactions on photosynthesis and photorespiration. Elevated photosynthetic rates were observed during two periods of the growing season and correlated with the fruiting process. The first period of increased photosynthetic rates was during the bloom period, when spur leaves on flowering shoots exhibited up to 25% higher photosynthetic rates than vegetative spur leaves on a leaf area basis. CO2 assimilation rates were also higher in fruiting trees than nonfruiting trees during the period of rapid fruit growth from July to September. Photorespiration, dark respiration, leaf resistance , and transpiration exhibited no seasonal changes which correlated to the presence or absence of fruit. These data represent the first comprehensive examination of the effects of flowering/fruit formation on photosynthesis and photorespiration in perennial plants. Regulation of photosynthesis by fruit growth occurs in many plants, with photosynthetic promotion or reduction depending on sink demand. Other studies, however, have shown no correlation between photosynthetic rates and sink strength (cf 7). In apple (Malus domestica Borkh.), fruiting trees were shown to have reduced leaf weights (25) and leaf areas (1), but more total dry matter than nonfruiting trees (12), suggesting greater photo-synthetic efficiency. In other studies, fixation of '4CO2 was usually greatest in leaves nearest the fruit, although sometimes the second nearest or even fruits further away received much of the labeled assimilates (10). Kazaryan et al. (18) also found the highest photosynthetic activity in leaves attached directly to the base of the apple pedicel and lower photosynthetic rates in leaves of nonfruiting branches of the same age. Wardlaw (29) noted that developing apple fruits always seemed to have priority demand over assimilates from adjacent leaves, whereas flowers and small or mature fruit had no effect on photosynthetic rates and attracted very little assimilates (4). From mid-June, the fruits in apple become strong sinks and may absorb nearly all assimilates from leaves on the same short shoot (spur). This suggests a pronounced fruit effect on photosynthesis during the period of intensive fruit growth (9, 13). while Avery and Moore (3) found fruiting apple trees had lower dark respiration than nonfruiting trees, along with the lower stomatal and mesophyll resistances and increased transpiration rates. Others also recorded lower stomatal resistance (26) and higher transpiration rates (12) in fruiting apple trees. Photorespiration occurs three to five times faster than dark respiration in C3 plants and is a major factor determining net photosynthetic rates. Experiments utilizing shading or partial defoliation of leaves to alter the source-sink ratio have shown that the increase in photosynthesis of the remaining leaves is not due to a decrease in photorespiration. In soybean (Glycine max), where shading 63% of the leaf area caused a 25% increase in photosynthesis and lower stomatal resistance in the uncovered leaves, a proportionate increase in photorespiration was observed (22, 24). Thus, changes in net photosynthesis due to shading were not accounted for by changes in photorespiration. Another study ofpartial shading of soybean leaves showed a 50% increase in both net photosynthesis and photorespiration (28). When the shaded leaves were uncovered, photosynthetic rates in the source leaf dropped. Similar increases in both net photosynthesis and photorespiration were also seen upon partial defoliation of lu-cerne (Medicago sativa) (17). In a preliminary report by Lenz (19), however, photorespiration was lower in fruiting trees of Citrus madurensis. In a subsequent report by Monselise and Lenz (23), photorespiration was similar in apple trees with and without fruit. Previous studies on the effects of sinks on photosynthesis in fruit trees have not been comprehensive and most often consist of a few measurements conducted once during the growing season. In this study, seasonal changes in photosynthesis were followed to gain a better understanding of the influence of fruit on photosynthesis and photorespiration in apple. MATERIALS AND METHODS Branches from the south side of 10-year-old, biennial-bearing "Starkrimson' apple trees (Malus domestica Borkh.) were cut between 8 and 10 AM and brought into the laboratory. Ends were recut under water and the branches transferred to containers with water. The branches were allowed to equilibrate under 1000-w lucalux lamps for 45 to 60 min before the first photosynthetic readings were made. All measurements were taken within 6 h of branch removal and made on individual, attached leaves of vegetative spurs ('vegetative spur leaves') or spurs with fruits ('fruiting spur leaves'). Measurements made in 1982, used in Figures 5 and 7 to supplement 1981 measurements, were carried out on the same block of trees and used the same methods as in 519