Root respiration has important implications for understanding plant growth as well as terrestrial carbon flux with a changing climate. Although soil temperature and soil moisture often interact, rarely have these interactions on root respiration been studied. This report is on the individual and combined effects of soil moisture and temperature on respiratory responses of single branch roots of 1-year-old Concord grape (Vitis labruscana Bailey) vines grown in a greenhouse. Under moist soil conditions, root respiration increased exponentially to short-term (1 h) increases in temperature between 10 degrees C and 33 degrees C. Negligible increases in root respiration occurred between 33 degrees C and 38 degrees C. By contrast to a slowly decreasing Q(10) from short-term temperature increases, when roots were exposed to constant temperatures for 3 d, the respiratory Q(10) between 10 degrees C and 30 degrees C diminished steeply with an increase in temperature. Above 30 degrees C, respiration declined with an increase in temperature. Membrane leakage was 89-98% higher and nitrogen concentration was about 18% lower for roots exposed to 35 degrees C for 3 d than for those exposed to 25 degrees C and 15 degrees C. There was a strong interaction of respiration with a combination of elevated temperature and soil drying. At low soil temperatures (10 degrees C), respiration was little influenced by soil drying, while at moderate to high temperatures (20 degrees C and 30 degrees C), respiration exhibited rapid declines with decreases in soil moisture. Roots exposed to drying soil also exhibited increased membrane leakage and reduced N. These findings of acclimation of root respiration are important to modelling respiration under different moisture and temperature regimes.

Below-ground carbon allocation represents a substantial fraction of net photosynthesis in plants, yet timing of below-ground allocation and endogenous and exogenous factors controlling it are poorly understood.Minirhizotron techniques were used to examine root populations of Vitis labruscana Bailey cv. Concord under two levels of dormant-season canopy removal and irrigation. Root production, pigmentation, death and disappearance to a depth of 110 cm were determined over two wet and two dry years (1997-2000).There was continual root production and senescence, with peak root production rates occurring by midseason. Later in the season, when reproductive demands for carbon were highest and physical conditions limiting, few roots were produced, especially in dry years in nonirrigated vines. Root production under minimal canopy pruning was generally greater and occurred several weeks earlier than root production under heavy pruning, corresponding to earlier canopy development. Initial root production occurred in shallow soils, likely due to temperatures at shallow depths being warmer early in the season.Our study showed intricate relationships between internal carbon demands and environmental conditions regulating root allocation.

Temperature is a main environmental factor affecting the net carbon exchange (NCE), carbon balance and carbon partitioning in the apple tree. Although shoot and fruit growth and gas exchange have been correlated to temperature in several studies, this research project began with the aim of documenting simultaneously, the effects of temperature on growth early and during the season. Groups of potted 'Royal Empire' apple trees were put in growth chambers every week for a 6-day period for 4 consecutive weeks starting 15 days after full bloom (DAFB) when the average fruit size was about 6 mm in diameter. Fruit growth and shoot growth were measured four times: -3, 0, 3 and 6 days after starting treatments. The day/night temperature regimes were 12/7, 19/14, 26/21 and 33/28degreesC. Relative humidity and light were set to 80 % +/-15 and 14-hour daylength of about 650 mumol PAR m(-2) s(-1) at the top of the trees, respectively. The results indicate that fruit growth is highly sensitive to temperature early in the season, but significantly less responsive later. The highest temperature regimes promoted the highest fruit growth during the first week of study, but later the treatment of cool temperatures (19/14degreesC) induced the highest fruit growth during the next 2 weeks, and, finally, in the last week, when the initial fruit size was about 27 mm (36 DAFB), fruit growth was relatively insensitive to temperature. Shoot growth was higher with the cool temperatures 19/14 and 26/21degreesC. The highest temperature (33/28degreesC) seems to be detrimental for fruit and shoot growth later than 3 weeks after full bloom. It is not clear whether the impact of high air temperature on potted trees was related to temperature only, to temperature-induced water stress and/or high soil temperature.

Field chemical thinning studies were conducted in 2000, 2001 and 2002 with 6 to 8-year old 'Gala' and 'McIntosh' trees on M.9 rootstock and 'Delicious' apple trees on M.26 rootstock. Single application sprays of a tank mix of 75 mg l(-)1 of 6-benzyladenine (BA) (formulation VBC-30001) plus 600 mg l(-1) of Carbaryl (formulation Sevin XLR Plus), or 1 7.5 mg l(-1) of Naphthaleneacetic Acid (NAA) (formulation Fruitone N) plus 600 mg l(-1) of Carbaryl were applied at 3 or 4 day intervals beginning at petal fall until 28 days after petal fall (DAPF). In all three years, a prolonged cool period following petal fall resulted in slow fruit growth and poor thinning during the first 10 DAPF. Thinning effectiveness was poor when fruits were smaller than 10 mm and improved when fruits were relatively large between 12 and 20 mm. In 2000, the best thinning efficacy coincided with a warm period, but in 2001 and 2002 the best thinning efficacy coincided with one of the coolest periods during the experiment. Weather data from each year was used to calculate daily carbon supply by the tree and demand by the shoots using a carbon fixation model. The difference between estimated carbon supply and estimated demand by the shoots was an estimate of the carbon available to support the growing fruits. Comparisons of the estimated carbon available for fruit growth with the curves of thinning efficacy showed that in the first 10 DAPF, when thinning response was poor, estimated carbon supply to the fruits appeared to be adequate (non-limiting) for fruit growth despite the cool temperatures. Although total estimated carbon supply was reduced due to the cool temperatures, estimated shoot demand was also reduced resulting in an adequate supply to the fruits. From 10-20 DAPF was a period when estimated carbon supply for the fruits was less than adequate due to higher estimated demand by the shoots. This coincided with the timing of the best thinning response. A possible explanation for the thinning efficacy responses is that a deficit in carbon supply supporting young fruitlets growth is a necessary pre-requisite for the fruit thinning chemicals to induce fruit drop in the developing fruitlets. In the 3 years of our study, the best thinning and the greatest deficits in carbon supply both occurred once fruit size was greater than 10 mm. These results may indicate that in cool years it is better to wait for relatively large fruit sizes before fruit thinners are applied.

For irrigation design and scheduling, water use of crops is commonly estimated from grass reference evapo-transpiration (ETo) multiplied by published crop coefficients (Kc). This method is assumed to adjust crop values in different climates. However, the simple application of Kc may not be accurate in cool, humid climate, especially for tall crops well coupled to bulk air. The aim of this work was to measure actual transpiration in an apple orchard in New York, and to test the values against published Kc values. Measurements of water use in dwarf apple trees were made with heat pulse sap flow gauges calibrated with whole-tree gas exchange chambers (to check or correct potentials errors related to the velocity-to-flow assumptions). Daily ETo was estimated from meteorological data acquired nearby. Published Kc values generally overestimated the measured water use rates, suggesting the inadequacy of using Kc values directly from arid climates in our cool and humid climate. Also, the crop coefficient concept implies that grass and orchards behave the same in all climates; this does not seem to be true in cool humid climates. Grass-to-tree differences in advective flows, boundary layer conditions and stomatal regulation me be the reason for variation in ET ratios. Modifications of the Penman-Montieth equation were made to take into account the specific characteristics of apple tree canopy (like light interception and coupling to the bulk air), and stomata regulation by light and VPD. Results are in good agreement with field measures.

This paper focuses on the processes occurring in the fruit between the stages of fertilization and harvest, with particular emphasis on some physiological, energetic and modelling aspects affecting the growth of this organ. As fruits transition from flower to actively growing fruitlets, cell division sets the basis for final fruit size, which is the result of the product cell number x cell volume. If cell volume is relatively constant, a fruit with more cells should be larger at harvest. The importance of the cell division phase has been confirmed in apples, which show some degree of correlation between fruit growth during this early stage and final fruit size. Fruit growth rates have in turn been correlated to daily temperature regimes: warmer temperatures can induce faster growth in the initial stages, and can thus result in larger fruit at harvest. Cell volumes may also vary in fruits adding variation to final size. The interplay of length of season and temperature regimes needs to be integrated with other factors that influence fruit development; in particular the type of leaves that support fruit growth, light microclimate, crop load, vegetative growth and changes in the fruit anatomy/physiology. Along with endogenous hormones, nutrients and water relations involved in the control of fruit growth, the plant carbon balance plays a major role. The energy required for fruit growth can be expressed by its cost of production per gram of fruit, which is often fairly constant during most of the growing season. The cost of production of organs provides a useful expression for comparison with photosynthetic energy production. In conclusion, there are many factors capable of influencing the growth of the fruit, and also the quality attributes that the fruit attains after harvest. Knowledge and integration of these factors is important if the goal is to produce large amounts of top quality fruit.

Summer pruning increases canopy light penetration and re-exposes spur leaves of the interior canopy of apple trees ( Malus × domestica Borkh .). However, we hypothesized that leaf photosynthetic ability is determined by the pre-pruning light environment, and the re-exposure intensity after summer pruning is incapable of restoring the photosynthesis efficiency of shaded leaves. To test this hypothesis, a commercial-type thinning-cuts pruning was applied to mature central leader `Empire'/M.26 apple trees. Changes in light availability, leaf net photosynthesis (Pn), photosystem II efficiency, and specific leaf weight (SLW) were recorded periodically before and after pruning. Leaf photosynthesis declined slightly through the growing season and was well correlated with pre-pruning light availability until late September. Although Pn decreased more substantially late in the season on exterior leaves than on interior leaves, Pn of leaves in the inner and middle canopies was lower than exterior leaves until late October. Maximum efficiency of photosystem II of dark-adapted leaves, measured by chlorophyll fluorescence (Fv/Fm), was not related to prior exposure or re-exposure. Specific leaf weight was well correlated with pre-pruning light availability and with leaf Pn in August but not in October. Results suggested that commercial summer pruning significantly increases light environments in the inner and middle canopies. However, light availability at interior and middle canopy sites was still much lower than exterior canopy and, consequently, leaf photosynthetic ability did not increase after summer pruning.

Summary While many undesirable effects of summer pruning on apple (Malus domestica Borkh.) growth and development have been reported, the results are inconsistent and difficult to interpret. This study resolves the inconsistency by supplying a model that integrates pruning effects with tree physiological crop load, i.e. canopy net carbohydrate exchange rate per fruit. Our previous study suggests that the potential impact of reducing canopy photosynthesis after summer pruning depends on the balance of carbohydrate supply and demand. To test the hypothesis that summer pruning affects carbohydrate balance, we measured fruit growth, fruit quality, return bloom, and root growth in 20 year old slender spindle ‘Empire’/M.9 apple trees in response to different severities of summer pruning. Results were interpreted in relation to pruning severity, fruit number per tree, and the physiological crop load. Within commercial cropping ranges, light and moderate summer pruning had slight influences on fruit size and fresh weight. Summer pruning did not affect fruit colour, soluble solids content, starch, firmness, and internal breakdown after storage. Summer pruning alone did not affect return bloom or root growth. However, the potential negative effects of summer pruning on fruit growth, return bloom, and fine root survival can be predicted through their relationships with physiological crop load. This study suggests that the carbohydrate supply and demand balance model feasibly explains summer pruning influences. In addition, the impact of carbohydrate shortage after summer pruning is likely to be mediated by the reduction in canopy transpiration. However, the interaction between canopy carbon balance and water status after summer pruning is also likely to be manipulated by annual weather pattern.

• Minirhizotron techniques were used to examine root lifespan in Vitis labruscana (Concord grape) for roots born in four different years that varied in rainfall. • Experimental vines were given irrigation (irrigated or not) and canopy pruning treatments (minimal or balanced). Root survival was assessed from 1997 through 2000 and analysed using Cox proportional hazards regression. Model covariates included pruning, irrigation, vine yield, soil depth, root diameter, time of root birth, and numbers of neighboring roots. • Soil depth, root diameter and time of birth consistently influenced root lifespan in all years (P < 0.05). Deeper and coarser roots had longer lifespans. Roots born near bloom were shorter-lived than roots born later in the season. Pruning and irrigation influenced root lifespan in some years but their effects seemed to vary with growing-season environmental conditions. • These data underscore the value of long-term studies in distinguishing factors that consistently affect root lifespan from those that change annually with environmental conditions, and emphasize the diversity in life histories of fine roots within a species.

Summary Canopy size control is one of the major purposes of summer pruning. However, reducing canopy size might also result in less light interception, consequently decreasing canopy photosynthetic efficiency and carbohydrate production, which might lead to the imbalance of carbohydrate supply and fruit demand. To document the effectiveness of summer pruning on canopy control and the impact on canopy gas exchange, pruning treatments at four levels of intensity (unpruned, light, moderate, and severe) were carried out on mature ‘Empire’/M.9 slender spindle apple trees (Malus domestica Borkh.) on 30 July 1998 and 4 August 1999. Changes in canopy leaf area after summer pruning were estimated. Canopy net carbon exchange rate (NCER) and canopy transpiration before and after summer pruning were monitored. Canopy growth was suppressed by summer pruning and the post-pruning regrowth was insignificant. Canopy NCER was reduced in proportion to the amount of leaf area removed by summer pruning. The result suggests that commercial pruning intensity similar to the moderate to severe treatments in this study could cause a significant reduction in canopy NCER and carbohydrate production. In addition, canopy transpiration was reduced in proportion to pruning intensity. Lower water consumption and improved water status during the growing season after summer pruning might benefit fruit growth and relieve the potential detriment due to carbohydrate shortage.

A basic tenet of integrated pest management (IPM) is that many crop plants can tolerate some threshold level of leaf injury from foliar pests before the crop is affected. In practice, foliar pests are monitored, and pesticides are used only when pests reach a density that is thought to cause too much damage. Sound monitoring methods for many pests have been developed, but our understanding of plant responses to pests is weak. A problem is that pest thresholds are usually used independently for each pest, and do not take into account other factors that affect the crop. We propose that in apple trees; (1) foliar pest injury primarily reduces leaf carbohydrate production and effects of multiple pests are additive, and (2) the effects of foliar pests and other factors on the crop can be integrated by the balance of the tree's carbohydrate supply from the leaves to the demands for crop and tree growth. A dry matter production model developed by Lakso and Johnson in 1990 and subsequently expanded was tested as an integrator of the effects of pests, environmental variations, and cultural practices (initially crop load adjustments) on fruit development. The model simulated the measured effects of European red mites (Panonychus ulmi Koch) on apple fruit growth at different crop levels. Simulations suggest that a threshold response of fruit growth to foliar pests occurs with light crop load, but may not at high crop loads. This is in agreement with empirical studies of pest stress interactions with tree crop load. The model provides a plant-based integration of multiple foliar pests, environment and cultural practices of apples.

Effects of short- and long-term climatic variation and foliar pests were modeled with an improved version of a simplified apple tree carbon balance model (Lakso and Johnson, Acta Hort. 276:141, 1990). New and improved submodels were developed on (a) spring leaf area development and autumn leaf fall, (b) partitioning of carbon to the organs of the tree, and (3) fruit growth and abscission based on carbon balance. Simulations of known effects of low light and foliar pest damage on apple fruit growth and abscission were realistic and similar to experimental results. Similarly, long-term simulations were run of leaf area development, light interception, canopy photosynthesis and canopy respiration in different climates using temperature and radiation data from New York, New Zealand and Washington State. Simulations suggest that mid-to-late-season differences were the most distinguishing among climates, and that early season differences were less when expressed as time after budbreak. Total canopy photosynthesis over the season was estimated to be approximately 18, 23 and 25 kg fixed CO2 for a mature slender spindle tree at 2000 trees/ha using NY, WA and NZ weather, respectively. Seasonal canopy respiration was simulated to be about 13-15% of the fixed carbon for all climates. Simulations of short-term responses to low light and foliar pests and the long-term climatic simulations suggest that the model in general is behaving realistically.

During several years of hand and chemical apple thinning trials, individual fruit diameter was monitored on many apple fruits to examine the immediate and season-long effects on fruit growth and abscission as affected by different methods (hand versus several chemicals) and timing of fruit thinning at different times after bloom. A general response was found between fruit abscission and reduced fruit growth rates (FGR) after shade and chemical treatments that cause fruit abscission. Among chemical thinners the most severe reductions in growth rates of the fruits ultimately retained to harvest occurred with Naphthaleneacetic acid (NAA) treatments applied at 15 days after bloom (DAB) with fruits about 10-15 mm diameter. Increases in final fruit weights in relation to crop reduction were greatest for hand thinning up to 20 DAB and for bloom or 5 DAB. NAA, benzyladenine and carbaryl at 15 DAB all inhibited fruit growth too much to allow maximum response to crop reduction. Fruit diameters did not recover to equal that of the non-thinned fruits for about 30 days. Benzyladenine and carbaryl also caused reductions in FGR of retained fruits but the reductions were less severe than with NAA and fruits from these treatments recovered to equal that of the non-thinned fruits by about 20 days.

A 2-year field study of `Mutsu' apple [ Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] on `Malling 9' (M.9) rootstock was conducted to observe root growth in situ, and compare patterns of root growth, root maturation and turnover rates, and soil-root respiration. Rhizosphere respiration was monitored with a portable chamber connected to an infrared gas analyzer; root emergence, browning, and turnover rates were measured by direct observation through minirhizotron tubes inserted in the root zone. Negligible root growth was observed before the onset of shoot growth in mid-May. In both years, a main peak of new root emergence in late June and early July coincided partially with major phases of shoot and fruit growth. A smaller peak of root emergence during August to September 1997 consisted primarily of new roots at 20 to 45 cm soil depths. Most roots remained <1 mm in diameter and developed in the upper 25 cm soil profile; no roots were observed at any time below 50 cm, due to a compacted soil layer at that depth. The cumulative survivorship of new roots was 38% in 1996 and 64% in 1997, and 50% of emergent white roots turned brown or senesced within 26 days in 1996 and 19 days in 1997. Root turnover rates were highest in mid-August both years. Rhizosphere respiration was correlated ( r 2 = 0.36 and 0.59, P = 0.01 and 0.004) with soil temperatures in 1996 and 1997, with Q 10 values of 2.3 in both years. The Q 10 for root-dependent respiration (the difference between soil only and combined soil-root respiration) in 1997 was 3.1, indicating that roots were more sensitive than soil microflora to soil temperature. The temporal overlap of high rates of shoot, root and fruit growth from late May to mid-July suggests this is a critical period for resource allocations and competition in temperate zone apple trees.

A series of experiments were conducted on container-grown Pinot Noir and Sangiovese grapevines to investigate whether changes in berry water loss at veraison influence the pattern of sugar accumulation. Berry transpiration was induced to vary either by changing the vapour pressure deficit (VPD) around bunches through temperature or relative humidity (RH) manipulations, or by applying emulsion accelerating drying or a hydrophobic coating (vaseline) over the berry skin. Transpiration rates were derived either from measurements of attached bunches using a custom-built gas-exchange system or from weight loss calculated for single excised berries. Berry development and ripening were monitored throughout each experiment as deformability, fresh weight and sugar concentration and content. Berries either did not respond to VPD-enhancement or showed reduced water loss when bunches were subjected to high temperature. Low berry transpiration in the latter treatment led to lower sugar content per berry up to harvest, and berry transpiration and net sugar intake were linearly correlated up to 0.20-0.25 mmol m-2s-1. When berry transpiration was restricted by applying vaseline, sugar accumulation was retarded. Low values of VPD, induced by raising the RH around the bunches, lowered sugar concentration but not sugar content per berry.

The study evaluated the relationship of spur vs. extension shoot leaf area and light interception to apple ( Malus { XtimesX } domestica Borkh.) orchard productivity. Fifteen-year-old `Marshall McIntosh'/M.9 trees had significantly greater leaf area and percentage of light interception at 3-5 and 10-12 weeks after full bloom (AFB) than did 4-year-old `Jonagold'/Mark trees. Despite significant increases in leaf area and light interception with canopy development, linear relationships between total, spur, and extension shoot canopy leaf area index (LAI) and 1) light interception and 2) fruit yield were similar at both times. Mean total and spur canopy LAI and light interception were significantly and positively correlated with fruit yield; however, extension shoot LAI and light interception were poorly correlated with yield. In another study total, spur and extension shoot canopy light interception varied widely in five apple production systems: 15-year-old central leader `Redchief Delicious' MM.111, 15-year-old central leader `Redchief Delicious' MM.111/M.9, 16-year-old slender spindle `Marshall McIntosh' M.9, 14-year-old `Jerseymac' M.9 on 4-wire trellis, and 17-year-old slender spindle `MacSpur' M.9. Yields in these orchards were curvilinearly related to total and extension shoot canopy light interception and decreased when total light interception exceeded 60% and extension shoot interception exceeded 25%. Fruit yields were linearly and highly correlated ( r 2 = 0.78) with spur light interception. The findings support the hypothesis that fruit yields of healthy apple orchards are better correlated with LAI and light interception by spurs than by extension shoots. The results emphasize the importance of open, well-illuminated, spur-rich tree canopies for high productivity.

Defining root death in studies of root dynamics is problematic because cell death occurs gradually and the resulting effects on root function are not well understood. In this study, metabolic activity of grape roots of different ages was assessed by excised root respiration and tetrazolium chloride reduction. We investigated changes in metabolic activity and patterns of cell death occurring with root age and changes in root pigmentation. Tetrazolium chloride reduction of roots of different ages was strongly correlated to respiration ( R 2 = 0.786). As roots aged, respiration and tetrazolium chloride reduction declined similarly, with minimum metabolic activity reached at six weeks. Tetrazolium chloride reduction indicated that the onset of root browning corresponded to a 77% reduction in metabolic activity ( P < 0.001). Anatomical examination of roots at each pigmentation stage showed that even though some cells in brown roots were still alive, these roots were functionally dead. The effect of using different definitions of root death in relation to root survivorship was determined in a study of ‘Concord’ grapes with two pruning treatments, using three criteria for root death: browning, blackening or shriveling, and disappearance. There was no effect of vine pruning on root life span when life span was defined as the time from first appearance to the onset of browning. However, if death was judged as the point when roots either became black or shriveled or disappeared, vine pruning decreased root life span by 34% and 40%, respectively ( P < 0.001), and also increased the decay constant for root decomposition by about 45% ( P < 0.001). We conclude that the discrepancy among determinations of root life span assessed with different definitions of death might be partly caused by the latter evaluations of root life span incorporating a portion of root decomposition in definitions of root death.

The simplified apple dry matter production model developed by Lakso and Johnson (1990) was modified by inputing tree-specific parameters from a study of seasonal growth and gas exchange of 4-year-old Empire/M.9 apple trees, and light and temperature response curves developed for Empire apple organs. Measurements of the seasonal trend of diurnal net CO2 and canopy water vapor exchange were made at intervals on three four-year-old 'Empire'/M.9 slender spindle apple trees in the orchard from 10 days after bloom until 25 days after harvest. The tree canopies were enclosed in crests plastic "balloon-type" chambers (similar to Corelli and Magnanini, 1993) that was monitored continuously for more than 40 days with an automated control/datalogging system. The measurements over the season under different weather conditions and with late-season reductions in leaf photosynthesis due to pests gave a good range of values with which to test the model. In general, the model simulations showed the same seasonal patterns of gas exchange as the measurements, and gave actual values quite close to those measured. Variation in the canopy gas exchange rates after pest damage were not adequately reflected in the sampled single leaf gas exchange rates that were used as model inputs, suggesting that leaf sampling patterns should be adjusted for non-uniform pest damage.

Historically, Concord (Vitis labruscana) native-American fresh juice grapes have not been irrigated when grown with standard heavy pruning in the cool, humid climate and soils of New York. The advent of minimal pruning of vines has: led ro more rapid canopy development and heavier crops that may need more water than conventionally-pruned vines. An experiment was established with minimal versus conventional pruning with and without supplemental drip irrigation. Differences in light interception were documented showing a much more rapid light interception by the minimally-pruned vines although by mid-season differences were small. In a very dry warm year comparisons were made of vine growth and leaf gas exchange responses over the season. The primary difference in response to drought was that the minimally-pruned vines used more soil water early in the season due to the higher early light interception and therefore depleted the soil water supply earlier than in the conventionally pruned vines. This led to earlier reduction in leaf photosynthesis, reduced berry growth earlier during the cell division period and later and eventual partial defoliation in the drier sites. Numbers of live shoots the following year in the minimally-pruned vines was reduced as was the subsequent crop, while the subsequent crop of conventionally-pruned vines was only slightly reduced.

Apples have very high record yields (about 140 tons/ha sustained) that demand large amounts of carbon to be produced and partitioned into both fruit and vegetative structures. Even though large quantities of dry matter can be produced, profitability depends on the management of the carbon production and partitioning to produce the optimal balance of yield and fruit quality. The productivity is mostly related to moderate photosynthesis rates per leaf area, long leaf area duration, high seasonal radiation interception, relatively low respiration, and very high harvest index. Due to the perennial nature and large size, few good estimates of seasonal carbon balance are available. Models have been developed, but are not wellvalidated yet, but general seasonal trends are apparent. Daily net CO 2 exchange begins negative with early spring growth, reaches zero near bloom, peaks about 6 to 10 weeks after bloom, then gradually declines until leaf fall. The demand of the fruit appears to increase exponentially during cell division, then levels off to a relatively constant demand until harvest. Experiments and modeling suggests that if fruit development is limited by carbon availability, the probability increases in heavily cropping trees, and will occur at about 2 to 4 weeks after bloom and before harvest. Best carbon balance appears to occur in relatively cool temperatures and in very long seasons.