Spring Flush Research Articles

Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under the following conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) the decapitation was in the previous season's overwintered wood, and (3) the point of decapitation was below the upper large irrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, the terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there is evidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-order branching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in the three species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual in promoting bud growth.

The Citrus Leafminer Phyllocnistis citrella (Lepidoptera: Gracillariidae)in South Texas: Incidence and Parasitism

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), was first reported in the Lower Rio Grande Valley of Texas in August, 1994. We surveyed about 40 orchards in 1995 and 20 in 1996. Percentage of leaf infestation by the leafminer was lowest on the spring flush, and increased significantly in the early summer (May-July) and late summer flushes (Aug.-Oct.) through to late fall (Nov.-Dec.). Numbers of citrus leafminer immatures usually ranged from 0-6.8 per leaf. Several native parasite species were identified from the surveys, including 9 species of parasites from 3 families, Eulophidae, Proctotrupidae and Ceraphronidae. The most abundant native parasitoid was Zagrammosoma multilineatum (Ashmead) (Eulophidae). Less dominant parasitoids were the eulophids Horismenus sp., Closterocerus sp., Neochrysocharis sp., Pnigalio sp., and Tetrastichus sp. Percentage parasitism by native parasitoids usually ranged from 5-10%. The exotic parasitoid Ageniaspis citricola Logvinoskaya (Encyrtidae) was released in February-April 1995 and August-October 1996.

Seasonal Reproductive and Vegetative Growth Patterns and Resource Allocation during Cactus Pear Fruit Growth

Flowers and stems (cladodes) of cactus pear [Opuntia ficus-indica (L.) Mill.] appear simultaneously in spring, and a second vegetative and reproductive flush can be obtained in early summer by completely removing flowers and cladodes of the spring flush at bloom time. The seasonal growth patterns of cactus pear fruits and cladodes were examined in terms of dry-weight accumulation and cladode extension (surface area) to determine if cladodes are competitive sinks during fruit development. Thermal time was calculated in terms of growing degree hours (GDH) accumulated from bud burst until fruit harvest. Fruits of the spring flush had a 25% lower dry weight and a shorter development period than the summer flush fruits, and, particularly, a shorter duration and a lower growth rate at the stage when most of the core development occurred. The duration of the fruit development period was better explained in terms of thermal rather than chronological time. The number of days required to reach commercial harvest maturity changed with the time of bud burst, but the thermal time (40 × 103 GDH) did not. Newly developing cladodes may become competitive sinks for resource allocation during most of fruit growth, as indicated by the cladode's higher absolute growth rate, and the fruit had the highest growth rate during the final swell of the core, corresponding to a consistent reduction in cladode growth rate. Cladode surface area extension in the first flush ceased at the time of summer fruit harvest (20 Aug.), while cladodes continued to increase in dry weight and thickness until the end of the growing season (November), and, eventually, during winter. The growth of fruit and cladodes of the summer flush occurred simultaneously over the course of the season; the cladodes had a similar surface area and a lower (25%) dry-weight accumulation and thickness than did first flush cladodes. The proportion of annual aboveground dry matter allocated to the fruits was 35% for the spring flush and 46% for the summer flush, being similar to harvest increment values reported for other fruit crops, such as peach [Prunus persica (L.) Batsch.]. Summer cladode pruning and fruit thinning should be accomplished early in the season to avoid resource-limited growth conditions that could reduce fruit and cladode growth potential.

Genetic variation and productivity of Populus trichocarpa and its hybrids. X. Trait correlations in young black cottonwood from four river valleys in Washington

A common-garden study of Populus trichocarpa Torr. & Gray was established in spring 1986 with 128 clones collected from sites along two mesic (Hoh and Nisqually) and two xeric (Dungeness and Yakima) river valleys in Washington. Two replicate plantations, one in Puyallup and the other in Wenatchee, Wash., were established with this material. Over 2 years data were taken on stem growth, leaf/crown characters, spring/autumn phenology, and the incidence of Melampsora occidentalis leaf rust. Combining clones from all four sources, correlation/regression analyses were used to examine clonal stability of traits between test sites and trait relationships with stem growth; broad-sense heritabilities (H 2) and genetic correlations revealed the genetic strength of these traits. At Puyallup, many leaf/crown traits predicted stem growth moderately to very well (r 2>0.50), e.g., total leaf area (TLA) to diameter gave an r 2 of 0.91 and current-terminal leaf size, of 0.79. Some regressions were quadratic, suggesting a threshold level in a trait (e.g., leaf size) beyond which stem growth levels off. Upper-crown TLA was more closely related to height than TLA of the lower sylleptics, but the reverse was true for diameter. A decline in r 2 values from upper to lower crown positions was sharper for correlations of TLAs with height than with diameter. Thus, leaf area allocation seems to differentially affect stem growth. When autumn leaf fall (LF) and rust incidence (R) were regressed with growth, r 2 values ranged from 0.58 to 0.71, but those of spring flush (SF) were only 0.10 to 0.12. Early LF and high R, both negatively affecting growth, had a strong geographic component as it occurred mainly on lower-elevation Yakima clones. At Wenatchee, field conditions were harsher and microsites more variable, so trait/growth relationships were weaker. Genetic correlations with growth revealed similar trends as phenotypic analyses. Unlike leaf/crown traits, clonal scores of LF, SF, and R were fairly stable across the two test sites (r 2: 0.58–0.80). These traits also showed strong genetic control (H 2: 0.96–0.98). The trait/growth relationships as well as trait stability within clones have implications for selecting clonal stock in poplar culture and conservation.

Ecological correlates of mortality of roe deer fawns in a predator-free environment

The mortality of roe deer (Capreolus capreolus) fawns in summer was studied on Storfosna Island (10 km²), a predator-free island 2 km off the west-central coast of Norway, during a period of rapid population increase. Between 1991 and 1994 the population density increased from about 10 to 40 roe deer/km2. During this period 285 live and 36 stillborn fawns were found and the live fawns were all subsequently radio-collared. All observed mortality occurred within the first 35 days of life. The main causes of death were stillbirth, starvation/hypothermia, drowning, car accidents, and falls. The average annual total mortality for all 4 years was 18%. Although there was a large increase in mortality between 1991 and 1992 (from 10 to 18%), there was little subsequent increase in the last 3 years, resulting in an overall weak density dependence in total mortality. However, the proportion of stillbirths (5% in 1991 to 15% in 1994) showed a clear and significant trend toward an increase with increasing density. Low temperatures in April, which strongly affect the timing of the spring flush of high-quality forage during late gestation, also tended to contribute to higher mortality. Fawns that died were found to have reduced individual growth rates prior to death compared with fawns that survived. Fawns born to relatively light mothers or in triplet litters had higher mortality rates than those born to heavy mothers or in smaller litters. Maternal age and date of birth or sex of fawn had no effect on mortality.

Evolution of endogenous gibberellins at different stages of flowering in relation to return bloom of cactus pear ( Opuntia ficus-indica L. Miller)

The effect of flower emasculation, flower injection with paclobutrazol and the complete removal of the spring flush of flowers and cladodes on the changes in GA 3 concentration at different stages of bloom in flowers and fertile cladodes of Opuntia ficus-indica has been determined. Throughout the bloom period, the concentration of GA 3 was higher in intact flowers and the cladode than in flowers emasculated or injected with paclobutrazol. With the onset of blooming, GA 3 started to accumulate both in the flower and in the fertile cladode, and the highest concentration was measured at full bloom in the flower and in the cladodes whose flowers were not removed. Flower emasculation or removal similarly affected GA 3 concentration at any stage of bloom. Until petal shedding GA 3 concentration was higher in the flowers than in the fertile cladodes. Consistent return bloom occurred only when the spring flush was removed before bloom. Fruit resulting from flowers emasculated or treated with paclobutrazol had a smaller size, percent flesh and seed content than fruits produced by untreated flowers.

Effects of nitrogen fertilization of grapefruit trees on soil acidification and nutrient availability in a Riviera fine sand

Nitrogen (N) fertilizer applied in the NH4+ form results in some degree of soil acidification, which could influence nutrient availability to plants and nutrient losses through leaching. Effects of various N rates (0 – 168 kg N ha-1 yr-1) on soil acidification and nutrient availability were investigated in a Riviera fine sand with 26-year-old white Marsh grapefruit (Citrus paradisi MacFadyen) trees. Soil pH significantly decreased with increasing NH4–N rates. Application of 112 kg N ha-1 yr-1 for four years decreased the pH by 0.7 to 1.7 unit. Soil acidification was greater when the NH4+ form of N fertilizer was applied as dry soluble granular material compared to fertigation or controlled release forms. The marked effect of NH4–N fertilization on the pH of the Riviera fine sand was due to its low buffering capacity. Soil acidification increased the concentration of extractable Fe and P but decreased that of K, Zn and Mn. Soil pH was positively correlated with concentration of Ca, but negatively with concentrations of Fe, Mn and Zn in six-month-old spring flush leaves of the grapefruit trees. Leaf P concentrations, however, were poorly correlated with soil pH. This study also demonstrated an increase in leaching of P and K below the grapefruit trees rootzone with a decrease in soil pH.

A Rapid Nondestructive Technique to Predict Leaf Nitrogen Status of Grapefruit Tree with Various Nitrogen Fertilization Practices

The objective of this study was to evaluate the use of a leaf chlorophyll meter (SPAD-502) to predict nitrogen status and yield response of grapefruit trees. The study included two irrigation treatments, three sources, and four rates of fertilizer [dry soluble granular fertilizer broadcast or through fertigation with N rates either 50, 100, 150 lb/acre/yr (56, 112, or 168 kg.ha-1.yr-1)], and controlled-release fertilizer with N rates either 25, 50, 100 lb/acre. Irrigation treatments did not affect SPAD readings and leaf N concentrations. Correlations between SPAD readings and leaf N were greater for the spring flush (r2 = 0.61) than for the summer flush (r2 = 0.49). High correlations of SPAD meter readings with either fruit yields or leaf N suggests that SPAD meter reading is an acceptable index of N status and fruit production of the trees. SPAD readings can be made rapidly without destructive sampling of the leaves. This study demonstrates that the SPAD meter can be used to evaluate the tree N status and improve a N fertilization program for grapefruit trees.

Adventitious shoot production from strips of stem in the Dutch elm hybrid ‘Commelin’: plantlet regeneration and neomycin sensitivity

The regenerative ability of small strips of stem of the Dutch elm hybrid ‘Commelin’ was tested as well as its sensitivity to neomycins. Cambium explants (1 mm thick), were excised from woody stems collected in the field. Up to 20 buds/explant were induced within 2–3 weeks giving 2–5 rootable shoots/explant after 5–6 weeks. Shoot excision every week from week three improved the yield up to 7 shoots per explant. Fourteen and 2.9 μM GA3 promoted shoot growth. Cytokinins (1 μM zeatin or 5 μM BA or 0.05 μM TDZ) completely inhibited shoot production and promoted callus formation. Kanamycin and paromomycin at between 240 and 360 μM inhibited shoot formation as did geneticin at 80 μM. The shoot-forming ability of the explants was high from leaf fall in the autumn to the spring flush, but could be maintained up to September by using cold storage (5–7 °C). Ninety-six percent of the shoots rooted with 0.5 μM IBA and were successfully acclimatized despite having a large basal callus.

Response characteristics of DOC flushing in an alpine catchment

The spatial distribution of source areas and associated residence times of water in the catchment are significant factors controlling the annual cycles of dissolved organic carbon (DOC) concentration in Deer Creek (Summit County, Colorado). During spring snowmelt (April–August 1992), stream DOC concentrations increased with the rising limb of the hydrograph, peaked before maximum discharge, then declined rapidly as melting continued. We investigated catchment sources of DOC to streamflow, measuring DOC in tension lysimeters, groundwater wells, snow and streamflow. Lysimeter data indicate that near-surface soil horizons are a primary contributor of DOC to streamflow during spring snowmelt. Concentrations of DOC in the lysimeters decrease rapidly during the melt period, supporting the hypothesis that hydrological flushing of catchment soils is the primary mechanism affecting the temporal variation of DOC in Deer Creek. Time constants of DOC flushing, characterizing the exponential decay of DOC concentration in the upper soil horizon, ranged from 10 to 30 days for the 10 lysimeter sites. Differences in the rate of flushing are influenced by topographical position, with near-stream riparian soils flushed more quickly than soils located further upslope. Variation in the amount of distribution of accumulated snow, and asynchronous melting of the snowpack across the landscape, staggered the onset of the spring flush throughout the catchment, prolonging the period of increased concentrations of DOC in the stream. Streamflow integrates the catchment-scale flushing responses, yielding a time constant associated with the recession of DOC in the stream channel (84 days) that is significantly longer than the time constants observed for particular locations in the upper soil. © 1997 John Wiley & Sons, Ltd.

Response characteristics of DOC flushing in an alpine catchment

The spatial distribution of source areas and associated residence times of water in the catchment are significant factors controlling the annual cycles of dissolved organic carbon (DOC) concentration in Deer Creek (Summit County, Colorado). During spring snowmelt (April–August 1992), stream DOC concentrations increased with the rising limb of the hydrograph, peaked before maximum discharge, then declined rapidly as melting continued. We investigated catchment sources of DOC to streamflow, measuring DOC in tension lysimeters, groundwater wells, snow and streamflow. Lysimeter data indicate that near-surface soil horizons are a primary contributor of DOC to streamflow during spring snowmelt. Concentrations of DOC in the lysimeters decrease rapidly during the melt period, supporting the hypothesis that hydrological flushing of catchment soils is the primary mechanism affecting the temporal variation of DOC in Deer Creek. Time constants of DOC flushing, characterizing the exponential decay of DOC concentration in the upper soil horizon, ranged from 10 to 30 days for the 10 lysimeter sites. Differences in the rate of flushing are influenced by topographical position, with near-stream riparian soils flushed more quickly than soils located further upslope. Variation in the amount of distribution of accumulated snow, and asynchronous melting of the snowpack across the landscape, staggered the onset of the spring flush throughout the catchment, prolonging the period of increased concentrations of DOC in the stream. Streamflow integrates the catchment-scale flushing responses, yielding a time constant associated with the recession of DOC in the stream channel (84 days) that is significantly longer than the time constants observed for particular locations in the upper soil. © 1997 John Wiley & Sons, Ltd.

Occurrence of pesticides in the Arno River and in potable water--a survey of the period 1992-1995.

Water samples were collected from the river entering the drinking water treatment plant of Florence and from the finished drinking water. Water sampling was performed biweekly during winter months (February, January, November and December) and weekly in spring, summer and autumn, and as frequently as every day during storm runoff in May and June in anticipation of increased pesticide concentrations related to application timing and spring flush. Water samples were collected in 1-L baked-glass bottles in Teflon-lined caps and extracted within 4 h. Empore solid-phase extraction (SPE) disks (C 18-bonded silica, 47 mm diameter, containing about 500 mg of adsorbent, obtained from J. T. Baker, Phillipsburg, NJ) and an SPC DISC 6 SPE membrane workstation (Stepbio, Bologna, Italy), equipped with a Unijet II vacuum pump, were used for extraction of the analytes

Postharvest Heat Conditioning Effects on Early Ripening `Gialla' Cactus Pear Fruit

The influence of postharvest heat conditioning at 38 °C for 24, 48, or 72 hours on ripe `Gialla' cactus pear [Opuntia ficus-indica (L.) Miller] fruit produced by the spring flush was investigated during 21 days of storage at 6 °C and 90%-95% relative humidity (RH) followed by 7 days at 20 °C and 70%-75% RH (simulated marketing). Conditioning for 24 to 72 h reduced by 50% the severity of chilling injury (CI) on cactus pears following exposure to cold storage. Treatment for 24 to 72 h was also effective in reducing decay, with conditioning for 24 h being the most effective. Overall visual quality was better in heat-conditioned compared with control fruit. Mass loss was significantly reduced by all heat conditioning treatments. Respiration rate was not affected by heat treatment. Ethylene evolution was lower in fruit heat-conditioned for 48 or 72 h than for 0 h. Conditioning for 72 h resulted in the highest fruit ethanol levels. The influence of conditioning on juice pH, titratable acidity, soluble solids concentration and ascorbic acid was negligible. Prestorage heat treatment provides some measure of CI and decay control without detrimental effects to visual quality of early ripening cactus pear fruit and may offer an alternative to fungicide treatments.

Chlorpyrifos in the Air and Surface Water of Chesapeake Bay: Predictions of Atmospheric Deposition Fluxes

A 1993 survey including eight stations down the center of the Chesapeake Bay mainstem, four times during the year, resulted in detection of chlorpyrifos in 100% of air and water samples. Water concentrations were higher in March and April, coinciding with the spring flush from the Susquehanna River (1.67−0.25 ng L-1). The lowest water concentrations were found in September (0.25−0.045 ng L-1). Air concentrations were lowest in March (2−3 pg m-3). Air concentrations were highest in June in the southern Bay region (95 pg m-3). Atmospheric loadings of chlorpyrifos to the Chesapeake Bay mainstem were estimated for the four sample collection periods using fugacity-based equations. Net volatile loss of chlorpyrifos across the surface area of the Bay was predicted in March and April at 147 and 145 g day-1, respectively, while net deposition was predicted for June and September at 85 and 56 g day-1, respectively. A comparison of atmospheric loadings to the Bay with total load within the surface water illustrates that, during the mid to late summer, atmospheric loadings become an important contributor to the Bay chlorpyrifos budget.

Macronutrient Allocation to Leaves and Fruit of Mature, Alternate-bearing Pistachio Trees: Magnitude and Seasonal Patterns at the Whole-canopy Level

Estimates of leaflet and fruit macronutrient (N, P, K, Ca, and Mg) accumulation and resorption were developed in six (three heavily cropping, on-year and three noncropping, off-year) mature pistachio (Pistacia vera L. `Kerman') trees over three growing seasons during three stages of phenology [the spring growth flush (April to June); seed fill (late June to September); and leaf senescence (September to November)]. Crop load influenced total nutrient content per tree in annual organs (leaves and fruit), the relative allocation of nutrients between leaves and fruit, temporal patterns of nutrient accumulation in annual organs, and the magnitude of net leaf nutrient resorption per tree prior to leaf fall. In off-year trees, macronutrient accumulation in annual organs (leaves) was concentrated during the spring flush of growth. In contrast, significant macronutrient accumulation in annual organs of on-year trees (leaves plus fruit) occurred not only during the spring flush of growth but also during seed fill. Duration and magnitude of macronutrient accumulation were greater in on-year vs. off-year trees. Fruit N and P demand during seed fill was partially met by a net decrease in the N and P contents of the pericarp. These decreases in pericarp nutrient content during seed fill were equivalent to 32% and 26% of embryo accumulation of N and P, respectively. Fruit demand for N, P, and K during the spring flush of “on” years was accompanied by reduced leaf N, P, and K contents per tree. Net leaf N, Ca, and Mg resorption per tree during leaf senescence differed with crop load. Net leaf N resorption was significantly greater in off-year trees than on-year trees. Leaf N resorption presumably represents an important component of the N pool stored in perennial tree parts during dormancy. The greater leaf N resorption following “off” years was a function of greater leaf N concentration and greater leaf biomass per tree. In contrast, net leaf resorption of Ca and Mg was greater in on-year vs. off-year trees. Experimental validation of the magnitude and periodicity of nutrient uptake by mature pistachio trees is needed during the alternate-bearing cycle, especially in light of the potential contribution of current fertilization practices to groundwater contamination.

Assessment of nitrogen, phosphorus, and potassium uptake capacity and root growth in mature alternate-bearing pistachio (Pistacia vera) trees

We examined interrelationships between crop load, nitrogen (N), phosphorus (P), and potassium (K) uptake, and root growth in mature, alternate-bearing pistachio (Pistacia vera L.) trees. Pistachio trees bear heavy (on-year) and light (off-year) fruit crops in alternate years. Uptake and partitioning of N, P, and K among tree parts were determined during (a) spring flush (mid-March to late May), (b) nut fill (late May to early September), and (c) postharvest-leaf senescence (late September to early December). Nutrient uptake occurred primarily during nut fill in both on-year and off-year trees. In on-year trees, N and K uptake increased by 35 and 112%, respectively, during nut fill compared with off-year trees. During this period, nutrients were allocated largely to embryo development in on-year trees and to storage in perennial tissues in off-year trees. Nutrient uptake was negligible between harvest and leaf senescence. Although root growth was reduced during nut fill in on-year trees compared with off-year trees, there was no relationship between root growth and the uptake of N, P or K from the soil. Our data support the hypothesis that sink demand regulates the uptake and distribution of N, P, and K in pistachio trees.

Morphology and Anatomy of Stems and Pedicels of Spring Flush Shoots Associated with Citrus Fruit Set

Flowering and vegetative shoots of ‘Shamouti’ orange [ Citrus sinensis(L.) Osbeck] and ‘Marsh’ seedless grapefruit ( Citrus paradisiMacf.) were examined for correlation of their morphology and anatomy with fruit set. Fruit set is favoured on leafy inflorescences whereas abortion is nearly complete on leafless inflorescences. Leafless inflorescences of ‘Shamouti’ with one flower were found to have a very thin stem which contained few vascular bundles, whereas those with three flowers had better-developed vascular systems. The vascular system of leafy inflorescences is significantly different from that of leafless ones and contains a distinct central xylem cylinder. The vascular area of leafless inflorescences is only about one-quarter of that of the leafy ones. The vascular system of grapefruit resembles that of the ‘Shamouti’ orange. This study emphasizes the importance of the dimension of the vascular system for fruit set and provides a possible explanation for the better fruit set on both leafy and leafless inflorescences with several flowers compared with single-flowered inflorescences.

Genetic variation and productivity of Populus trichocarpa and its hybrids. IX. Phenology and Melampsora rust incidence of native black cottonwood clones from four river valleys in Washington

A common-garden study of Populus trichocarpa Torr. and Gray was initiated in 1985 when material from 128 trees was collected along two mesic (Hoh and Nisqually) and two xeric (Dungeness and Yakima) river valleys. In spring 1986 cuttings were used to establish two replicate plantations, at Puyallup and at Wenatchee, WA. For 2 years trees were assessed for timing of spring flush, autumn budset and leaf fall, and incidence of Melampsora occidentalis leaf rust. Seasonal patterns varied with trait, plantation, year, and origin of clones. Significant differences were found mainly between clones from the lower and upper elevations of the Nisqually (LN, UN) and Yakima (LY, UY). At both plantations, clones from the cooler UN flushed later than those from the LN. In contrast, cool canyon climates of some sites along the LY resulted in later spring flush of lower than upper clones. Factors affecting patterns of autumn phenology differed between trees from those river valleys. Later budset in LN vs. UN trees was again related to cold affecting the growing season length, but earlier budset and leaf fall in LY than UY clones at Puyallup was brought about by higher rust infection. The greater rust susceptibility of LY material likely reflects lower rust pressure in the more arid lower Yakima valley. Soil moisture deficit and the presence/absence of rust at the plantations also contributed to autumn phenology patterns. Genetic variances were large for all traits at Puyallup. In Yakima trees, the large group component of the genetic variance (elevational group, clone) in leaf fall and rust points to a steep selection gradient along the Yakima. A similarly steep gradient seems also to influence spring flush along the Nisqually. Heritabilities indicate moderately to very strong genetic control in these traits. The patterns of genetic variation have implications for short-rotation intensive-culture forestry and management of natural populations.

Estimation and Significance of Nutrient and Starch Storage Pools in Mature, Alternate-Bearing Pistachio (Pistacia vera. L.) Trees

The effect of crop load on nutrient and starch storage in mature, alternate-bearing pistachio trees was examined. Tree storage pools were estimated from the differences in nutrient and starch contents of perennial tree parts between dormancy (the period of highest nutrient and starch content) and following spring flush (the period of lowest nutrient and starch content). Following a lightly cropping (off) year, trees contained significantly larger N, P, and K storage pools than following a heavily cropping (on) year. The relative contribution of leaf nutrient resorption to tree nutrient storage pools varied depending on the crop load. Nutrient storage is a function of net leaf nutrient resorption and current uptake from the soil. Leaf nutrient (N, P, and K) resorption was a more significant component of nutrient storage in on-year than off-year trees. The contribution of nutrient storage to shoot (i.e., leaves, fruit, current-year wood) nutrient contents was evaluated following the spring flush (May) and nut fill (September) periods. Nutrient storage pools are an important source of nutrients during the spring flush of growth, but nutrient demands during nut fill are met primarily by current nutrient uptake from the soil. The relationships between nutrient storage and uptake are discussed.

Using Leaf Chlorophyll Meter to Predict Nitrogen Status and Yield of Grapefruit Trees

Rapid and accurate diagnosis of plant nutrient deficiency is critical for growers to use fertilizer economically and to minimize environmental concern. The objective of this study was to evaluate the use of leaf chlorophyll meter (SPAD-502) to predict nitrogen status and yield response of grapefruit trees. The study includes two irrigation treatments, three fertilizer sources, and four fertilizer rates. No significant differences in SPAD readings and leaf N were found between two irrigation treatments. Correlations between SPAD readings and leaf N were higher for the spring flush (r = 0.61) than that for summer flush (r = 0.49). There were poor relations between SPAD meter readings and the extractable chlorophyll content in the spring flush on a dry-weight basis (r = 0.25). Higher correlations between SPAD meter readings and N rates or fruit yields than between leaf N concentrations and N rates or fruit yield.