Detecting different levels of drought stress in apple trees (Malus domestica Borkh.) with selected biochemical and physiological parameters

Rational irrigation scheduling based on sensing drought stress directly in plants is becoming more important due to increasing worldwide scarcity of fresh water supplies. In order to evaluate a set of potential biochemical and physiological stress indicators and select the best drought stress markers in apple trees, two experiments with potted trees and an experiment with intensive orchard grown apple trees ‘Elstar’ and ‘Jonagold Wilmuta’ were conducted in early summer in tree following years. Biochemical parameters: ascorbic acid, glutathione, tocopherols, chlorophylls, carotenoids, free amino acids, soluble carbohydrates, and physiological parameters already known as stress indicators in apple trees: predawn and midday leaf water potential, net photosynthesis (Pn), stomatal conductance (g_s), transpiration (Tr) and intercellular CO₂ concentration (Ci) were measured in leaves of apple trees subjected to different intensities of slowly progressing drought or no drought. Our study pointed out zeaxanthin and glutathione as the best drought stress markers in apple trees. Ascorbate and sorbitol appeared to be reliable indicators of moderate drought only. Responses of other tested biochemical parameters were not consistent enough to prove their role as drought stress markers in apple trees. Relative air humidity should be taken in consideration when physiological parameters g_s, Pn, Tr and Ci are used as drought stress markers in apple trees. Our study revealed that in situations where low relative air humidity affects g_s and with g_s connected physiological parameters, biochemical markers may be better tool for determination of drought stress intensities in apple trees.     

Water relations of pistachio (Pistacia vera L.) as affected by phenological stages and water regimes

Pistachio is a drought tolerant fruit tree that can be cultivated in rainfed and irrigated conditions. The water requirements of the tree, however, are considerable so in most of the commercial orchards deficit irrigation is a common practice. Regulated deficit irrigation in pistachio trees has been described in several works, which reported that the phenological stage of shell hardening, so called stage II, is the most drought tolerant. This paper proposes that such drought resistance is related to changes in water relations linked to the phenological stages, even in conditions of no water stress. In order to evaluate such changes, the daily pattern of stem water potential and gas exchange (net photosynthesis, P_n, and leaf conductance, g_s) was measured, determining also the pressure–volume curves, in three different phenological stages of mature pistachio trees (Pistacia vera cv Kerman on P. terebinthus L. rootstock.). The daily pattern of stem water potential and gas exchange were performed in three different irrigation treatments: control, regulated deficit irrigation and rainfed. The pressure–volume curves were made only in the control and rainfed treatments. Significant differences were found in the daily pattern of stem water potential in all the phenological stages considered, while only in the last one the net photosynthesis was affected by water stress. The daily pattern of gas exchange at the beginning of the season was not affected by the evaporative demand, with a constant value when radiation was not limiting. Moderate levels of water stress during the last measurement date reduced the maximum values of g_s and P_n resulting also in a clear change in the pattern of the daily curve, with maximum values only at the beginning of the day. The relationships between stem water potential and gas exchange parameters were different during stage II and almost the same in stages I and III. The parameters drawn from the pressure–volume curves also indicated a change in the elastic modulus of the leaf cells in stage II. In addition, differences in the osmotic adjustment (OA) index suggested different degree of osmotic adjustment of the phenological stages in the response to water stress. The results showed that different mechanisms of drought resistance are operating in the different phenological stages in pistachio trees.     

Seasonal trends in hydraulic conductance of field-grown ‘Valencia’ organge trees

Leaf water potential (ψ₁), whole tree transpiration (F) and leaf surface conductance (g), together with a number of environmental parameters, were measured from early summer until late winter in a well-watered mature ‘Valencia’ orange (Citrus sinensis (L.) Osb.) orchard during the growing season of 1977/78. F and g showed a seasonal trend, with maximum values reached during the late summer. Tree conductance (C_tree), calculated as the ratio between transpiration and the water potential difference between the wet soil and the sunlit leaves, was found to vary seasonally and showed maximum values during late summer. The soil temperature (T_s) showed a seasonal trend similar to C_tree. Analysis of this and other data from the literature confirmed that the sensitivity of tree conductance to soil temperature is related to the climatic origin of the plant species.The linear regression fitted to the relationship between C_tree and T_s was used to calculate soil temperature at apparent zero C_tree. This datum can be used as an index of plant sensitivity to root medium temperature.     

Genotypic variation in water relations and gas exchange of urban trees in Detroit,Michigan, USA

Increasing tree species diversity has become a key underpinning for communities to improve resilience of urban and community forests. Increasingly, urban forestry researchers are examining physiological traits to aid in selecting trees for urban sites. Knowledge of physiological responses also has implications for understanding species’ resilience to increased stresses associated with climate change. Here, we compare growth, leaf SPAD chlorophyll index, water relations, and gas exchange of seven genotypes of shade trees planted in two locations in downtown Detroit, MI, USA. Genotypes included Redpointe® maple (Acer rubrum ‘Frank Jr.’), Flashfire® maple (Acer saccharum ‘JFS-Caddo2′), Pacific Sunset® maple (Acer truncatum x platanoides ‘Warrenred’), Emerald City® tulip tree (Liriodendron tulipifera ‘JFS-Oz’), Chanticleer® pear (Pyrus calleryana ‘Glen’s Form’), swamp white oak (Quercus bicolor), and Emerald Sunshine® elm (Ulmus propinqua ‘JFS-Bieberich’). Trees were planted in either Lafayette Plaisance Park (Park), a large urban greenspace, or on the median of St. Aubin Avenue (Median), a nearby major thoroughfare. Tree height growth and leaf SPAD index were higher for trees planted in the Park location than on the Median. However, genotypic variation was larger than the effects of location or the interaction of Genotype × Location for most traits. Across measurement dates, midday leaf water potential was lowest for Pyrus trees and highest for Ulmus and Liriodendron trees. Pyrus and Quercus trees had relatively high rates of net photosynthesis (A) and stomatal conductance (g_s) while Liriodendron, Acer saccharum, and Ulmus trees had low rates of A and g_s. Liriodendron trees closed their stomata rapidly as leaf water potential (Ψ_w) declined (isohydric response), while Pyrus and Quercus trees maintained g_s across a range of leaf Ψ_w (anisohydric response). Liriodendron trees also had the highest relative growth rates, suggesting that drought stress avoidance through isohydry is a viable drought tolerance mechanism in urban trees.     

Effect of tree and branch thinning on growth,yield, and fruit quality of persimmon trees in a high-density orchard

A study was conducted to evaluate the effect of branch and tree thinning on growth, yield and fruit quality in 8-year-old non-astringent persimmon (Diospyros kaki L.f.) trees in a high-density orchard over 6 years in Korea. Adjacent trees in the rows were removed in 2008 (first-year thinning) or in 2010 (gradual thinning). Trees in the second treatment also had 25% of the main scaffold branches removed in the first two seasons. Control plots were left unthinned. First-year thinning elevated leaf macro-nutrient concentrations, increased the production of short fruiting shoots (less than 20 cm) per tree, decreased water sprout development and weight of pruned biomass, and increased fruit set compared with values recorded on control trees and with gradual thinning. First-year thinning gave higher photosynthetic photon flux in the centre of the canopy and better fruit quality. There were generally only small differences in total yield across the three treatments in individual years, but much higher total marketable yield from 2010 to 2013 in the first-year thinning plots. First-year thinning improved overall tree productivity, and the gross returns were 80% higher than those of control from 2008 to 2013 when the extra cost of the thinning was taken into account.     

Reponses of mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding in calcareous soil

Physiological and growth responses of ‘Pantin’ and ‘Magana’ mamey sapote (Pouteria sapota) trees to continuous and cyclical flooding were studied in a series of experiments. Trees were grown in containers in a very gravelly loam soil and were subjected to continuous flooding of the root zone for 30–66 days (Experiments 1 and 2) or alternating flooding–unflooding cycles for 50 days (Experiments 3–5). For all experiments, the control treatment consisted of nonflooded trees. Net CO₂ assimilation (A) and stomatal conductance (g_s) decreased within 3 days of continuous flooding and internal CO₂ concentration was significantly higher in leaves of flooded than nonflooded plants. In the cyclic flooding experiments, trees were flooded in 3- to 6-day cycles and then unflooded for the same time periods. Stomatal conductance and A decreased within 3 days of flooding, leaf epinasty occurred between days 5 and 10, leaf senescence and abscission occurred between days 15 and 30, and branch dieback and tree death occurred between days 30 and 60. Three cycles of 3-day flooding and 3-day recovery of trees had little effect on leaf gas exchange of ‘Magaña’ trees. Similarly, ‘Pantin’ trees survived 3 cycles of 6 days of flooding interspersed with 3–6 days of recovery despite consistent decreases in g_s and A during flooding. Stomatal conductance and A of both mamey sapote cultivars decreased within a few days of flooding and this species appears to have intermediate flooding tolerance compared with other tropical fruit crops based on tree survival.     

Water,Osmotic and Pressure Potential Relationships in Apple Leaves

Water (ψ_w) and osmotic (ψ_s) potentials were measured in leaves of irrigated and unirrigated Cox’s Orange Pippin apple trees on MM. 104 rootstock during the summer seasons of 1969 and 1970. Turgor pressure potential (ψ_ρ) was assessed from the difference between and ψ_s.

Intercept values for ψ_s derived from regressions of ψ_s on ψ_w reflected an increasing intracellular osmotic concentration throughout the season and an increasing ability of the trees to withstand environmental stress in respect to turgor. In 1970 ψ_s decreased about 5 bars from July to September.

The slope relating ψ_s and ψ_w remained nearly constant after June and reflected the property of the leaf cells to expand or contract with increasing or decreasing ψ_w. Since the slope was similar throughout both seasons, a general relationship between ψ_w, ψ_s and ψ_p was evaluated, from which it was deduced that a diurnal decrease in ψ_w resulted from the sum of the depressions in ψ₃ and ψ_ρ in the ratio of 3 : 4.

The rate at which ψ_w decreased with increasing evaporation demand appeared to be under stomatal control; the latter was maximal when ψ_ν equalled the value of ψ_s at full turgor. The value of ψ_p at this point was about 8 bars. Further depression of ψ_w led to yellowing and dropping of the basal leaves of shoots and spurs—an effect only present in the unirrigated trees.

Under moderate conditions of evaporation demand the effect of irrigation on ψ_w appeared to be negligible while the soil moisture tension within any part of the root zone remained within the tensiometer measurement range. When soil moisture stress became greater than this, irrigation maintained ψ_w at between 5.3 and 6.3 bars higher than it was in the leaves of the unirrigated trees. Subsequently, the unirrigated trees defoliated completely several weeks earlier than the irrigated trees.     

Water-use efficiency and evapotranspiration of mango orchard grown in northeastern region of Brazil

Knowledge of evapotranspiration (ET) and water-use efficiency (WUE) is essential in crops management mainly in arid and semiarid regions where water resources are scarce for irrigation. Field experiments were conducted at a commercial farm to obtain the WUE and ET of mango orchard growth in a semiarid environment of northeastern region of Brazil. Measurements were performed within a randomly selected experimental plot with the spacing of 10 m × 5 m between rows and plants. Soil water balance method was used to obtain the mango orchard evapotranspiration while the Penman–Monteith method (FAO/56) was used for determination reference evapotranspiration (ET_o). Soil water content was determined by six tensiometer sets installed at 0.20 m layer intervals from the soil surface down to 1.20 m soil depth. The experimental plot was irrigated with a sprinkler irrigation system based on four irrigation levels (T1 = 70%, T2 = 80%, T3 = 90% and T4 = 100% of ET_o. Results showed that ET and WUE are strongly influenced by soil water availability. Mango yield varied from a minimum value of 28.06 ton/ha in treatment T4 to a maximum value of 31.06 ton/ha in treatment T3. Such difference was found to be statistically significant (P < 0.05) by Tukey's test. Results also indicated that WUE values based on irrigation and evapotranspiration were maximum and minimum for low (treatment T1) and high (treatment T4) water levels, respectively.     

Photosynthetic responses of greenhouse tomato plants to high solution electrical conductivity and low soil water content

SUMMARY

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in a peal-moss based substrate and supplied with nutrient solutions of high (4.5 mS cm^-1) or low (2.3 mS cm^-1) electrical conductivity (EC) and under high (95 ± 5%) or low (55 ± 8% of capillary capacity) soil water content, to elucidate how EC and soil water status affect plant photosynthesis and related physiological processes. Two weeks after beginning the treatments, photosynthesis (Pn) was measured during changes of photo-synthetic photon flux (PPF) from 0 to 1200 u.mol m^-2 s^-1 using a gas exchange method. The rectangular hyperbolic model (Pn = P_max KI (1-KI)^-2 -r) provided a good fit for the photosynthetic light-response curve. High EC treatment changed the curve by increasing the initial slope (quantum yield) and decreasing photosynthetic capacity at high PPF. However, soil water deficit not only decreased the photosynthetic capacity, but also decreased quantum use efficiency. Depression of Pn was attributed to decreased stomatal (gs) and mesophyll (_gm) conductances, but g_s was depressed more than g_m. The ratio of _gm/(g_m + g_s), an indicator of water use efficiency and a measure of relative control of Pn by carboxylation and C02 supply, was higher for high-EC treated plants. Chlorophyll content was increased by high EC treatment, and was consistent with quantum yield. Leaf water potential was decreased by high EC and/or low soil water content and the decreases in leaf water potential ultimately accounted for the Pn depressions. The effects of high EC and soil water deficit were additive on photosynthesis and most related physiological processes.     

Water use and fruit yield of mango (Mangifera indica L.) grown in a subtropical Mediterranean climate

Mango (Mangifera indica L.) is one of the most important fruit crops in tropical and subtropical regions worldwide. On the coast of Granada and Malaga (SE Spain), irrigated subtropical fruit species have been introduced and cultivated on terraces with a considerable economic importance as the only European production region. The subtropical fruit production in this zone is possible with intensive irrigation on terraces, which are economically more profitable than traditional rainfed crops (almond and olive), which have been replaced or abandoned. A 2-year monitoring period was conducted using drainage lysimeters to determine the crop coefficients (Kc) and fruit yield in mango (Mangifera indica L. cv. Osteen) orchards. Also, some quality parameters such as titratable acidity, total soluble solids, and vitamin C were evaluated under these conditions. The averaged Kc values of mango trees varied within production cycle of 0.43, 0.67, and 0.63 at flowering, fruit set, and fruit growth, respectively. In this study, the fruit yield under full water requirements (100% ET_C) averaged 24.1 kg tree^?1, amounting to 21.2 kg ha^?1 mm^?1 in terms of water-use efficiency. The quality parameters of the mango fruits harvested in the study area were satisfactory. Thus, this study highlights the need to optimize the irrigation-water use according to actual mango requirements, thereby achieving more sustainable Mediterranean subtropical farming in orchard terraces.     

Aesthetic growth of a native tree species with desirable characteristics for urban green areas in arid and semiarid environments

Having an aesthetic form is one of the desirable aspects of trees used to create or reforest urban green areas, which is often achieved by procedures that require a great effort. The understanding of factors affecting the architecture of plants in natural conditions may be useful to promote a desirable shape during the growth process. Under the hypothesis that trees in high luminosity conditions have a more aesthetical growth, in this study we explore the relationship between plant shape and photosynthetically active radiation (PAR) in Bursera fagaroides, a characteristic species of the tropical dry forest and xerophytic shrub-lands useful for urban green areas in semiarid environments. To evaluate trees shape we calculate a symmetry index (I_S) by using two diameters of the crown, perpendicular to each other, and the total height. Measurements were made over three periods (before, during and after the growth period); and PAR was also measured for each tree. Linear regressions were used to analyze the relationship between the symmetry of trees and PAR received during the growth period. The post-growth symmetry of trees showed a positive relationship with the PAR received by trees during the growth period, suggesting that PAR effects can be harnessed to develop post-germination strategies for the production of trees with a more aesthetical growth in the studied species. Effect of PAR on the shape of plants may represent a practical option to promote aesthetical growth of trees and shrubs with importance for creation and restoration of urban green areas.     

Seasonal changes in CO2 assimilation in leaves of five major Greek olive cultivars

Leaf CO₂ assimilation rate, stomatal conductance (g_s), internal CO₂ concentration (C_i), chlorophyll (a + b) content, specific leaf weight (SLW) and stomatal density were measured during the season, under field conditions, for five major Greek olive cultivars, ‘Koroneiki’, ‘Megaritiki’, ‘Konservolia’, ‘Lianolia Kerkiras’, and ‘Kalamon’, with different morphological and agronomic characteristics and diverse genetic background. Measurements were taken from current-season and 1-year-old leaves, and from fruiting and vegetative shoots, throughout the season, from March to November in years 2001 and 2002. CO₂ assimilation rates showed a substantial seasonal variation, similar in all cultivars, with higher values during spring and autumn and lower values during summer and late autumn. Stomatal conductance (g_s) followed similar trends to leaf CO₂ assimilation rates, increasing from March to July, following by a decrease during August and increasing again in autumn. ‘Koroneiki’ had the highest leaf CO₂ assimilation rate and g_s values (21 μmol m⁻² s⁻¹ and 0.45 mol m⁻² s⁻¹, respectively) while ‘Lianolia Kerkiras’ and ‘Kalamon’ showed the lowest leaf CO₂ assimilation rate and g_s values (13–14 μmol m⁻² s⁻¹ and 0.22 mol m⁻² s⁻¹, respectively). One-year-old leaves had significantly higher leaf CO₂ assimilation rate than current-season leaves from April to June, for all cultivars. From August and then, leaf CO₂ assimilation rate in current-season leaves was higher than in 1-year-old leaves. There were no significant differences in leaf CO₂ assimilation rate between fruiting and vegetative shoots. Total chlorophyll (a + b) content increased with leaf age in current-season leaves. In 1-year-old leaves chlorophyll content increased in spring, then started to decrease and increased slightly again late in the season. Chlorophyll content was higher in 1-year-old leaves than in current-season leaves throughout the season. Total specific leaf weight (SLW) increased throughout the season for all cultivars. Stomatal density in lower leaf surface was lowest for ‘Koroneiki’ (399 mm⁻²) and highest for ‘Megaritiki’ (550 mm⁻²). Our results showed differences in leaf CO₂ assimilation rate among the five different olive cultivars, with a diverse genetic background, ranging from 12 to 21 μmol m⁻² s⁻¹. From the five cultivars examined, ‘Koroneiki’, a drought resistant cultivar, performed better and was able to maintain higher leaf CO₂ assimilation rate, even under high air vapor pressure deficit. All cultivars had a pronounced seasonal variation in leaf CO₂ assimilation rate, affected by date of the year, depending on ambient conditions. The high temperatures and high air vapor pressure deficit occurring during summer caused a reduction in leaf CO₂ assimilation rate in all cultivars. Leaf CO₂ assimilation rate was also affected by leaf age for all cultivars, with old leaves having significantly higher leaf CO₂ assimilation rate than young leaves early in the season.     

基于图像处理技术的柑桔园估产系统研制

通过利用柑桔园代表性植株成熟期普通数码照片,运用计算机图像处理、图形识别、智能推衍等技术识别植株挂果数,建立柑桔园产量估算模型,进而对拟测产目标果园取样单株进行图像分析处理,估算出单株产量和单位面积产量,最终实现果园产量的准确估算.     

Reduction of apple tree height (Malus domestica Borkh) cv. Ultra Red Gala/MM111 does not decrease fruit yield and quality

In Chile, like in other countries, a high percentage of apple (Malus domestica Borkh) orchards are grafted on vigorous or semi-vigorous rootstocks. The need to decrease the amount of labor involved and increase efficiency has motivated this study on the effect of reducing the height of cv. Ultra Red Gala/MM111 trees in a commercial orchard in the Maule Region of Chile. Apple trees were planted in 2003, and their tree heights were adjusted to 2.5, 3.0 and 3.6 m prior to blooming in 2006. The reduction of plant height from 3.6 m to 2.5 m determined a significant reduction in canopy volume (26% and 29% in 2007/2008 and 2008/2009, respectively), but no differences were detected in leaf area index (LAI) and the photosynthetic active radiation (PAR) transmitted through the canopy. However, a higher proportion of the canopies of the shorter trees had over 30% of incident PAR, the threshold for the production of good fruit quality and flower-bud differentiation. There were no differences in fruit production among plant heights, accumulating 131 tonnes ha⁻¹ in the three growing seasons. Over the three seasons, the shortest plants (2.5 m) required 19, 57 and 42% less labor time at harvest, respectively, than the plants maintained at a height of 3.6 m. There were no evident differences in fruit quality among the treatments; flesh firmness varied between 73.0 and 74.0 N, soluble solids between 11.6 and 12.6°Brix, mean weight between 180 and 200 g, and the percentage of red coloring exceeded 59% of the Premium fruit in the three growing seasons. Our results suggest that it is possible to manage plants with reduced height on semi-vigorous rootstocks and thus reduce the time necessary for harvesting, without affecting fruit yield and quality.     

Yield and oil quality of intensively trained trees of three cultivars of olive (Olea europaea L.) under different irrigation regimes

Summary

The aim of the present study was to evaluate the qualitative and quantitative yield response to irrigation of olive in southern Italy. The olive cvs Kalamata, Ascolana Tenera, and Nocellara del Belice were tested in a factorial combination with four irrigation levels: a rain-fed control (T₀) and three treatments (T₁, T₂ and T₃) irrigated daily with an amount of 33%, 66% and 100%, respectively of crop evapotranspiration. Soil water content remained near field capacity in treatment T₃ with no difference with respect to treatment T₂. In T₀ and T₁, the soil moisture decreased during the summer with the lowest value (20.8%) found in T₀ on 12 August, after which rainfall restored the soil moisture to field capacity. During the season the relative water content in the leaves was higher in the irrigated treatments than in the rain-fed control. Yields were higher in all the cultivars with irrigated treatments than in the rain-fed control. The yield increase with treatment T₁ in `Nocellara del Belice' was 200% compared with the rainfed control and with T<sub>2</sub> in `Ascolana tenera' and `Kalamata' the yield was 233% and 47% greater than in the control. The higher oil yield obtained in the irrigated treatments was mainly due to the increase in fruit yield, since the pulp-stone ratio and the quantity of triglycerides accumulating in the fruits were similar for all treatments. The fatty acid composition of the oils was not affected by irrigation, while there was a decrease in the content of polyphenolic substances with irrigation. This decrease could be attributed to different enzymatic activity, caused by the water deficit, rather than to different degrees of fruit ripening. The decrease of polyphenols did not influence the oil quality in terms of organoleptic parameters or oil shelf-life.     

Seasonal variations in leaf gas exchange of plantain cv. Hartón (Musa AAB) under different soil water conditions in a humid tropical region

The seasonal effect of soil water availability on leaf gas exchange of plantain plants cv. Hartón growing on two different texture soils (loamy and clayey) were evaluated. Soil water deficits corresponded to 48, 24 and 4 days without precipitation. Daily measurements of leaf gas exchange and microclimatic conditions were carried out at 2 h intervals in a humid tropical environment south of Maracaibo Lake, Venezuela. The results show that cv. Hartón is sensitive to conditions of low water deficit on loamy and to a much greater degree on clayey soils. A marked reduction in leaf conductance (g_s) was observed under severe as well as moderate deficit (below 50 mmol m⁻² s⁻¹) on clayey soils. Under low deficit g_s increases to values between 60 and 100 mmol m⁻² s⁻¹. The same trend was observed in plants on loamy soils but higher g_s for all conditions were obtained compared with plants on clayey soil. Stomatal closure produced a reduction of 85 and 55% of total assimilation (A_tot) for severe and moderate deficit in plants on clayey soils, respectively. While plants on loamy soil exhibited a 65 and 35% reduction, respectively. Water use efficiency (WUE) consistently decreased as available soil water decreased on both soil types. Independently of soil water conditions, higher WUE were always obtained for loamy soils. This suggests that cv. Hartón does not have the ability to adjust the CO₂ assimilation to transpiration ratio in order to optimize gas exchange. This evidences the importance of maintaining high conditions of available soil water in order to avoid lower assimilation rates that probably influence negatively on yield and fruit quality.     

Effects of Vegetation-Free Strip Width and Irrigation on Newly Planted Peach

Field experiments were conducted at two locations (Clayton and Jackson Springs, NC) to determine the influence of vegetation-free strip width (VFSW) and irrigation on newly planted peach growth and yield in a low-density orchard with a volunteer weedy ground cover. The experiments included VFSW of 0, 0.6, 1.2, 2.4, 3, or 3.6 m under irrigated or nonirrigated conditions. Seasonal variation in the orchard floor vegetation was observed as different weed species reported in summer and winter. However, this difference was not apparent with respect to VFSF and irrigation. At Jackson Springs, NC, the predicted irrigated VFSW which would produce the same trunk cross-sectional area (TCSA) as the grower standard (3-m nonirrigated) was 1.5, 1.3, and 0.8 m for one-, two-, and three-year-old trees, respectively. The predicted irrigated VFSW which would produce the same yield as the grower standard was 1.16 m. At Clayton, TCSA and fruit yield were not different by irrigation, but did increase linearly with VFSW. At both locations, leaf nitrogen (N) concentration was lower in irrigated trees than nonirrigated trees. Leaf N, leaf area, and SPAD were positively related to VFSW at Jackson Springs. In contrast, leaf N concentration was not different by VFSW at Clayton. However, leaf area and SPAD were positively related to VFSW at Clayton. These results suggest that a 1.5 m VFSW combined with proper irrigation and fertilization will produce tree growth and yield in newly planted orchard with volunteer weedy vegetation similar to the current grower standard in the southeastern USA.     

Role of healthy nursery plants in orange yield during eight years of Citrus Variegated Chlorosis epidemics

The production and commercialization of citrus seedlings inspected and produced in protected screenhouses has become mandatory in São Paulo State, Brazil since January 2003. This law was intended to avoid the dispersion of Citrus Variegated Chlorosis (CVC), disease caused by Xylella fastidiosa. Our objective was to compare the yield over 8 years of ‘Natal’ sweet orange trees grafted onto Rangpur lime obtained from healthy nursery plants and from plants artificially inoculated with X. fastidiosa. Yield was evaluated in an orchard planted in February 1999 with two treatments: (i) trees from healthy nursery plant, and (ii) trees from plants artificially inoculated with X. fastidiosa. The mean yield was 21% higher in trees from healthy nursery plants, as compared to trees from inoculated nursery plants. This difference represents a gain of approximately 203 boxes of 40.8 kg each, considering a planting density of 550 plants per hectare.     

Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent. I: GA3 increases the yield of commercially valuable ‘Nules’ Clementine Mandarin fruit only in the off-crop year of an alternate bearing orchard

Worldwide, gibberellic acid (GA₃) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA₃ concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2 mm) of ‘Nules’ Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard. GA₃ applied during the period of intense flower abscission failed to reduce the total number of abscised flowers in both the light off- and heavy on-bloom years. No GA₃ treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA₃ in the high yield on-crop year had fewer abscised fruit than untreated control trees (P = 0.0188) and GA₃ applied 10 days after 75% petal fall and in July increased the number of fruit retained on tagged branches >20% compared to control trees (P = 0.0005). Maximum air temperature was not related to flower or fruit abscission. In the off-crop year (548 fruit per untreated control tree), it was necessary to apply 15 or 25 mg L⁻¹ GA₃ at 60% bloom, 90% bloom, 75% petal fall and 10 days after 75% petal fall to significantly increase the number of fruit per tree and yield of commercially valuable fruit (kilograms and number per tree) (P < 0.0001) above that of control trees, with no reduction in total kilograms per tree. In the following on-crop year, it was better not to apply GA₃: no treatment increased total yield or fruit size and five of seven GA₃ treatments tested reduced total yield as kilograms and number of fruit per tree (P = 0.0003). The results provide strong evidence that GA₃ efficacy is crop load-dependent and dictate that crop load should be considered when using GA₃ to increase fruit set or fruit size of mandarins.