Michael V. Mickelbart and Mary Lu Arpaia
Effect of salinity (1.5, 3.0, 4.5, or 6.0 dS·m-1) on growth and physiology of 1-year-old `Hass' avocado (Persea americana Mill.) trees on one of three rootstocks, `Thomas', `Toro Canyon', or `Duke 7', was investigated to determine the relative salinity tolerance of these rootstocks and to determine possible reasons for any observed differences in tolerance. Leaves of trees on `Thomas' rootstock had the highest leaf Na+, Cl-, and necrosis compared to trees on the other two rootstocks. Exposure to salinity resulted in decreased growth of shoots on all rootstocks, but was greatest on `Thomas' and least on `Duke 7'. The oldest leaves on all rootstocks had the highest proportion of leaf necrosis, whereas younger leaves exhibited almost no necrosis. Salinity reduced net CO2 assimilation (A) and chlorophyll concentrations of scion leaves on all rootstocks, but more in older leaves than in younger leaves. Although the effects of salinity on A were greater for trees on `Thomas' on one measurement date, overall, rootstock differences in A were not significant for any leaf age. Differences in response to salinity among rootstocks were noted primarily in morphological traits such as growth and leaf necrosis, rather than physiological traits such as gas-exchange and water relations. Based on overall growth and physiological response to salinity, trees on `Thomas' performed poorest, whereas trees on `Duke 7' exhibited the greatest salt tolerance. The relative tolerance of the various rootstocks appeared to be due primarily to their ability to exclude Na+ and Cl- from the scion.
Michael V. Mickelbart and Mary Lu Arpaia
Salinity effects on `Hass' avocado was studied on three rootstocks, `Thomas' (TH), `Toro Canyon' (TC), and `Duke 7' (D7). Four levels of salinity (1.4, 3.0, 4.5, and 6.0 dS·m–1) were applied to 1-year-old trees grown in sand culture for 10 weeks. Increased salinity resulted in decreased trunk cross-sectional area and reduced shoot growth. Specific leaf area and dry weight decreased linearly with increased salinity. TH was significantly more affected than TC of D7. Leaf necrosis was also greatest in TH. Older leaves had the highest percentage of leaf necrosis, while younger leaves of TH exhibited symptoms only in the 6.0 dS·m–1 treatment. TH had the highest Cl leaf levels. TC maintained the lowest Na levels in the scion plant organs, indicating an ability to sequester Na in the rootstock. TC also had the lowest Na:K ratio in leaf tissue, indicating that TC can utilize K as an osmoticum. Predawn xylem potential decreased linearly with increased salinity in all rootstocks. Leaf osmotic potential decreased with increased salinity; however, leaf age moderated the response, indicating an adjustment to the stress. No rootstock differences were observed. Net CO2 assimilation (A) decreased with time only in trees exposed to 4.5 or 6.0 dS·m–1. Reduction in A due to increased salinity was less in younger leaves. No rootstock differences were noted. Chlorophyll per leaf area decreased with increased salinity to the greatest degree in TH.
Craig E. Kallsen, Blake Sanden, and Mary Lu Arpaia
The objective of this study was to measure effects of late-season water stress on fruit yield, size, quality, and color of an early-maturing navel orange cultivar, Citrus sinensis (L.) Osbeck ‘Beck-Earli’. Three irrigation regimes were initiated in August in the southern San Joaquin Valley of California in 2006, 2007, and 2008. Increasing levels of water stress resulted in decreasing midday shaded leaf water potential (SLWP) ranging from –1.4 MPa in early September to a minimum of –2.5 MPa at harvest. Generally, over the course of the 3 years, late-season water stress decreased fruit grade and increased soluble solids concentration (SSC), titratable acidity (TA), the BrimA index, and orange color. Fruit juiciness and SSC:TA ratios were unaffected by late-season water stress. The intensity of the water stress in 2007 decreased fruit yield by number and weight and decreased the percentage of large fruit. When trees exposed to 2 years of late-season water stress were fully irrigated the next year, fruit yield and quality were similar to trees that had not experienced late-season water stress for the 3 years of the study.
David Obenland, Sue Collin, James Sievert, and Mary Lu Arpaia
Navel oranges were subjected to high-temperature forced-air (HTFA) treatment to evaluate the effect on quality and sensory attributes as well as flavor volatiles of a treatment protocol designed to disinfest citrus of Anastrepha spp. fruit flies. The treatment consisted of heating the fruit to a core temperature of 44 °C and then holding it there for 100 min, after which the fruit were placed into storage for 4 weeks. The fruit were removed from storage and evaluated for surface injury, soluble solids concentration (SSC), titratable acidity (TA), and then judged for sensory characteristics by a semiexpert panel. In a separate experiment, fruit were removed at 30-min intervals from the treatment chamber and sensory quality as well as flavor volatiles determined to obtain an estimate of when the flavor changes occurred. It was found that the HTFA treatment caused a significant loss in flavor quality that was most closely linked to a loss in the fresh flavor of the fruit. The HTFA-treated fruit were also determined by panelists to be less sweet, although the SSC/TA ratio was increased by treatment. Neither storage nor waxing after treatment appeared to alter the HTFA effect, although waxing before treatment greatly enhanced the negative effect on flavor. Flavor began to be significantly affected during the final 30 min of treatment. The flavor changes occurred at the same time as large increases in the amount of four esters, two of which were present in concentrations exceeding aroma thresholds and are likely involved in the loss in flavor quality induced by HTFA treatment.
Xuan Liu, James Sievert, Mary Lu Arpaia, and Monica A. Madore
Avocado (Persea americana Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO2 fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated d-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g-1 fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.
Robert L. Heath, Michael V. Mickelbart, Mary Lu Arpaia, Claudia Fassio, and Ruby Miller
Vapor pressure deficit (VPD) is the driving force for plant water loss. However, air relative humidity (RH) can be used as a surrogate for VPD. While plants can adapt to environments with varying RH, little is known about how they respond to sudden shifts in RH. Areas of Southern California can experience drastic shifts in RH, from 60% or greater to less than 20% in just a few hours. The effect of these shifts on avocado (Persea americana Mill.) tree productivity is a major concern to growers. We studied the effect of shifts in RH on `Hass' avocado leaf stomatal conductance (g s) and branch sap flow in trees grafted on Duke 7 clonal rootstock. Under many conditions, the avocado assimilation rate is governed by g s. When g s is high in morning (>150 mmol·m-2·s-1), the water loss generally leads to some stomatal closure in the afternoon (50% or more). Conversely, low morning g s results in a higher g s rate in the afternoon (10% to 20% stomatal closure). This relationship between morning and afternoon g s is intensified by a shift from high to low RH in the afternoon. Therefore, in a drier atmosphere in the afternoon, the afternoon depression in g s is greater, leading to an impaired assimilation capacity. We hypothesize that the afternoon decrease in g s is due to low root/shoot hydraulic conductivity since soil water is readily available. While it is possible that low hydraulic conductivity on g s is exacerbated at the graft union, sap flow of grafted trees in greenhouse studies was nearly equal to trees on their own roots (ungrafted); in fact, often the depression in the afternoon was less on grafted trees. This suggests that while avocado is not suited to areas with low RH, water flow through the roots could be an additional criterion in selecting improved rootstocks.
Dana F. Faubion, Mary Lu Arpaia, F. Gordon Mitchell, and Gene Mayer
Optimum controlled atmosphere (CA) storage conditions were evaluated over a two year period for California-grown `Hass' avocado (Persea americana). Fruit harvests corresponded to early, middle and late season commercial harvests. Various temperatures and CA conditions were tested. The results indicate that the storage life of `Hass' can be extended from 3 to 4 weeks in 5C air, to 9 weeks in 5C CA if they are held in 2% oxygen and 2 to 5% carbon dioxide. Loss of quality as determined by chilling injury expression and flesh softening was greatly reduced in these conditions. Fruit maturity influenced the response to CA storage. Late season fruit had greater loss of quality in storage than earlier fruit. In 2% oxygen and 2.5% carbon dioxide, continuous exposure to ethylene levels as low as 0.1 ppm significantly increased quality loss. Delays in cooling and CA atmosphere establishment of up to three days after harvest did not effect quality.
Nydia Celis, Donald L. Suarez, Laosheng Wu, Rui Li, Mary Lu Arpaia, and Peggy Mauk
Avocado (Persea americana Mill.) is one of the most salt-sensitive crops and one of the highest value crops per acre. In the United States, avocados are grown primarily in California, in regions experiencing both scarcity of freshwater and salinization of available water supplies. Thus, our objectives were to evaluate avocado rootstocks for salt tolerance and evaluate the relationship between leaf ion concentrations, trunk diameter, leaf burn, and fruit yield. Our field experiment evaluated the salt tolerance of the Hass scion grafted onto 13 different avocado rootstocks using the Brokaw clonal rootstock technique. The experiment consisted of 156 trees arranged in a randomized complete block design with six replications of each saline [electrical conductivity (EC) = 1.5 dS·m–1, Cl– = 4.94 mmol·L–1] and nonsaline (EC= 0.65 dS·m–1, Cl– = 0.73 mmol·L–1) irrigation water treatment. We collected soil samples and leaves, then analyzed them for major ions. The rootstocks R0.06, R0.07, PP14, and R0.17, which had high concentrations of Cl and Na in the leaves, were the least salt tolerant, with 100% mortality in the rows irrigated with saline water for 23 months. The rootstocks R0.05, PP40, R0.18, and Dusa, which had low concentrations of Cl ions in the fully expanded leaves, were least affected by salinity, and these rootstocks exhibited the greatest yields, largest trunk diameters, and greatest survival percentages in the saline treatment. Yield and growth parameters correlated well with leaf Cl concentration, but not Na, indicating that salt damage in avocado is primarily a result of Cl ion toxicity. Under arid inland environments, no variety performed satisfactorily when irrigated with an EC = 1.5 dS·m–1 water (Cl– = 4.94 mmol·L–1). However, the more tolerant varieties survived at soil salinity levels that would apparently be fatal to varieties reported earlier in the literature.
David Obenland, Paul Neipp, Jim Sievert, Sue Collin, and Mary Lu Arpaia
Maturity standards that determine when navel oranges can be harvested in California are currently based upon the ratio of soluble solids content (SSC) to titratable acidity (TA) and the rind color of the fruit. These standards may be inadequate to describe the quality of the fruit, which is important given the increased competition from other commodities in the marketplace and declining consumption of fresh citrus. To reevaluate the basis of the maturity standard, navel oranges were harvested at intervals throughout the season and evaluated for SSC, TA, juice ethanol concentration, percent juice, peel coloration, and sensory characteristics. Three varieties of navel oranges, representing early-, mid- and late-season maturities, were used. SSC: TA ratios averaged 6.3 at the beginning of the season and steadily increased to 23.4 at the end of the season. Changes in the hedonic rating, or likeability of the fruit taste as rated by the sensory panelists, were closely related to the SSC: TA ratio and ratings of sweetness and tartness. These relationships showed a similar pattern for all of the navel varieties. A hedonic rating of 6 (like slightly) was not reached until the SSC: TA ratio exceeded the current legal minimum of 8:1, suggesting that the standard should be raised. Juice ethanol levels and percent juice did not have any apparent influence on the sensory ratings. Fruit that were run over a packing line and waxed developed higher amounts of ethanol during storage than control fruit but did not differ substantially from them in hedonic rating.