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- Author or Editor: Mark Rieger x
Root and shoot characteristics related to drought tolerance were studied for Prunus persica, P. andersonii, P. besseyi, P. maritima, P. subcordata, and P. tomentosa. In general, shoot characteristics were more closely associated with drought adaptation than root characteristics across species. The most xeric species, andersonii, had the most xerophytic leaf morphology, highest rates of leaf gas exchange, high root length/leaf area and root weight/leaf area ratios, but had root length and hydraulic conductivity similar to that of more mesic species. Water use efficiency (WUE) increased as water potentials (ψ) dropped to -3.0 to -4.0 Mpa during a 5-7 day drought for the xeric andersonii and subcordata. However, after an initial increase, WUE decreased with declining ψ in the other 4 species, indicating that carboxylation was affected by stress in the -1.5 to -3.0 range of ψ for besseyi, maritima, persica and tomentosa. CO2 assimilation (A) decreased linearly with ψ during drought in all species, but the ψ at which A reached zero was not well correlated with drought adaptation. Root hydraulic conductivity was similar for all species, indicating a lack of importance of this parameter for drought tolerance. The data suggest that introduction of xerophytic shoot characteristics into commercial cultivars of Prunus would improve drought tolerance to a greater extent than using drought tolerant species as rootstocks.
Growth, gas exchange, root hydraulic conductivity, and drought response of seedling and rooted cuttings of Lovell and Nemaguard peach [Prunus persica (L.) Batsch], and Carrizo (Poncirus trifoliata × Citrus sinensis) and sour orange (C. aurantium L.) citrus rootstocks were compared to determine the influence of propagation method on these characteristics. Rooted peach cuttings had a higher proportion of root biomass in fibrous roots (≤ mm in diameter) and lower root: shoot ratios than seedlings, although this did not occur in citrus. Net CO2 assimilation (A) was higher for peach seedlings than for cuttings, but similar for `Redhaven' (RH) scions on either seedling- or cutting-propagated rootstocks, suggesting that leaf-associated factors were responsible for differences. As in peach, A was higher for Carrizo seedlings than for cuttings, but A was not affected by propagation method in sour orange. Peach seedlings maintained higher A than cuttings as water potentials declined during short-term soil drying, although in citrus this occurred only for Carrizo. RH scions on either root type exhibited similar declines in A as soil dried, indicating the lack of a rootstock effect. Root hydraulic conductivity (Lp) was similar between seedlings and cuttings of all cultivars when expressed on a length basis. Leaf conductance and osmotic adjustment were similar for RH scions on seedling- or cutting-propogated rootstocks during 45 days of drought stress, indicating the lack of a rootstock effect on long-term stress response.
At the University of Georgia, HORT 3020 (Introduction to Fruit Crops) is a two-credit survey of the botanical characteristics, taxonomy, and production practices of the world's major fruit crops. It is offered via traditional classroom instruction, and as a distance education (DE) course through the University System of Georgia Independent Study program. The DE version of the course is designed to be identical in content, final exam, and grading scale. However, due to the nature of independent study, the end-of-topic evaluations are open-book, written assignments in the DE course, whereas students in the classroom version have closed-book quizzes at the end of each topic. Student performance in the two versions of the course was compared over a 3-year period (May 1998 to May 2001) by analyzing scores on end-of-topic evaluations, final exams, and overall course grades. Students in the DE version had higher scores on end-of-topic evaluations in all 3 years, higher scores on a comprehensive final exam in 2 of 3 years, and consequently higher overall course grades than classroom students in all 3 years. Better performance of DE over classroom students may have been related to 1) qualitative differences in end-of-topic evaluations (written assignments versus quizzes), 2) differences in student demographics (nontraditional students in DE, traditional undergraduates in classroom), 3) the elective (DE) versus required (classroom) nature of the courses, or 4) differences in course duration (1 year for DE, 15 weeks for classroom). Equal or better performance of DE students suggests that survey courses such as Introduction to Fruit Crops can be offered via distance education without compromising learning outcomes.
Spring frost events reduce fruit production in the southeastern United States more than any other factor, with some losses occurring in 5 out of 7 years. Orchard heaters, wind machines, and overhead irrigation are sound methods of reducing losses, but their relatively high cost is a major deterrent for fruit growers (Castaldi, 1990). A potentially leas-costly and more water- efficient approach to frost protection is overtree microsprinkling. Microsprinkler irrigation was applied either beneath or onto canopies of 4-year-old `Loring' peach [Prunus persica (L.)] trees at a rate of 38 liters/h per tree to evaluate the relative efficacy of low-volume undertree and overtree microsprinkling for frost protection. Overtree microsprinkling maintained flower bud temperatures 2C during a calm, radiative frost on 20-21 Mar. 1990 (minimum air temperature -4.4C), whereas undertree sprinkling provided 0.5C of air temperature elevation at a comparable height in trees (2 m). Twelve days later, fruit set was lower for nonirrigated and undertree-irrigated trees (none to one fruit/m of shoot length) than for trees irrigated with overtree microsprinklers (eight to nine fruit/m of shoot length). Economic analysis showed that capital costs of overtree microsprinkler systems increased annual costs of peach production by 8% to 13%, which required increased yield (or price per unit yield) of 17% to 20% before profits exceeded those of nonirrigated orchards, assuming all else equal. The estimated 1% increase in annual production costs of overtree microsprinkling compared to undertree microsprinkling appears to be justified by the increased efficacy of the overtree system.
Pressure-induced flow (PIF) and transpiration-induced flow (TIF) methods were used to obtain estimates of root hydraulic resistance (Rroot), stem hydraulic resistance (Rstem), and total hydraulic resistance (Rtotal = Rroot + Rstem) of ‘Lovell’, ‘Nemaguard’‚and ‘Siberian C’ peach [Prunus persica (L.) Batsch] rootstocks. Estimates of Rtotal did not differ significantly between the PIF and TIF methods, and Rroot differed between methods for ‘Siberian C only. However, Rstem was about three times lower and velocity of flow was greater in the PIF than in the TIF method for all three rootstocks. Lower Rstem in the PIF than the TIF method was not due to water moving in non-xylem pathways (i.e., pith, cortex) of stems in PIF determinations. Rroot and Rtotal were highly correlated in all cases, except TIF determinations with ‘Nemaguard’. Rroot accounted for 64% to 73% of Rtotal in TIF determinations, but significantly more (90% to 91%) of Rtotal in PIF determinations.
Paclobutrazol (PBZ) was supplied in nutrient solution culture to `Nemaguard' peach rootstock [Prunus persica × P. davidiana] at concentrations of 0, 0.001, 0.01, 0.1, and 1.0 mg·liter-1. PBZ increased root: shoot ratio and decreased root length by ≈ 5-fold over the range of PBZ concentrations tested. Root tip diameter, stele diameter, and width of the root cortex were not significantly affected by PBZ. Root hydraulic conductivity decreased log-linearly with increasing PBZ concentration; however, this decrease did not affect midday leaf conductance or net photosynthetic rate. Foliar levels of N, P, K, Fe, and Mo were reduced, whereas levels of Ca, Mg, B, and Mn were increased by PBZ. The magnitude of changes in foliar nutrition were proportional to the degree of growth suppression. Chemical name used: (2RS,3RS)-l-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol).
One-year-old kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et R. Ferguson var. deliciosa] vines were grown under 8- and 16-hour photoperiods to study the influence of photoperiod on cold acclimation and determine the potential level of hardiness that young vines attain. Vines were acclimated by reducing growth chamber temperature at 2-week intervals, beginning at 31/20C (16 hours/8 hours) and ending with 15/5C after 8 weeks. Vines receiving an 8-hour photoperiod were more cold hardy than vines receiving a 16-hour photoperiod after 4 weeks of acclimation as determined by electrolyte leakage from stem tissues. Moreover, vines receiving an 8-hour photoperiod survived freezing at – 9C at the end of the 8-week acclimation period, whereas those receiving a 16-hour photoperiod were killed at – 6C. Vine survival and electrolyte leakage of sterns were highly correlated (r = – 0.79 to – 0.90).
Estimates of root hydraulic conductivity (Lp) were obtained on intact peach (Prunus persica × P. davidiana `Nemaguard') and sour orange (Citrus aurantium L.) rootstock over a broad range of transpiration rates. Within a species, Lp was lower when estimated using the Ohm's law analog than the reciprocal of the slope of the linear regression between transpiration (E) and stem xylem water potential (Ψ). Nonzero y-intercepts in linear regressions of Ψ vs. E resulted in the lack of agreement between Lp estimates. Removal of the root system caused xylem Ψ to rapidly approach zero in both species when E ≈ 0, suggesting that factors responsible for nonzero y intercepts resided within roots. Lp was 2.2 and 3.5 times lower for sour orange than peach when calculated by the Ohm's law and regression methods, respectively.
Rooted cuttings of Nemaguard peach [Prunus persica (L.) Batsch.] were grown in 0.18-, 0.36-, 0.90-, and 2.40-liter containers for 16 weeks to study the influence of root confinement on growth, gas exchange, water uptake, and leaf carbohydrate and nutrient content. An automatic nutrient-solution dispensing system was used to ensure uniform fertility among treatments and to prevent drought stress. Leaf area and stem length were reduced by root confinement 6 to 7 weeks after transplanting, and differences among treatments increased throughout the experiment. Final tree dry weights were reduced by 51% over a 13-fold reduction in rooting volume, but dry weight partitioning was largely unaffected. A temporary limitation to CO2 assimilation (A) and leaf conductance (g) was observed just after budbreak, but consistent reductions in A and g for confined trees did not occur until after week 11. Sorbitol and starch accumulated earlier in leaves of trees in smaller containers than larger containers. Despite similar fertility, concentrations of all nutrients except N and Cu were reduced ≈2-fold for trees in 0.18-liter containers compared to other treatments. However, characteristics of nutrient deficiency were not observed on any trees, and growth reduction with no change in leaf nutrient content was observed in other treatments. It was concluded that the initial mechanisms limiting growth were not gas exchange rates, levels of nonstructural carbohydrates, or drought stress, although nutrient deficiency may have contributed to growth limitation in trees with severely confined root systems.
Photosynthesis is the very essence of agriculture. Previous photosynthetic and transpirational studies of onion (Allium cepa) have been limited to specific developmental stages. Our study measured photosynthesis and transpiration in sixteen plants of a single short-day cultivar over an eleven week period containing both non- and bulb inductive photoperiods. Differences in weekly means for photosynthesis, leaf conductance, water use efficiency, and intercellular CO, were highly significant. Weekly photosynthetic means increased under a non-inductive photoperiod and peaked one week after initiating a bulb inducing photoperiod. A decrease and leveling period occurred as bulbs developed followed by a decrease as foliage lodged. Weekly photosynthetic and leaf conductance means were correlated and highly significant. Water use efficiency and intercellular CO, means remained fairly constant throughout the study suggesting that photosynthesis in unstressed onions was controlled by internal mechanisms instead of stomata.