Citrus orchard evapotranspiration: Comparison between eddy covariance measurements and the FAO-56 approach estimates

Abstract

The aim of this study was to use the FAO-56 single and dual crop coefficient approaches to estimate actual evapotranspiration (ET_a) over an irrigated citrus orchard under drip and flood irrigations in Marrakech, Morocco. The results showed that, by using crop coefficients suggested in the FAO-56 paper, the performance of both approaches was poor for two irrigation treatments. The Root Mean Squared Error (RMSE) between measured and simulated ET_a values over the citrus orchard under drip irrigation was about 1.43 and 1.27 mm/day for the single and dual approaches, respectively, while the corresponding statistics for the orchard irrigated by the flooding technique was 1.87 and 2.48 mm/day. After determination of the appropriate values of the crop coefficient (K_c) based on eddy covariance measurements of ET_a, the performance of both approaches greatly improved. The obtained K_c values were lower than the FAO-56 values by about 20%. The low K_c values obtained reflect the practice of drip irrigation for one field and the low value of cover fraction for the other field. Additionally, the efficiency of the irrigation practices was investigated by comparing the measured K_c for two fields. The results showed that a considerable amount of water was lost by direct soil evaporation from the citrus orchard irrigated by flooding technique.     

Antagonists of Plant-parasitic Nematodes in Florida Citrus

In a survey of antagonists of nematodes in 27 citrus groves, each with a history of Tylenchulus semipenetrans infestation, and 17 noncitrus habitats in Florida, approximately 24 species of microbial antagonists capable of attacking vermiform stages of Radopholus citrophilus were recovered. Eleven of these microbes and a species of Pasteuria also were observed attacking vermiform stages of T. semipenetrans. Verticillium chlamydosporium, Paecilomyces lilacinus, P. marquandii, Streptomyces sp., Arthrobotrys oligospora, and Dactylella ellipsospora were found infecting T. semipenetrans egg masses. Two species of nematophagous amoebae, five species of predatory nematodes, and 29 species of nematophagous arthropods also were detected. Nematode-trapping fungi and nematophagous arthropods were common inhabitants of citrus groves with a history of citrus nematode infestation; however, obligate parasites of nematodes were rare.     

Comparative proteomic analysis of Xanthomonas citri ssp. citri periplasmic proteins reveals changes in cellular envelope metabolism during in vitro pathogenicity induction

Citrus canker is a plant disease caused by Gram‐negative bacteria from the genus Xanthomonas. The most virulent species is Xanthomonas citri ssp. citri (XAC), which attacks a wide range of citrus hosts. Differential proteomic analysis of the periplasm‐enriched fraction was performed for XAC cells grown in pathogenicity‐inducing (XAM‐M) and pathogenicity‐non‐inducing (nutrient broth) media using two‐dimensional electrophoresis combined with liquid chromatography‐tandem mass spectrometry. Amongst the 40 proteins identified, transglycosylase was detected in a highly abundant spot in XAC cells grown under inducing condition. Additional up‐regulated proteins related to cellular envelope metabolism included glucose‐1‐phosphate thymidylyltransferase, dTDP‐4‐dehydrorhamnose‐3,5‐epimerase and peptidyl‐prolyl cis–trans‐isomerase. Phosphoglucomutase and superoxide dismutase proteins, known to be involved in pathogenicity in other Xanthomonas species or organisms, were also detected. Western blot and quantitative real‐time polymerase chain reaction analyses for transglycosylase and superoxide dismutase confirmed that these proteins were up‐regulated under inducing condition, consistent with the proteomic results. Multiple spots for the 60‐kDa chaperonin and glyceraldehyde‐3‐phosphate dehydrogenase were identified, suggesting the presence of post‐translational modifications. We propose that substantial alterations in cellular envelope metabolism occur during the XAC infectious process, which are related to several aspects, from defence against reactive oxygen species to exopolysaccharide synthesis. Our results provide new candidates for virulence‐related proteins, whose abundance correlates with the induction of pathogenicity and virulence genes, such as hrpD6, hrpG, hrpB7, hpa1 and hrpX. The results present new potential targets against XAC to be investigated in further functional studies.     

Environmental conditions for Diaphorina citri Kuwayama (Hemiptera: Liviidae) take‐off

Environmental factors that influence flight activity of Diaphorina citri Kuwayama (Hemiptera: Liviidae) may have implications for Huanglongbing spread and management. In this work, four studies were conducted to evaluate the effect of environmental conditions on D. citri take‐off. In the first, insects were transferred to sweet orange seedlings and confined inside an acrylic cage to verify the take‐off periodicity and the effect of environmental factors on this process. In the second, take‐off temperature threshold was estimated by recording the number of insects that initiated flight from a platform when subjected to gradual temperature increases from 15 to 39°C. In the other studies, we evaluated the effect of different photoperiods and temperature regimes (third study) and of constant temperatures (fourth study) on the propensity for D. citri flight. Insects were confined in clear plastic bottle cages with tubes of 50 ml placed on the cab, to collect emerged adults that initiated flights. Results showed that a small portion of the tested population (maximum 10%) tends to take off from plants and this behaviour is more prevalent in the afternoon (14:00–16:00 h), coinciding with daytimes of lower humidity and higher thermal amplitude. Adults that were submitted to lower temperatures (18°C) and short light periods (10 h) showed less propensity to flight. In contrast, at constant 27°C, the insects were more prone to flight, and this result was confirmed when individuals were submitted to increases in temperature, indicating that 27.14°C is the take‐off temperature threshold of D. citri. Results show that temperature plays an important role in the flight activity of D. citri and suggest that control measures of the insect may be more effective in the morning and in temperatures below 27°C, when the probability to take off from a host is lower.     

Leaf age affects the efficacy of insecticides to control Asian citrus psyllid,Diaphorina citri (Hemiptera: Liviidae)

Huanglongbing (HLB), the most destructive citrus disease worldwide, is associated with the bacteria transmitted by the psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae). One of the main tactics used to manage this disease is the chemical control of D. citri. In this study, the efficacy of four insecticides commonly used by the citrus growers against the D. citri adults was assessed in two vegetative growth stages of flush: young shoots (~ 10 cm in length) and mature leaves. The bioassays were carried out under the greenhouse and field conditions. The insecticides bifenthrin, dimethoate, imidacloprid and thiamethoxam were tested at the recommended rates of 15, 450, 40 and 25 mg/L of water, respectively. The insecticides were sprayed onto the leaves of each stage until runoff. Psyllid mortality was assessed at different periods after insecticide application (0, 1 and 2 weeks). In both greenhouse and field trees, the period of control was shorter in the young shoots than in the mature leaves. In addition, the efficacy of insecticides rapidly decreased in the young shoots over time, mainly for imidacloprid and bifenthrin (less than 1 week). Thus, these results suggest that the number of spray applications to control D. citri should be more frequent during the flushing period.     

The influence of ethylene on in vitro rooting of GF 677 (Prunus persica × Prunus amygdalus) hybrid peach rootstock

Summary Treatments differing from each other for the type of tube closure (i.e., cotton plug for free gas exchange, airtight rubber cap, and rubber cap with ethysorb) and/or rooting culture medium (i.e., enriched or not by 25 to 100 μM acetylsalicylic acid) were compared for their effects on gaseous composition of the culture atmosphere and microcutting rooting of the GF 677 (Prunus persica × Prunus amygdalus) hybrid. Rubber capping, which leads to rapid ethylene accumulation inside tubes, strongly reduced rooting time and in some cases enhanced final rooting percentage over that of cotton plugs. Ethysorb almost completely absorbed ethylene produced by shoots, which showed lower rooting percentages within 9 d than microcuttings cultured in the absence of ethysorb. In contrast, no significant difference in rooting was found between the two treatments after 14 d. Carbon dioxide concentration was similar in all treatments within 5 to 9 d and seemed to be ineffective for rooting. The influence of acetylsalicylic acid on rooting was unclear. Root number and length were not significantly influenced by the treatments. These results demonstrate that the use of airtight closures, leading to rapid ethylene accumulation, can reduce time of rooting expression for GF 677 microcuttings. However, free gas exchange towards the end of the rooting period (from Day 9 to Day 14) is advisable to prevent leaf yellowing. No significant difference in plantlet survival and growth after transfer ex vitro was found among treatments.     

Production of ROL gene transformed plants of Rosa hybrida L. and characterization of their rooting ability

Transgenic plants of the rootstock Rosa hybrida L. cv. Moneyway were produced via a two-step procedure. First, kanamycin-resistant roots were generated on stem slices from micropropagated shoots, which were cocultivated with Agrobacterium tumefaciens containing the neomycin phosphotransferase II (NPTII) gene for conferring kanamycin resistance, together with individual ROL genes from A. rhizogenes. Root formation was quite efficient and up to two kanamycin-resistant roots per stem slice were produced. In the second step, these roots were used to regenerate transgenic plants via somatic embryogenesis. Although regeneration lasted up to 12 months, production of several transformants was successfully accomplished. Untransformed escapes were not found, indicating that the initial selection on kanamycin resistance was reliable.The presence of a combination of ROLA, B and C genes enhanced adventitious root formation on micropropagated shoots and explants of stems and leaves. It appears that the auxin sensitivity was increased to such a degree that cells were able to respond even to endogenous auxins present in shoots and leaves. Rooting experiments in greenhouse demonstrated that adventitious root formation on cuttings was improved threefold upon introduction of these ROL genes. It is concluded that a method was developed for the production of ROL gene transformed roses with improved rooting characteristics.     

Effects of temperature on biology and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae)

The development, survivorship, longevity, reproduction, and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama were evaluated at 10°C, 15°C, 20°C, 25°C, 28°C, 30°C and 33°C. The populations reared at 10°C and 33°C failed to develop. Between 15°C and 30°C, mean developmental period from egg to adult varied from 49.3 days at 15°C to 14.1 days at 28°C. The low‐temperature developmental thresholds for 1st through 5th instars were estimated at 11.7°C, 10.7°C, 10.1°C, 10.5°C and 10.9°C, respectively. A modified Logan model was used to describe the relationship between developmental rate and temperature. The survival of the 3rd through 5th nymphal instars at 15–28°C was essentially the same. The mean longevity of females increased with decreasing temperature within 15–30°C. The maximal longevity of individual females was recorded 117, 60, 56, 52 and 51 days at 15°C, 20°C, 25°C, 28°C and 30°C, respectively. The average number of eggs produced per female significantly increased with increasing temperature and reached a maximum of 748.3 eggs at 28°C (P<0.001). The population reared at 28°C had the highest intrinsic rate of increased (0.199) and net reproductive rate (292.2); and the shortest population doubling time (3.5 days) and mean generation time (28.6 days) compared with populations reared at 15–25°C. The optimum range of temperatures for D. citri population growth was 25–28°C.