Net CO2 assimilation (A) and stomatal conductance for CO2 (gs) were determined for flooded and nonflooded avocado plants (Persea americana Mill.) with different severities of phytophthora root rot (caused by Phytophthora cinnamomi Rands). Under nonflooded conditions, root necrosis (primarily due to P. cinnamomi) of <50% generally had little effect on net gas exchange, whereas root necrosis ≥50% resulted in decreases in A and gs of up to 65% when compared to noninfested controls. However, root necrosis ≥20% greatly reduced A and gs when plants were flooded for 7 days. Net CO2 assimilation and gs were not reduced when plants without root rot were flooded for 7 days.
Net CO2 assimilation (A), stomatal conductance for CO2 (gc), water-use efficiency (WUE), specific leaf density (Wa), and leaf chlorophyll and nitrogen (N) content were determined for mango (Mangifera indica L.) leaves that developed in full sun and 25%, 50%, and 75% shade. Leaves that developed in full sun had greater A and gc than leaves that grew in 75% shade. Leaf chlorophyll content and Ν content (dry-weight basis) increased as percent shading increased, but WUE and Wa were not significantly affected by shading. Light response curves for A had a steeper initial slope for leaves that developed in full sun and 25% shade than for leaves that developed in 50% and 75% shade. The data indicate a greater quantum-use efficiency for leaves developed in high light levels than for those developed in shade.
`Mark' rootstock is a relatively new dwarfing rootstock that induces precocity in apple trees. While `Mark' has desirable horticultural characteristics, it has been difficult to propagate in some areas of the United States. To determine the optimum budding date at two climatically diverse locations, `Jonagold' buds were chip-budded onto `Mark' rootstock on 20 July, 10 Aug., 31 Aug. and 21 Sept. 1989 at Atlas, Illinois and Wenatchee, Washington. Prior to budbreak, unions were sampled from each budding date and the callus, bud plate and rootstock were measured and photographed. Trees budded and grown in Illinois had more callus growth than those budded in Washington. In Illinois, the callus of trees budded on 20 July averaged 3.2 mm., whereas those budded on 21 Sept. averaged 1.0 mm. Trees grown in Washington had 0.4 mm of callus at both budding dates. Callus growth will be correlated with union compatibility and strength in Nov. 1990.
The influence of shoot age on 14C partitioning in potted avocado (Persea americana var. americana Mill.) trees was determined. The oldest leaf of actively growing shoots and the youngest leaf of previously matured shoots were exposed to 14CO2 18 and 34 days after budbreak (DABB) of new shoots. At these times, treated leaves had a positive net CO2 assimilation rate and, therefore, were considered to be net C exporters. Sixteen days after 14C exposure, separate plant tissues were harvested, dried, weighed, and oxidized. The percentage of 14C in each tissue was determined by liquid scintillation spectrometry. Photoassimilates were translocated acropetally and basipetally from all treated leaves. However, at 18 DABB, developing leaves of actively growing shoots seemed to be the strongest sink for C assimilated by the oldest leaf of these shoots, whereas the roots were the strongest sink for C assimilated by the youngest leaf of the previously matured shoots. By 34 DABB, roots were the strongest sink for C assimilated by leaves of new and previously matured shoots. These data are useful in developing improved management strategies for controlling phytophthora root rot (incited by Phytophthora cinnamomi Rands) in avocados by systemic phosphonate fungicides translocated in the photoassimilate pathway. Thus, phosphonates should be applied after shoots have matured and most of the canopy is in a quiescent state for maximum translocation to the roots.
Primed acclimation (PA) is a regulated deficit irrigation (RDI) strategy designed to improve or maintain yield under subsequent drought stress. A previous study showed photosynthetic increases in papaya in response to a PA treatment. The present study was undertaken to test the duration of the PA effect when papaya plants were challenged with severe drought stress. Potted plants were stressed at 1, 2, and 3 months after conclusion of a PA treatment consisting of 3 weeks at soil water tension (SWT) of −20 kPa. Measurements included leaf gas exchange, root growth, and organ dry mass partitioning. PA did not reduce net CO2 assimilation (A) during the deficit period. At the end of the PA period, total dry matter accumulation per plant and for each organ was unaffected, but proportional dry matter partitioning to roots was favored. After resuming full irrigation, A increased and whole plant water use was more than doubled in PA-treated plants. However, water use and A of PA-treated plants decreased to reconverge with those of control plants by 6 weeks after the PA treatment. Over the course of the study, PA plants maintained lower stem height to stem diameter ratios, and shorter internode lengths. However, these changes did not improve photosynthetic response to any of the water-deficit treatments. We conclude that papaya exhibits some signs of stress memory, but that rapid short-term acclimation responses dominate papaya responses to soil water deficit.
Fruit number and weight per tree were recorded for 3 years for 2 avocado (Persea americana M.) cultivars—‘Booth 8’ and ‘Peterson’. Sample variances were used to determine the number of years per experiment and trees per treatment required to detect differences in yield among treatments at the 5% and 10% significance levels for each cultivar. Yearly variability was greater than variability among trees for both cultivars. Yearly variability for total fruit weight was greater for ‘Booth 8’ than for ‘Peterson’. Fifteen trees per treatment were required to detect a 10–20% difference in total fruit weight per treatment at the 5% level of significance. Each additional tree per treatment decreased the percentage of detectable difference only slightly. Increasing the number of years per experiment decreased the percentage of detectable difference. However, after 3 years, additional years decreased the percentage of detectable difference only slightly for both an alternate- and a nonalternate-bearing cultivar. Efficient designs for avocado yield experiments for the cultivars tested should consist of 10 to 15 trees per treatment and should be conducted for at least 3 years.
Root ferric chelate reductase (FCR) activity in Annona glabra L. (pond apple), native to subtropical wetland habitats and Annona muricata L. (soursop), native to non-wetland tropical habitats, was determined under Fe-sufficient and Fe-deficient conditions. Four-month-old seedlings of each species were grown hydroponically in a complete nutrient solution containing 90 μm Fe or no Fe. The degree of tolerance of Fe deficiency was evaluated by measuring root FCR activity, chlorophyll and Fe concentration in recently matured leaves and plant growth. Root FCR activity was higher in soursop than in pond apple in the nutrient solution with Fe. However, there were no differences in root FCR activity between species under Fe-deficient conditions. Root FCR activity in pond apple and soursop was not induced in the absence of Fe. Leaf chlorophyll index and Fe concentration, and dry weights of pond apple were lower when plants were grown without Fe compared to plants grown with Fe. Leaves of pond apple grown without Fe became chlorotic within 3 weeks. Lack of Fe decreased the chlorophyll index and Fe concentration in young leaves less in soursop than in pond apple. In contrast, the Fe level in the nutrient solution had no effect on dry weights of soursop. The rapid development of leaf chlorosis and low FCR activity of pond apple may be due to its native origin in wetland areas where there is sufficient soluble Fe for plant growth and development.
The effect of soil water depletion on plant water potential and leaf gas exchange of carambola (Averrhoa carambola L. cv. Arkin) in Krome very gravelly loam soil was studied in an orchard and in containers in the field and in a greenhouse. The rate of soil water depletion was determined by continuously monitoring soil water content with multi-sensor capacitance probes. Stem water potential and leaf gas exchange of carambola in containers were reduced when the soil water depletion level fell below 50% (where field capacity = 100%). Although there was a decrease in the rate of soil water depletion in the orchard as the soil dried, soil water depletion did not go below an average of 70%. This was presumably due to sufficient rainfall and capillary movement of water in the soil. Therefore, soil water content did not decline sufficiently to affect leaf gas exchange and leaf and stem water potential of orchard trees. A decline in soil water depletion below 40% resulted in a concomitant decline in stem water potential of the container trees in the field and greenhouse to below -1.0 MPa. Stomatal conductance, net CO2 assimilation, and transpiration declined significantly when stem water potential was below -1.0 MPa. The reduction of net CO2 assimilation and transpiration was proportional to the decline in stomatal conductance of container trees in the field and greenhouse. Thus, soil water depletion in Krome very gravelly loam soil must be less than 50% before water potential or leaf gas exchange of carambola is affected. Based on these results, irrigation scheduling should be based on physiological variables such as stem water potential and stomatal conductance or the amount rather than the rate of soil water depletion.
In the article “ Effect of Fruit Removal on Net Gas Exchange of Avocado Leaves”, by Bruce Schaffer, L. Ramos, and S.P. Lara (HortScience 22:925-927, October 1987), ref. 2 was incorrect. The correct citation is: Annu. Rev. Plant Physiol. 33:317-345.