Some ecophysiological aspects of cashew (Anacardium occidentale L) with emphasis on possible flower manipulation in Maputaland.
There has been interest in developing a cashew industry in Maputaland, the far north-eastern corner of Natal/KwaZulu. Flowering and fruit development coincide with a rainy period, with accompanying serious flower diseases (Oidium anacardii and Colletotrichum gloeosporioides). Glasshouse studies were carried out at Pietermaritzburg, concurrently with field trials in Maputaland, in an attempt to manipulate flowering and growth of cashew trees. Two glasshouse trial were carried out. A factorial design with treatments 0, 3, 6 and 9 weeks of low temperatures (24°C day/9°C nights)(factor A) and 0, 3, 6 and 9 weeks of water deficit (Factor B) was used, with both factors in all combinations. During the second season the durations were increased to 0, 4, 8 and 12 weeks for both factors. No flowering occurred in this trial. Tree growth was not affected significantly by drought and/or cold duration. Temperature appeared to be the dominant factor at low temperatures, stomatal conductance and transpiration being suppressed by cold regardless of soil water potential. At more optimum temperatures for growth, stomatal conductance was dependent on soil water potential (r² = 0.756). Starch levels in the roots, dry matter production in the leaves, roots and stems, as well as leaf area were decreased significantly (P≤0.01) with increasing low temperature duration. Another glasshouse trial to test the effects of foliar urea at concentrations of 0, 1, 2, 4 and 8 g urea 100 l⁻¹, applied once, twice or thrice at fortnightly intervals was undertaken. The treatments were applied in late autumn/early winter of 1990 and 1991. Tree growth and flowering were monitored, and starch and leaf NH₃/NH₄⁺ analyses carried out. The highest urea concentration (8%) resulted in leaf scorch and abscission, extremely low stem diameter growth rates, and was too high for glasshouse trees. The starch contents of the 8% urea treatment were depleted significantly (P≤0.01) more than the other concentrations. The other urea treatments resulted in vigorous growth and high dry matter production. There were no significant effects of the number of sprays on cashew growth. Only seven trees flowered, and therefore no definite conclusions could be drawn regarding urea effects on flowering. Most hermaphrodite flowers (max. 76.8% hermaphrodite) opened soon after first anthesis of a panicle, and all terminal flowers of panicle branches were hermaphrodite. Flowers generally opened basipetally in a panicle, starting with hermaphrodite flowers and with progressively more male flowers. Urea sprays resulted in NH₃/NH₄⁺ build-up in the leaves, concentrations in flowering trees ranging from 100 to 700 μg g⁻¹ DM for approximately a month. A field trial at was carried out at Makatini Research Station to determine the effects of timing of a two month winter drought period on flowering and growth. An observational trial to determine the effects of girdling on growth and flowering was incorporated in the border rows of the irrigation trial. The trial tested five treatments (no irrigation during May and June, June and July, July and August, August and September, and a control treatment which received irrigation throughout Winter). Mean monthly temperatures were below 20°C, and mean minimum temperatures below 15°C for the 5 winter months during treatment application. There were no significant differences in tree growth, flowering, flushing, or yields between drought stressed treatments and control, indicating that, under the conditions at Makatini, autumn and winter temperature was the overriding factor controlling initial flower induction. Flowering occurred from early October (when mean temperatures exceeded 23 to 24°C) to late April (7 months - a prolonged flowering period), when mean monthly temperatures dropped below 23 to 24°C. Girdling of cashew trees in March and May, using girdle widths of 1, 5 and 10 mm was not successful in improving flowering and yields under the conditions of the trial. A field trial was carried out at Mosi Estate in Maputaland to test the following chemicals as tree and/or flower manipulators: foliar applied ethephon (50,100,200,500,2000 mg l⁻¹), KNO₃ (1 %, 2%, 4%), urea (1 %, 2%, 4%) and paciobutrazol (500, 1000, 2000 mg l⁻¹). A phenological model for cashew in Maputaland showed a dormant period during winter, followed by a generative flush, from which panicles and flowers were produced (peak November-January). The harvest period peaked in February and March. A strong post-harvest flush preceded the winter dormant period. Trunk starch levels were at their highest after the dormant winter period, and at their lowest following the harvest. Ethephon at high concentrations (500 and 2000 mg l⁻¹) resulted in excessive leaf drop, disturbed the root shoot balance and normal phenological patterns, and gave poor yields. The best ethephon concentrations were 100 to 200 mg l⁻¹. KNO₃had no significant effect on tree growth, flushing, flowering or yields when compared to control trees. Urea at 2% concentration gave a significant increase (P≤0.05) in flushing and simultaneous decrease in flowering. Paclobutrazol at 500 to 2000 mg l⁻¹ resulted in significantly lower growth rates, and early panicle production. The mean yields of all paclobutrazol treated treatments were higher than controls, despite a hail storm which damaged the flowers. From results of this trial, the use of these chemicals to improve yields and manipulate flowering may not be economically justified. The most promising chemical for further research was paclobutrazol.