Physiological studies on cocoa (Theobroma cacao) in Ghana
The research reported was conducted within a U.K. Technical Aid Research Project aimed at breeding improved cocoa cultivars for replanting cocoa areas of Ghana devastated by Swollen Shoot Virus Disease (CSSV). Tolerance of CSSV was minimal in exposed conditions and in soil conditions which restricted root aeration such as heavy texture and excess moisture. A proportion of infected plants effectively tolerated infection when grown in favourable shade and soil conditions. CSSV infection induced plant water stress but insufficiently to account for the observed reduction in growth. Photosynthetic rate was reduced by CSSV infection but carbohydrate accumulated in infected plants suggesting that utilisation was reduced to a greater extent than assimilation. The study of the water relations of cocoa included the development of stress, the effect of stress on physiological processes and the control of stress toy irrigation and other means. Plant water stress was initially assessed by the Relative Water Content (RWC) but there was no constant relationship between RWC Psiw and as measured by the pressure chamber, RWC being reduced to a greater extent by soil moisture stress than by transpirational stress. The osmotic potential (Psis) of cocoa leaves was generally -17 to -21 bars and varied by less than 2 bars diurnally. Drought which reduced pre-dawn Psiw to -3 bars reduced Psis to -24 bars. Leaf Psis increased as a leaf flush developed. Over a drought cycle, pre-dawn Psiw reflected soil water potential, and daytime Psiw reflected the balance between absorption and transpiration. Daytime stress developed faster in the morning and fell later in the early evening as soil moisture stress increased. Gradients of up to 5 bars in Psiw developed between stem and leaf during active transpiration, indicating considerable resistance to water flow in the leaf. Water stress equivalent to pre-dawn Psiw of -0.4 to -0.5 bars inhibited leaf development, induced leaf abscission and inhibited flowering. Fertilisation and setting were greatly reduced during severe Harmattan conditions. Cherelle wilt was less sensitive to water stress than most other processes. Beans produced in drought conditions had less than 10% viability. Irrigation was investigated with a view to vise in seed gardens to increase the flexibility in timing seed pod production. A scheme was set tip to irrigate 16 ha of the Bechem seed garden. Both PMA and Folicote were effective antitranspirants and in gauzehouse experiments prolonged seedling survival by up to 16 days. Screening programmes were initiated, for the three main components of establishment ability---tolerance of exposure, draught resistance and vigour. Tolerance of exposure varied widely among progenies and was a strong characteristic in the Scavina population and several of the Nanay clones. Drought resistance was less variable among progenies but significant differences were found. Photosynthetic rate (PR) was measured in the field using a 14 CO2 absorption technique. Light saturation occurred at 150 to 350 muE m-2 sec-1 depending on nutrition. In saturating light PR was related to stomatal resistance (rs). Maximum PR was low---7 mg CO2 dm-2 hour-1---and occurred in thick leaves (SLW > 7 mg cm-2) with an rs less than 1.2 sec cm-1. Stomata opened in low light intensity (30-60 muE m-2 sec-1) and rs in vigorous field cocoa increased gradually during the day, with little or no mid-day stomatal closure. Stomatal resistance was little affected by water stress until Psip fell to 4-5 bars, when rs increased rapidly. PR fell as the leaf aged, reaching 61% of the maximal value by the stage at which three further flushes had been produced on the shoot. Cherelle wilt was generally effective in matching pod load to nutritional status of the tree but the wilt mechanism could be overcome by heavy concentrated hand pollination on some trees. Some trees were debilitated by overcropping. In natural cropping, early pods grow fast and have a high survival rate compared with late pods. Pod survival patterns are given for Amelonado and shaded and unshaded Amazon. The increased yield response to hand pollination in mixed Amazon varied from 10 to 50% in a number of small-scale trials, though some floriferous trees produced an exceptionally heavy crop of pods following hand pollination. Various cropping patterns were imposed on mixed Amazon trees by cherelle removal and hand pollination with the objective of producing a late concentrated crop which might escape blackpod and simplify management. Late concentrated crops had relatively low blackpod losses but the yield of fermentable pods was not increased relative to the natural crops due to lower numbers of sets and the tendency of late pods to become deformed.