FLORAL INDUCTION IN TROPICAL FRUIT-TREES - EFFECTS OF TEMPERATURE ANDWATER-SUPPLY

Floral induction in tropical trees generally follows a check in vegeta tive growth. However, it is not easy to identify the environmental fac tors involved in flowering, which normally occurs during the dry seaso n when temperatures are also often lower. The separate and combined ef fects of temperature and water supply on floral induction were investi gated in 'Hass' avocado (Persea americana), 'Lisbon' lemon (Citrus lim on), 'Wai Chee' litchi (Litchi chinensis) and 'Sensation' mango (Mangi fera indica). Low temperatures (15-degrees/10-degrees-C or 15-degrees/ 10-degrees-C and 20-degrees/15-degrees-C compared with 30-degrees/25-d egrees-C and 25-degrees/20-degrees-C) generally decreased vegetative g rowth and induced flowering in well-watered avocado, litchi and mango. A pre-dawn leaf water potential (PSI(L)) of -1.7 to -3.5 MPa compared with -0.4 to -0.7 MPa in control avocado and litchi, and a pre-dawn r elative water content (R.W.C.) of 90-93% compared with 97% or above in control mango plants also reduced or eliminated vegetative growth, bu t did not induce flowering. Low temperatures (15-degrees-/10-degrees-C compared with 20-degrees/15-degrees-C, 25-degrees/20-degrees-C or 30- degrees/25-degrees-C) and water stress (pre-dawn PSI(L) of -2.0 to -3. 5 MPa compared with -0.7 to -0.8 MPa in controls) reduced or eliminate d vegetative growth in lemon. In contrast to the response in avocado, litchi and mango, flowering in lemon was very weak in the absence of w ater stress at 15-degrees/10-degrees-C or outdoors in Brisbane in subt ropical Australia (Lat. 28-degrees-S), and was greatest after a period of water stress. The number of flowers increased with the severity an d duration of water stress (two, four or eight weeks) and was generall y greater after constant rather than with cyclic water stress. In lemo n and litchi, net photosynthesis declined with increasing water stress reaching zero with a midday PSI(L) of -3.5 to -4.0 MPa. This decline in carbon assimilation appeared to be almost entirely due to stomatal closure. Despite the reduction in midday CO2 assimilation, starch conc entration increased during water stress, especially in the branches, t runk and roots of lemon. Leaf starch was uniformly low. The number of flowers per tree in lemon was strongly correlated with starch in the b ranches (r2 = 77%, P < 0.01) and roots (r2 = 74%, P < 0.001). In litch i, starch was lower than in lemon roots and was not related to floweri ng. In separate experiments to test the interaction between temperatur e and water supply, low day/night temperatures (23-degrees/18-degrees and 18-degrees/15-degrees-C compared with 29-degrees/25-degrees-C) red uced vegetative growth and induced flowering in avocado, litchi and ma ngo. None of these species flowered at 29-degrees/25-degrees-C or as a result of water stress (PSI(L) of -1.5 MPa compared with -0.3 MPa for avocado and -2.0 MPa compared with -0.5 MPa for litchi, and R.W.C. of 90-93% compared with 95-96% in mango). In contrast, in lemon, floweri ng was very weak (<10 flowers per tree) in the absence of water stress (pre-dawn PSI(L) of -2.0 MPa compared with -0.5 MPa) and was only hea vy (>35 flowers per tree) after stressed trees were rewatered. There w ere slightly more flowers at 18-degrees/15-degrees-C than at 23-degree s/18-degrees and 29-degrees/25-degrees-C in control plants, but no eff ect of temperature in stressed plants. Starch concentration in the roo ts of avocado, lemon, litchi and mango was generally higher at 18-degr ees/15-degrees-C and 23-degrees/18-degrees-C than at 29-degrees/25-deg rees-C. Water stress increased the starch concentration in the roots o f lemon and litchi and decreased it in avocado. There was no effect in mango. There was a weak relation (r2 = 57%, P < 0.05) between the num ber of flowers per tree in lemon and the concentration of starch in th e roots. In contrast, there was no significant relationship between fl owering and starch levels under the various temperature and water regi mes in the other species. In another experiment, only vegetative growt h in litchi and mango occurred at 30-degrees/25-degrees-C and only flo wering at 15-degrees/10-degrees-C. Six weeks of water stress (pre-dawn PSI(L) of -2.5 MPa compared with -1.0 MPa or higher in litchi, and R. W.C. of 90-93% compared with 95% or higher in mango) in a heated glass house (30-degrees-C days/20-degrees-C night minimum) before these temp erature treatments did not induce flowering. Temperatures below 25-deg rees-C for avocado and below 20-degrees-C for litchi and mango are ess ential for flowering and cannot be replaced by water stress. The contr ol of flowering in lemon over the range of day temperatures from 18-de grees-C to 30-degrees-C differed from that of the other species in bei ng mainly determined by water stress. Flowering was generally weak in well-watered plants even with days at 18-degrees-C. Starch did not app ear to control flowering.