Floral induction in tropical fruit trees: effects of temperature and water supply.
Chaikiattiyos S., Menzel C. M., Rasmussen T. S.
Author Affiliation: Sisaket Horticultural Research Centre, Sisaket 33000, Thailand.
Journal of Horticultural Science 69 : 397-415
Abstract : The separate and combined effects of temperature and water supply on floral induction were investigated in avocado cv. Hass, lemon cv. Lisbon, Litchi chinensis cv. Wai Chee and mango cv. Sensation. Low temp. generally decreased vegetative growth and induced flowering in well-watered avocado, litchi and mango. Reducing the pre-dawn leaf water potential ?L reduced or prevented vegetative growth, but did not induce flowering in avocado or litchi. Reducing the pre-dawn relative water content had similar effects in mango. Low temp. and water stress reduced or prevented vegetative growth in lemon. In contrast to the response in avocado, litchi and mango, flowering in lemon was very weak in the absence of water stress at 15°/10°C or outdoors in Brisbane in subtropical Australia (28°S), and was greatest after a period of water stress. The number of flowers increased with the severity and duration of water stress (2, 4 or 8 weeks) and was generally greater after constant rather than with cyclic water stress. In lemon and litchi, net photosynthesis declined with increasing water stress reaching zero with a midday ?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 concn increased during water stress, especially in the branches, trunk and roots of lemon. Leaf starch concn was uniformly low. The number of flowers per tree in lemon was strongly correlated with starch concn in the branches and roots. In litchi roots, starch concn was lower than in lemon roots and was not related to flowering. In separate experiments to test the interaction between temp. and water supply, low day/night temp. reduced vegetative growth and induced flowering in avocado, litchi and mango. None of these species flowered at 29°C/25°C or as a result of water stress. In contrast, in lemon, flowering was very weak (10 flowers per tree) in the absence of water stress and was only heavy (>35 flowers per tree) after stressed trees were rewatered. There were slightly more flowers at 18°C/15°C than at 23°C/18°C and 29°C/25°C in control plants, but temp. had no effect in stressed plants. Starch concn in the roots of avocado, lemon, litchi and mango was generally higher at 18°C/15°C and 23°C/18°C than at 29°C/25°C. Water stress increased the starch concn in the roots of lemon and litchi and decreased it in avocado. It had no effect in mango. There was a weak relationship between the number of flowers per tree in lemon and the root starch concn. In contrast, there was no significant relationship between flowering and starch concns under the various temp. and water regimes in the other species. In another experiment, only vegetative growth in litchi and mango occurred at 30°C/25°C and only flowering at 15°C/10°C. Six weeks of water stress in a heated glasshouse (30°C days/20°C night minimum) before these temp. treatments did not induce flowering. Temperatures below 25°C for avocado and below 20°C for litchi and mango are essential for flowering and cannot be replaced by water stress. The control of flowering in lemon over the range of day temp. from 18°C to 30°C differed from that of the other species in being mainly determined by water stress. Flowering was generally weak in well-watered plants even with days at 18°C. Starch concn did not appear to control flowering.