- DJ Roe1, J H Oosthuizen2 and CM Menzel3
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1Institute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit 1200, South Africa
Current address: Merensky Technological Services, P O Box 14, Duivelskloof 0835, South Africa
2lnstitute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit 1200, South Africa
3Maroochy Horticultural Research Station, Queensland Department of Primary Industries,
PO Box 5083, SCMC, Nambour Old 4560, Australia - 1995
The effects of water deficits on photosynthesis were investigated in young Mauritius litchi trees growing in pots, to develop irrigation strategies for this crop. Plants were firstly grown for various periods without water and measurements taken on a single day. In the other experiments, the rate of drying was varied by growing the plants in sand or in a clay soil. The onset of water deficits was much slower in the clay soil with greater water-holding capacity. Some plants were allowed to wilt, rewatered and given a second drying cycle to determine if they adapted during the previous water deficit.
There was a decline in leaf water potential, stomatal opening and photosynthesis measured at 09:00 hours, as plants went without water for 0, 5, 15, 19 or 22 days. Photosynthesis fell to 18% of the value in well-watered plants, when leaf water potential declined to -3,2 MPa, and plants wilted.
The values of leaf water potential for relative photosynthesis (photosynthesis of dry treatment/photosynthesis of wet treatment) of 0,5 and zero w e re -1,5 and -2,5 MPa for the short drying cycle in sand and shifted t o -2,0 and -3,0 MPa in a subsequent drying cycle, and to -2,4 and -3,5 MPa for the Jong drying cycle in a clay soil. The data of the authors demonstrate that there is no unique relationship between photosynthesis and tree water status in litchi. Results collected from potted plants with a rapid onset of drought and no previous water deficits may not necessarily be applicable to field grown trees which experience slow drying and intermittent drought.
Summary:
- The study investigated the effects of water deficits on photosynthesis in young Mauritius litchi trees grown in pots to develop irrigation strategies.
- Photosynthesis declined as the trees experienced water deficits, with rates dropping to 18% of well-watered plants when leaf water potential reached -3.2 MPa and wilting occurred.
- The relationship between photosynthesis and tree water status varied depending on the rate of soil drying and previous water deficit experience.
- Trees grown in sand dried out quickly, causing rapid onset of drought stress, while those grown in clay soil dried out more slowly due to greater water-holding capacity.
- Plants exposed to repeated drying cycles showed adaptation, maintaining photosynthesis at lower leaf water potentials than non-adapted plants.
- Photosynthesis recovery after drought took several days and was incomplete immediately after rewatering, indicating some lasting damage when plants wilted.
- The study concluded there is no unique relationship between photosynthesis and tree water status in litchi; data from potted plants with rapid drying do not reliably predict field-grown tree responses, which experience slower, intermittent drought.
- The article underscores the importance of considering the drying rate and adaptation effects when developing irrigation strategies for litchis in the field.
- The research was conducted under controlled conditions with variations in drying cycles and soil types (sand vs clay) to examine how these factors influenced photosynthesis and water potential.
- Overall, the findings suggest that field conditions with gradual drying and previous drought exposure alter the photosynthetic response compared to rapid drying in pots.