Pg. 17-19 Results from Potted Litchis under Drought not Necessarily Applicable to Field Grown Trees

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.

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