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Water deficits at anthesis reduce CO₂ assimilation and yield of lychee (Litchi chinensis Sonn.) trees

C. M. Menzel (1), J. H. Oosthuizen (2), D. J. Roe (2, 3) and V. J. Doogan (4)

1. Maroochy Horticultural Research Station, Queensland Department of Primary Industries, P.O. Box 5083, SCMC, Nambour, Queensland 4560, Australia / 2. Institute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit 1200, South Africa / 3. Merensky Technological Services, P.O. Box 14, Duiwelskloof 0835, South Africa / 4. Biometry, Queensland Department of Primary Industries, P.O. Box 46, Brisbane, Queensland 4001, Australia / Received August 9, 1994

Summary

Ten-year-old ‘Tai So’ lychee (Litchi chinensis Sonn.) trees growing on a sandy loam soil in subtropical South Africa (latitude 25° S) were watered weekly (well-watered treatment) or droughted from late July until January (drought treatment). After 16 weeks, at which time the trees obtained most of their water from below 150 cm, average soil water content at 0 to 150 cm depth was 14.5 ± 0.1% in the well-watered treatment and reached a minimum of 7.6% in the drought treatment. At Week 7, minimum leaf water potential (Ψ_L) in the morning and early afternoon declined to –2.6 and –2.8 MPa, respectively, in droughted trees compared with –1.5 and –2.2 MPa, respectively, in well-watered trees. From Week 9, stomatal conductance and net CO₂ assimilation rate ranged from 70 to 300 mmol m^–2 s^–1 and 3 to 13 µmol CO₂ m^–2 s^–1, respectively, in well-watered trees. The corresponding values for droughted trees were 50 to 180 mmol m^–2 s^–1 and 2 to 6 µmol CO₂ m^–2 s^–1. Five weeks after rewatering the droughted trees, gas exchange had not recovered to the rate in well-watered trees, although tree water status recovered within a week of rewatering. In the well-watered trees, water use (E_t) was 26 ± 1 mm week^–1 with evaporation (E_p) of 20 to 70 mm week^–1 indicating a crop factor (k_c = E_t/E_p) of 0.4 to 1.2. 

Before anthesis, tree water status did not affect extension growth of floral panicles or leafy shoots. In contrast, no vegetative shoots were initiated after fruit set in the droughted trees when Ψ_L in the morning declined to –2.5 MPa. Water deficits reduced initial fruit set by 30% and final fruit set by 70% as a result of fruit splitting (41.2 ± 4.0% versus 10.0 ± 1.3%). Water deficits did not alter the sigmoidal pattern of fruit growth, but reduced yield from 51.4 ± 5.5 kg tree^–1 in well-watered trees to 7.4 ± 3.3 kg tree^–1 in droughted trees.

Keywords: fruit growth, fruit retention, irrigation, leaf water potential, photosynthesis, soil water content, stomatal conductance, water use.

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