Pg. 24-28 Response of Litchi to Drought: Water Status, Photosynthesis, Growth and Yield of Trees Growing in a Sandy Loam Soil

The effects of drought stress on 10-year-old Mauritius litchi trees growing on a sandy loam soil at Nelspruit in subtropical South Africa (lat 25° S), were investigated over a single season to develop irrigation strategies for this crop. Trees were watered weekly to replace transpiration (wet), or dried out slowly from soon after panicle emergence in late July until after harvest in January (dry). Volumetric soil moisture was measured weekly to 150 cm soil depth with a neutron probe. The range in tree moisture status was monitored by measuring leaf water potential on the shaded side of the tree at 09:00 and on the exposed side of the tree at 14:00. Net photosynthesis was also measured at 09:00 on the exposed side of the tree. The growth of floral panicles and leafy shoots was measured before anthesis (flower opening), and the number and weight of fruit and its components after fruit-set.

Summary:

• The study investigated the effects of drought stress on 10-year-old Mauritius litchi trees grown in sandy loam soil at Nelspruit, South Africa, to develop irrigation strategies.
• Two treatments were compared: well-watered trees irrigated weekly to replace transpiration and drought-stressed trees allowed to dry slowly from flowering to after harvest.
• Soil moisture was monitored to 150 cm depth; trees extracted water from all depths and increasingly from deeper soil as surface moisture declined.
• Leaf water potential, an indicator of tree water status, declined progressively under drought, with significant differences between treatments after about eight weeks.
• Net photosynthesis in droughted trees was reduced to about 20% of well-watered trees when leaf water potential declined to -2.5 MPa, and did not fully recover five weeks after rewatering despite recovery in water status.
• Water stress did not affect early growth of floral panicles and leafy shoots before fruit set, but strongly suppressed vegetative shoot initiation and reduced fruit set by about two-thirds.
• Water stress increased fruit splitting from about 10% in well-watered trees to 38% in droughted trees and reduced total fruit weight by about one-third, mainly due to reduced skin and aril mass.
• Yield of droughted trees was drastically reduced (7.4 kg/tree) compared to irrigated trees (51.4 kg/tree).
• Sugar-acid ratio was not affected by water stress, indicating fruit quality in terms of sugar accumulation was maintained.
• The study highlights the sensitivity of litchi yield and growth to water deficits, with irrigation needing to maintain leaf water potential above -1.5 MPa in the morning for sustained productivity.
• Crop factors based on evapotranspiration were unreliable for scheduling irrigation due to climatic variability; soil moisture monitoring is recommended.
• The researchers suggest watering every two to three weeks in sandy loam soils and less frequently in clay soils with higher water holding capacity.
• The findings emphasize the importance of timely irrigation to prevent physiological damage and yield loss in litchi cultivation under drought conditions.