Yield decline in mechanically harvested clonal tea (Camellia sinensis (L) O. Kuntze) as influenced by changes in source/sink and radiation interception dynamics in the canopy

• Mechanical harvesting removes foliage indiscriminately and decreases tea yield.
• Desirable shoot number and mass are reduced under mechanical harvesting.
• The maintenance layer is depleted in mechanically harvested bushes.
• There are a greater number of strong sinks in mechanically harvested bushes.
• Sink/source dynamics altered by mechanical harvesting.

High labour costs and shortages and the cost of production has resulted in tea (Camellia sinensis (L) O. Kuntze) industries in central and southern Africa becoming unprofitable. This has led to the full mechanization of shoot harvesting, however, a reduction in yield has been observed with mechanical harvesting. It was hypothesized that the decline in yield as a result of mechanical harvesting is a result of the indiscriminate harvesting of shoots which leads to a change in sink/source and radiation interception dynamics within the canopy. As a result whole plant photosynthesis is impacted; which ultimately impacts tea bush productivity. Studies conducted at Tingamira estate, Chipinge, Zimbabwe showed significant yield differences between hand plucking and machine harvesting treatments, with higher yields under hand plucking across all seasons (43 945 kg green leaf ha−1) as compared to hand-held (35 114 kg green leaf ha−1) and ride-on machines (36 268 kg green leaf ha−1) (p < 0.05). This reduction in yield was associated with a decrease in both the number and mass of desirable shoots over each season. The cause of this change was largely attributed to the indiscriminate removal of foliage by the machines which resulted in the proliferation of immature shoots, with an associated increase in sink strength and competition for available photo-assimilates. In addition, the depletion of the maintenance layer in mechanically harvested bushes, as indicated by reduced fractional interception of photosynthetically active radiation in the top 10 cm in these bushes and reduced photosynthetic rates in these bushes, suggests that these bushes were also source limited, as compared to hand plucked bushes. Therefore the changes in tea bush architecture, as a result of mechanical harvesting, resulted in changes in sink/source dynamics which led to a proliferation of immature shoots which competed for limited photo-assimilates.