Physiological responses of banana (MusaAAA; Cavendish sub-group) in the subtropics. Y. Influence of leaf tearing on assimilation potential and yield

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SummaryA randomized block trial was planted to investigate the effect of leaf tearing on assimilation potential and yield of ‘Dwarf Cavendish’ banana plants over two crop cycles. Treatments comprised leaf tearing to leave strips of 12.5, 25, 50 and 100 mm widths. The treatments were artificially applied with specially designed combs to every leaf as soon as it was totally expanded, commencing at a plant height of ca. 1 m for the plant crop (P) cycle and after harvest of the P crop, for the R1 (first ratoon) cycle. To avoid further wind damage to the treatments, plants were protected by a series of 5 m high shadecloth windbreaks. Additional treatments with no artificial tearing were included, one inside the windbreak (NatIn), and another in a normal exposed plantation (NatOut) to investigate the effectiveness of artificial windbreaks in a subtropical environment. Leaf gas exchange and relative leaf folding were measured to study the interaction between leaf tearing and components of yield. Severe leaf tearing (12.5 and 25 mm) resulted in smaller plants with reduced leaf area, and in reductions of all bunch yield components. This in turn, caused up to 22% reduction in bunch mass. Untorn plants growing inside the windbreak (NatIn) had the highest yield and shortest cycle time, resulting in a 16% higher yield per annum than plants growing under prevailing wind exposure (NatOut). The latter plants showed yield characteristics comparable with the 12.5 mm and 25 mm tearing level inside the windbreak during the P and R1 cycles respectively. The most severe leaf tearing level (12.5 mm) applied over two crop cycles resulted in a bunch mass reduction of 17.9%, cycle time extension of 4% and an overall annual yield reduction of 20.9% compared with untorn leaves. Reductions in yield potential due to increased leaf tearing severity can be explained by a decreasing photosynthesis rate from the lowest (NatIn) to the highest (12.5 mm) tearing degree. With 12.5 mm tearing, low PAR at the leaf surface due to severe leaf folding, as well as boundary layer effects on the stomatal conductance and internal CO2 concentration of the narrow leaf strips, were the reasons for 26% lower photosynthesis rate, compared with the NatIn treatment during the P cycle. At leaf tearing severities of 25 and 50 mm, low PAR, increased leaf folding and low stomatal conductance were the factors limiting assimilation potential. No differences in photosynthesis were measured between 100 mm tearing and NatIn treatments, confirming that mild leaf tearing did not reduce physiological activity. Outside the windbreaks, photosynthesis was reduced by a combination of natural leaf tearing effects plus an additional constraint on stomatal conductance due to prevailing high wind speed. This resulted in 17% lower photosynthesis compared with untreated plants growing inside the windbreak (NatIn) during the P cycle. In the R1 cycle, NatIn plants showed only 3.5% higher photosynthesis than the NatOut treatment, because the larger ratoon leaf canopy contributed more to the mutual reduction of wind damage outside the windbreak. The windbreak was most effective in the P cycle when leaf area was too small to protect exposed plants from wind damage.