Why do bananas (Musa spp. AAA) remain green when ripened at tropical temperatures, but Plantains (Musa spp. ABB) do not? A study of physiological and horticultural aspects1
Maratree Plainsirichai
Mahasarakham University. Kantharawichai, Maha Sarakham. Thailand.
e-mail: maratreep@yahoo.com.au
1. Ph.D. dissertation, The University of Western Australia. Perth, WA. Australia.
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Abstract
Bananas (Musa spp.AAA) are tropical fruits that are generally consumed ripe. At tropical temperatures (>24oC), chlorophyll degradation is inhibited and the peel remained green in contrast to plantain (Musa spp.ABB), another group in the genus, in which the degreening is accelerated. The banana fruits that remain green during ripening have a low commercial value. The reason for the retention of chlorophyll in banana is poorly understood.
A series of experiments were undertaken to study physiological and horticultural aspects of banana and plantain peel during ripening. The L*a*b* colour system and RHS colour charts were used to measure changes in colour in the peel of banana and plantain ripening at 20oC. Concentration of chlorophyll a, b, a+b and total carotenoids was determined using a simple chemical method. The a* value (greenness) and hue angle were highly correlated with the concentration of chlorophyll a, b, or a+b in both banana and plantain but this association was sensitive to temperature. Within each colour chart number and chart intensity, there was a large range of concentrations of chlorophyll a, b, a+b and total carotenoids. A chromameter (with the L*a*b* colour system) was the most suitable method to measure colour. The RHS colour charts did not provide a useful indication of change in pigment concentrations in the peel of either banana or plantain.
Thin sections of fresh banana and plantain peel during ripening at 20 or 30oC were examined under a transmission electron microscope to explore changes in chloroplast ultrastructure. The emission of autofluorescence from chlorophyll was determined to investigate were chlorophyll was degrade in the peel. The chloroplasts of banana at 30oC, at day four of storage, retained their thylakoids whereas in chloroplasts of plantains at 30oC, The thylakoids had disappeared. Thylakoid membranes underwent slower degradation near the surface of the peel than in the inner layers in banana and plantain at both temperatures. Chlorophyll autofluorescence was retained only in the surface of the peel down to 8 cells depth in banana as it ripens at 30oC and it disappeared in banana ripened at 20oC and in plantain at both temperatures. Thus banana ripening at 30oC has the capacity to degrade chlorophyll in the inner layers of the peel. Chlorophyll and thylakoid membranes degraded from the inside to the outside of the peel in both cultivars at both temperatures. The retention of chlorophyll was related to the retention of thylakoid membranes.
Fluorescence was used to measure the functional activity of chlorophyll. At day five of banana ripening at 30oC where 38% of its chlorophyll was retained, Fv/Fm decreased over time in the same pattern as seen in plantain ripening at 30oC in which chlorophyll disappeared. Fv/Fm was zero in banana ripening at 30oC, and was 0.1 in plantain ripening at 30oC . In contrast to other treatments, F0 in banana ripening at 30oC increase with storage time. It was 0.06 at day 0 and was 0.16 at day five. In another experiment, fruits were exposed to temperatures of 20 or 30oC for short periods before storing at 30 or 20oC to complete ripening. This procedure determined critical steps of chlorophyll degradation that lead to the retention of green colour. If the temperature of the fruit was 20oC for the first few hours then full yellow colour was obtained, even in fruit subsequently ripened at 30oC. The temperature of the first few hours of ripening sets the control mechanism for the pattern for chlorophyll degradation in the peel of banana during ripening.
Exogenous hydrogen peroxide(H2O2), which is reactive oxygen species (ROS), and salicylic acid (SA), which induce H2O2, was applied to increase lipid peroxidation of thylakoid membranes and induce yellow colour in the peel of banana ripening at 30oC. Pretreating banana fruit with exogenous hydrogen peroxide or salicylic acid with a number of different approaches did not induce yellow colour in the peel of banana ripening at 30oC. This indicates that the retention of green colour might not be a result of the activity of ROS.
Banana remains green during ripening at 30oC because chlorophyll degradation is inhibited near peel surface (down to 8 cell layers depth). The first few hours of ripening are critical steps of chlorophyll degradation that leads to the retention of green colour. It is possible that protein that carries chlorophyll from that chlorophyll protein complex in the thylakoids can not synthesized near the peel surface of banana ripening at 30oC. Chlorophyll that is retained, however, is unable to photosynthesize because the reaction centre of photosystem II is disassociated from the antenna complex. If the exogenous reactive oxygen species applied reached the site of action or were not degraded before they were absorbed into the peel then the retention of green colour is unlikely to be a result of the activity of reactive oxygen species (ROS).