Role of ethrel in causation of floral malformation in mango cv. Amrapali: A scanning electron microscopy study

Abstract

Floral malformation is a main constraint to reduce fruit yield in mango plants. Recently, we report on the role of putrescine in normalizing the functional morphology of mango flower by reducing various adverse effects of ethylene. Here, ethrel, an ethylene releasing compound, was exogenously applied to mango plant cv Amrapali to evaluate the response of flower development under high level of ethylene. Scanning electron microscopy (SEM) study showed that ethrel treated flowers were observed to progressively be deformed and remain unbloom. The flower buds were not distinguishable and flower parts such as petals, sepals, anther and stigma were not properly developed. The stamen showed fused anther lobes and carpel depicted curved style with pointed stigma. The findings of present study suggest the involvement of ethylene to abort the functional morphology of flower and thereby development of malformation.

Keywords:

• ethrel
• flower
• mango
• malformation
• scanning electron microscopy

In spite of highest area coverage, mango productivity in India is still less owing to various factors.¹ Flower malformation, causing huge economic loss to mango production, first noticed in Darbanga (Bihar) in India. After that it has been reported in several other countries such as Pakistan, the Middle East, Egypt, South Africa, Israel, UAE, Bangladesh, and sultanate of Oman.² Malformed inflorescence possesses large number of male and rarely bisexual flowers. The ovary of malformed flowers is large and non-functional.³ There are indications that ethylene produced in mango plants either by stress⁴ or by Fusarium mangiferae⁵ enhances the endogenous ethylene level which could develop malformation disorder. Recently, we have reported that exogenous application of putrescine in mango flower could be resulted into nullifying the adverse effect of ethylene and thereby rescue of flower from malformation.⁶ The exogenous application of ethrel on mango flower to evaluate ethylene response to develop malformation in mango needs to be investigated.

Ethylene regulates various growth and developmental processes in mango flower including sex determination, senescence and abscission in responses to stress.⁷ Reports indicate that ethrel treatment led to increase in lipid peroxidation with simultaneously decrease in activities of enzymes such as catalase and superoxide dismutase.⁸ Ethylene application inhibited opening of floral buds and induced flower bud drop.⁹ Here, the exogenous application of ethrel was evaluated in mango flower cv. Amprapali under field condition to validate a possible role of ethylene in causation of mango malformation.

The experiments were conducted on 18 y old plants of mango cv Amrapali growing in orchard section of Department of Plant Physiolgy, GBPUA&T, Pantnagar, India. Ethrel (200 ppm) was individually applied by foliar application on floral buds and flowers. A total 20 panicles per plant was treated with ethrel and a total 3 plants of cv. Amrapali was evaluated. Three samples for ethrel treatment were analyzed individually by scanning electron microscope. The floral buds and flowers were plucked from ethrel treated plants with the help of a sterilized forceps and kept in gluteraldehyde overnight. Thereafter floral buds and flowers were dehydrated by graded ethanol series. The materials were allowed to dry by keeping the samples in acetone overnight. The dried samples were observed under scanning electron microscope (SEM) using an accelerating voltage of 5 kV and a spot size of 50 nm (JEOL, JSM-6610 LV). All images were processed using a computer.

We observed that ethrel treatment resulted into inhibition of opening of flower buds. Flower buds are less differentiated with fused sepals and petals (Fig. 1A and B). Similar trend was also revealed in flower in which the sepal, petal, stamen and carpel were poorly developed and deformed (Fig. 1C and D). The treated flowers showed aborted stamen with fused anther lobes (Fig. 1E and F) and deformed carpel with pointed stigma (Fig. 1G and H).

Figure 1. Evaluation of response of ethrel treatment in the development of malformation in flower of mango cv. Amrapali via SEM. Untreated flower bud, scale bar: 500 μm (A). Ethrel treated flower bud, scale bar: 500 μm (B). Untreated flower, scale bar: 500 μm (C). Ethrel treated flower, scale bar: 500 μm (D). Untreated stamen scale bar: 100 μm (E). Ethrel treated stamen scale bar: 100 μm (F). Untreated stigma scale bar: 100 μm (G). Ethrel treated stigma scale bar: 50 μm (H).

Mango plants bear healthy as well as malformed panicles parallel on the same tree. Some of the symptoms of mango malformation viz., loss of apical dominance, gummosis, latex flow, leaf epinasty, and abscission, formation of aerenchyma, swelling and shortening of internodes are the result of ethylene formation^7,10 and its higher level in malformed panicles compared to healthy in various developmental stages viz., fully swollen buds, bud inception, full grown panicles prior to full bloom and full grown panicles and full bloom suggest its involvement in the causation of disease.¹¹ Treatment of malformed panicles with 600 ppm AgNO₃ helped plants to bear normal panicles with increased number of fruit set, which shows the inhibitory effect of silver ion on ethylene action.² Malformation could be reduced by spraying 100–200 ppm naphthalene acetic acid (NAA) in the first week of October.² A role of putrescine in regulating ethylene endogenous pool and control of malformation is well documented.⁶ Earlier SEM work also revealed that malformed flowers have fused anther lobes and hooked stigma that are due to ethylene adverse effects.¹ Fusarium mangiferae is believed to enhance endogenous pool of ethylene by producing ethylene themselves.⁵ Low temperature stress ethylene has been reported in progression of disorder.⁴ Here, it is concluded that foliar application of ethrel could enhance the endogenous ethylene level in mango flower and augmented ethylene direct the transition of functional morphology of normal flower to deformed flower suggesting a role of ethylene in malformation disorder in mango.

Acknowledgments

GB is grateful to MPS's laboratory Department of Anatomy, College of Veterinary Sciences to carry out this work.

Funding

MWA is thankful to Department of Science and Technology for funding under DST fast track scheme of young scientist. AS is thankful to Dr. C P Singh for financial assistance.