Determination of Cultivar Differences of Coconut on Heat Tolerance by In Vitro Pollen Germination

Abstract

Coconut as one of the main commercial crops in Sri Lanka, it mainly grows in intermediate zone (around 50% of total land), wet zone (around 25% of total land) and the balance in the dry zone. Heat and drought stress are the main impacts of climate change on coconut production. Therefore, development of heat and drought tolerant coconut cultivars has been recognized as a major adaptation measure to climate change. Plant reproductive organs are more vulnerable to changes in short episodes of stress prior and during early stages of flowering (Ranasinghe et al., 2010). The major cause for failures in pollination under high temperature is reduced pollen germination at temperatures as high as 35 C to 39 C during some seasons. Therefore, it is imperative to develop tools for screening coconut for high temperature tolerance with respect to pollen germination. Several recent studies have used the in-vitro pollen germination and pollen tube growth under different temperatures to screen genotypes for high temperature tolerance. This type of pollen characteristics will provide useful insight into the reproductive tolerance of coconut to anticipated climate change. In-vitro pollen germination and pollen tube growth of typica and nana varieties were studied by Ranasinghe et al., (2010) and suggested that the response of in-vitro pollen germination to temperature will be an accurate method to screen coconut varieties to high temperature tolerance,. Therefore, this study focused on identifying the effect of temperature on pollen germination and pollen tube growth of new coconut hybrids.

Methodology

Six healthy coconut palms of Tall X Tall (TT), Dwarf Green X Tall (DGT), Tall x San Ramon (TSR), Brown Dwarf x Tall (DBT), Tall x Brown Dwarf (TBD), Brown Dwarf x Sa n Ramon (DBSR), Dwarf Green X San Ramon (DGSR) forms were selected randomly from Raddegoda and Mawathagama sites in Kurunegala, IL1a. The experimental design was Complete Randomized Design (CRD). Male flowers were collected from six palms of each cultivar and pollen of three randomly selected flowers was dusted into microfuge tubes with germination media and allowed to germinate in incubators. Incubators were maintained at predetermined temperatures from 16 C to 39 C within 2 C intervals (3 tubes per temperature regime). Pollen grains were counted for pollen germination (3 slides from each microfuge tube) after 22 hrs of incubation under light microscope. Germination percentage (% PG) was determined. The in- vitro elongation of pollen tubes was measured after 3 hrs of incubation by using an ocular micrometer fitted to the eye-piece of the microscope under a high power (x40). There were 18 pollen tubes per temperature regime for each variety. Maximum pollen germination percentage and pollen tube length recorded after incubation, at each temperature were analyzed using linear and non linear regression models (Ranasinghe et al., 2010). The bilinear equation (Equation 1) was used to estimate cardinal temperatures (Tmin ,Topt and Tmax) of all the varieties. Where; t is actual treatment temperature, and a, b1 and b2: equation constants. Topt: the optimum temperature for pollen germination or tube growth.