Successful fruit set in coconut depends on several reproductive processes including pollen quality, pollen germination percentage (%PG) and pollen tube growth processes. Three preliminary experiments were conducted at the Coconut Research Institute, Lunuwila, Sri Lanka to quantify the response of in vitro pollen germination in coconut to time of incubation, the response of in vitro pollen germination and pollen tube growth to temperature and to determine the cardinal temperatures (T min , T opt , T max) of coconut varieties for in vitro pollen germination. Pollen was collected from palms of six cultivars / varieties expressing variable tolerance to abiotic stresses. Pollen germination and pollen tube length were recorded after incubation in artificial growth media at the respective treatments. For pollen collected during October / November periods, cultivars differed for in-vitro pollen germination percentage with a mean values of 23%. The mean cardinal temperatures (T min , T opt , T max) averaged over cultivars were 20.0 ºC, 28.0 ºC, and 38.8 ºC, respectively, for pollen germination. The information generated in this study will be tested in different seasons of the year for consistency and used to develop a heat tolerance index for coconut. This index will be used to identify coconut varieties suitable for high-temperature prone areas. In addition, the identified cardinal temperatures and response functions could be incorporated into process-based coconut models to increase prediction accuracy under current extreme and projected future climates.

Abstract:The final yield of coconut (Cocos nucifera L.) is determined by early fruit set. Thus understanding the factors controlling fruit set including climate change is of importance for assessing the yield and its accurate prediction. This study assessed the survival pattern of female flowers produced in different months of the year, the effects of climate factors during early phase of flowering and the competition for assimilates from previously formed fruits on the fruit set of newly opening inflorescences in a 25 -26 year-old coconut plantation in the Intermediate Zone (IL 1a ) of Sri Lanka over a period of three years. The rate of fruit survival since the onset of flowering to maturity varied with the month of inflorescence opening. Abortion of fruits after three months from inflorescence opening was negligible. The optimum (T opt-in vitro ) and maximum (T max-in vitro ) temperatures for in vitro pollen germination were 28 ºC and 39.7 ºC, respectively. The fruit set of an inflorescence/s opened in a given month (number of set fruits palm -1 month -1 ) was negatively correlated with the total reproductive dry matter requirement of the palm (TDMR) of the month of inflorescence/s opening (first month), and the number of high temperature days (T max ≥ 33 ºC) during the first three months of inflorescence opening, whilst it was positively correlated with the number of female flowers produced in the inflorescence. There was a highly significant correlation between the monthly fruit set (at three month stage) and the corresponding harvested fruit yield palm -1 month -1. The lag period between flowering (opening of the female phase) and fruit maturing was eleven months.

The study estimated the carbon sequestration potential of 25 year old Tall x Tall coconut (Cocos nucifera L. variety typica) plantations under S 2 (highly suitable for coconut) and S 4 (moderately suitable for coconut) soils in wet (WL 3 , high moisture availability), intermediate (IL 1a , moderate moisture availability) and dry (DL 3 , low moisture availability) agro-climatic conditions during May to September 2009. Variation in total carbon stock (plant and soil), total carbon input (measured as Gross Primary Production of coconut, GPP), total carbon output (measured as plant and soil respiration) and net carbon balance of ecosystems were assessed. Eight coconut palms and sample plots per site were used for estimations (two factor factorial with eight replicates). There is a significant agro-ecological region (AER) x land suitability class (LSC) effect on all the components of the carbon balance in a coconut plantation. The total ecosystem carbon stock (B tot-eco ) reduces along a decreasing moisture gradient from WL 3 to DL 3 and decreasing soil fertility gradient from S 2 to S 4 . The GPP and R tot-eco do not show a reduction along a decreasing moisture gradient from WL 3 to DL 3 on S 2 , whilst it shows a reduction from WL 3 to DL 3 on S 4 . The net C balance reduces from WL 3 to DL 3 on S 4 , whilst it does not reduce from WL 3 to DL 3 on S 2 . C stock of the ecosystem varied between 32 and 72 Mg C ha -1 whilst the net carbon balance varied between 0.4 and 1.9 Mg C ha -1 month -1 under different growth conditions. Of the measured components, GPP and maintenance respiration of coconut palms and soil respiration had greater contributions to the overall C balance of the system. This is the first report on carbon sequestration potential in coconut plantations of Sri Lanka.

The net assimilation rate, carbohydrate content in leaf and trunk tissues and quantum of carbon removed in sap-producing (SP) and nut-producing (NP) coconut palms were compared. The correlations between sap (toddy) yields of SP palms, and their leaf and trunk carbohydrate content; net assimilation rate; and the pre-tapping phase nut yields, were investigated as possible criteria for selecting coconut palms with potential for high toddy yields. Thirty-five-year-old coconut palms of the Tall variety (Cocos nucifera L., var. typica), at Bandirippuwa Estate, Lunuwila, Sri Lanka were used for the study. Total soluble sugar content (TSS) in leaf and trunk tissues was higher (62-73 mg g dw -1 ) than their starch content (24-41 mg g dw -1 ) in both SP and NP palms. In SP palms, TSS of leaf tissue was higher, and trunk tissue was lower, than in NP palms. The total carbohydrate (TC) content in the trunk was generally higher than in leaves of both SP and NP palms. In SP palms, the ratio of TSS : starch was higher in leaves, and lower in the trunk, than in NP palms. Net assimilation rates and carbon removal by the produce (nut or sap) was similar in NP and SP palms.There was no significant correlation between sap yields and TSS and starch contents of leaf or trunk tissues of palms, before and during tapping; and the nut yields before; and the NAR during tapping. These parameters are therefore not of predictive value for selecting coconut palms for high sap yield.

SUMMARYSuccessful fruit set in coconut depends on several reproductive processes including pollen germination and pollen tube growth. High temperature (˃33 °C) during flowering reduces fruit set in coconut. Therefore, identification and development of coconut varieties or hybrids with high reproductive heat tolerance will benefit the coconut industry in view of the climate changes. This experiment was conducted to quantify the response of pollen germination and pollen tube growth of seven coconut hybrids to increasing temperature from 16 to 38 °C. A Principal Component Analysis (PCA) was carried out to classify coconut hybrids on the basis of their temperature tolerances to pollen germination. Pollen germination and pollen tube length of the hybrids ranged from 56 to 78% and 242 to 772 µm, respectively. A modified bilinear model best described the response to temperature of pollen germination and pollen tube length. Cardinal temperatures (Tmin, Topt and Tmax) of pollen germination and pollen tube length varied among the seven hybrids. PCA identified Tmax for pollen germination and Topt for pollen tube length as the most important parameters in describing varietal tolerance to high temperature. PCA also identified SLGD × Sri Lanka Tall and Sri Lanka Brown Dwarf × Sri Lanka Tall as the most tolerant hybrids to high temperature stress and Sri Lanka Tall × Sri Lanka Tall and Sri Lanka Green Dwarf × San Ramon as less tolerant ones based on cardinal temperatures for pollen germination and pollen tube length. Tmax for pollen germination of the most tolerant and less tolerant hybrids were 41.9 and 39.5 °C, respectively. Topt for pollen tube length in the most tolerant and less tolerant hybrids were 29.5 and 26.0 °C, respectively.