Pollen morphological study and temperature effect on the pollen germination of cashew (Anacardium occidentale L.) varieties

Abstract

Temperature is one of the main environmental factors that affects plant growth and development. Flowering phenology mainly pollination is highly sensitive to temperature extremes. Thus, pollen tests can be used as a tool to screen temperature tolerant crops. In present study, the in vitro pollen germination method was used for screening cashew varieties at different temperatures. The pollens of five cashew varieties comprising three early (VRI-3, Vengurla-4 and Ullal-3), one mid (Bhaskara) and one late (Madakkathara-2) were screened in field (in vivo) as well as at controlled temperatures from 5 to 50°C. The morphological features of pollen grains were also studied using Scanning Electron Microscopy (SEM). Significant differences in polar axis (P), equatorial axis (E) and P/E ratio were observed among five cashew varieties. Early varieties had high P and E. Pollen germination under in vivo was high for early types with optimum temperature (Topt) of 25°C while it was low in mid and late types with Topt of 30°C. The in vitro study further confirmed this finding, pollen germination at 25°C was 75% in early types while it was 57.8 and 50.5% for mid and late types respectively at 30°C. However, mid and late varieties exhibited better tolerance to high temperature beyond 40°C suggesting their wider adaptability to high temperature. The high reducing sugars mainly glucose and fructose in pollens of early varieties may be correlated to high pollen germination in response to temperature. Overall, the varietal differences in pollen behaviour can be used to select cashew varieties for evaluating their adaptability to temperature extremes during flowering across various regions.