Influence of Storage Duration and Storage Temperature on In-Vitro Pollen Germination of Citrus Species

Abstract

Citrus is one of the world’s important fruit crops which is widely grown in most areas with suitable climates between latitude 35°N~35°S. Citrus is the common term used for Genus, a flowering plant of rue family, Rutaceae. It falls under subfamily Aurantioideae and tribe Citreae. Although the origin of citrus is precisely not known, researchers [1-4] believe that it has the origin from south and southeast tropical and subtropical region of the world. Citrus comprises around 60 species, most of which are cultivated throughout the tropics and subtropics. They are indigenous in some parts of India, China, Northern Australia and New Caledonia [5]. Citrus in India is grown in 1.07 million ha area with a total production of 11.14 million tonnes. The most important commercial citrus groups or cultivars in India are the mandarin (C.reticulata Blanco) followed by sweet orange (C. sinensis Osbeck) and acid lime (C.aurintifolia Swingle) sharing 41, 23 and 23% of the respectively [6]. One of the routine tasks in plant evolutionary biology, pollination biology and crop breeding research is determining male fitness functions by quantifying total pollen grains per flower and proportion of viable pollen [7] [8]. The development of reliable methods for determining the functional quality of pollen helps in monitoring pollen vigour during storage, genetics and pollen-stigma interaction studies, crop improvement and breeding, and incompatibility and fertility studies [9]. The quality of pollen is assessed on the basis of viability and vigour of the pollen grain. Pollen vigour refers to the speed of germination of pollen grains and the rate of pollen tube growth [10]. In the breeding of fruit species it is very important to use a suitable method for determining pollen viability [11]. Several methods of pollen storage have been tried of which the most important factors are controlled temperature and humidity [12-14] and [15]. Different investigators have same consensus that low temperature and humidity are the two major influencing factors in storage of pollen grains for a long period of time [16, 17]. Pollen physiology especially germination and viability has received considerable attention for its application in plant breeding, conservation, adaptation and understanding of the physiological behaviour of the fertilizing pollen grains. There are several reports on pollen viability of different taxa [18-22] with varied aims and objectives. [23] Correlated pollen Abstract The present work was aimed to study the invitro pollen viability of five citrus species viz., mandarin (citrus. reticulata Blanco) sweet orange (C. sinensis L. Osbeck) grapefruit (C. paradisi Macf.) pummelo (C. maxima Burm. Merr.) and tangelo (C. reticulata × c. maxima or × C. paradise). The pollen at different temperatures (4oC, -20oC, and -196oC) and the viability was checked from 4 th up to 48 weeks. Pollen stored at low temperature (-196°C and -20°C) showed better viability percentage as compared to pollen stored at 4°C and room temperature. The maximum pollen viability was found in cultivar Mosambi (90.30%, 85.00%) followed by W.Murcott (86.30%, 79.00%) when stored at (196°C and -20°C) respectively, and it was significantly higher than all other genotypes. Minimum viability of (50.30%, 46.00%) was found in marsh seedless stored at -196°C and 20°C respectively. The lowest viability (16.30%) was found in marsh seedless at 4oC at 48 th week of storage interval week which was followed by 20.00% and 30.30% at -20oC and 196oC respectively. At room temperature storage, the pollen viability becomes zero after the first week. The results indicate that pollen viability gradually decreased upto 48 th week and pollen collected and stored at sub-zero temperatures from early blooming citrus varieties can be stored for very long period without any appreciable loss of viability and can successfully be used for throughout the blooming season for hybridization programmes by fruit breeders, for the development of new strains so as to widen the genetic base, create variability in citrus.