EFFECTS OF PRE-GERMINATION TREATMENTS PART 3
EFFECTS OF PRE-GERMINATION TREATMENTS,
DESICCATION AND STORAGE TEMPERATURE ON
GERMINATION OF CARISSA EDULIS, VANGUERIA
MADAGASCARIENSIS AND XIMENIA AMERICANA SEEDS
Indigenous wild fruits in Kenya provide an important supplement to diets of local people (Muok et al. 2001). Some fruits are rich in sugars, essential vitamins and proteins (Maundu et al.1998). In southern Africa, the sale of Zizyphus mauritana fruits is beginning to contribute to commercial trade (Kwesiga et al. 1996), whereas fruits of Vitellaria paradoxa and Parkia bigloea are harvested and marketed (Cooper et al. 1996). The expansion of agriculture into habitats previously occupied by undomesticated species and over-exploitation of these species are threatening this genetic resource with extinction as trees can no longer recover through natural regeneration (Cunningham & Mberikum 1993). In addition, wild fruit trees have received little attention in research and development compared with exotic varieties because they are perceived as slow growing, producing fruits after several years and are irregular in fruiting. There is a need to gather, cultivate, select and
improve these wild fruits in order to exploit their potential and conserve a valuable genetic pool. This requires understanding of the propagation requirements and seed behaviour under storage. The three main categories of seed storage behaviour based on physiological storage potential are orthodox, recalcitrant and intermediate seeds (Hong & Ellis 1996, Schmidt
2000). Orthodox seeds can be stored at low seed moisture content and low temperatures (Dullooet al. 2000) and longevity of seeds increases with decrease in seed storage moisture content (Roberts 1973). Recalcitrant seeds are shed when
seed moisture is high and usually start germinating soon after shedding and often before shedding (Rajeswari & Kaveriappa 2000). These seeds are sensitive to desiccation and loseviability on drying (Berjak & Pammer 1995). Chilly temperatures also damage seeds of tropical species. Intermediate seeds exhibit some degree of desiccation tolerance and freezing sensitivity. This categor y of seeds is damaged after desiccation to seed moisture content of 7–12%. Seeds of tropical species die more rapidly when temperatures are below 0 °C and in some cases temperatures below 10 °C kill seeds (Hong et al. 1998). The objective of this study was to determine the germination requirements and seed longevity
under varied temperature and moisture regimes for Carissa edulis, Vangueria madagascariensis and Ximenia americana seeds. These species are among the important indigenous dryland fruit species that require domestication. They are threatened with over-exploitation and in addition there is widespread destruction of their natural habitats for human settlement (Kigomo 2001).
MATERIALS AND METHODS
Seeds were collected from Kisanana division of Koibatek district (0° 25' S, 0° 25' N; 35° 30 W', 35° 15' E; 880–1000 m asl) in the Rift Valley, Kenya. Rainfall ranges from 300–750 mm per annum and is erratic and unreliable. Ripe fruits were harvested by hand. A total of 5 kg of C. edulis, 10 kg of V. madagascariensis and 20 kg of X. americana fruits were harvested from at least 25 trees maintaining an isolation distance of 100 m between trees. The fruits were transported in perforated polythene bags to the Kenya Forestry Research Institute Seed Centre where seeds were extracted and cleaned. Fruits were de-pulped by hand to minimize mechanical damage. Carissa edulis seeds were squeezed through a 2 mm sieve leaving the pulp; V. madagascariensis fruits were soaked in cold water for two days and the nut cracked using a knife to release the seed; while
X. americana fruits were rubbed against a wire under continuous running water until all the pulp was removed. Seed quality was assessed visually whereby damaged, infested or deformed seeds were discarded. Experiment 1: Germination capacity after 0 and 3 months of storage Two hundred seeds each of C.edulis and V. madagascariensis and 80 of X. americana were subjected to the following treatments: T1––control; T2––wa s h e d i n t a p w a t e r ; T3––nipped, i.e. seeds were nipped at the distal end taking care that no injury occurred to the embryo; T4––soaked in tap water for 24 hours at
room temperature; T5––soaked in tap water for 24 hours and then washed in running tap water; T6––nipped then soaked in tap water for 24 hours; T7––soaked in hot water for 1 hour, i.e. seeds were soaked in water previously heated to boiling point;T8––soaked in hot water for 1 hour and then washed in running tap water and T9––nipped then soaked in hot water for 1 hour. Seeds from each treatment were sub-divided into two lots. Seedlot A was used to test germination after applying the pre-germination treatments, while seedlot B was used to test germination after three months of storage at room temperatures (20–25 °C). Seeds were soaked in 1% sodium hypochloride solution for 10 min to sterilize the surface then
dried using blotter sheet prior to germination test. Seeds were germinated on 1% agar solution in a germination cabinet set at 20–25 °C. The design was a complete randomized design with four replicates. Ten seeds were used per replicate for X. americana due to lack of seeds and 25 seeds for the other species (ISTA 1993a, b). The numbers of germinated seeds were removed after counting and observations terminated when no further germination occurred.
Experiment 2: Effects of desiccation on seed germination
The initial and target seed moisture contents (TSMC) were determined according to Hong and Ellis (1996). Anhydrous silica gel at the ratio of 1 seed: 2 silica gel by weight was used to desiccate the seeds: C. edulis––29.7, 25.3, 20.1, 14.7, 9.7 and 5.1%; V. madagascariensis––17.7, 11.5, 9.9, 5.8 and 3.1% and X. americana––22.5, 12.5, 9.5, 5.4 and 3.8%. Germination test was conducted on 25 seeds per species replicated five times and the procedures were as in experiment 1.
Experiment 3: Effects of storage temperature on seed longevity
Seeds at TSMC were stored in airtight plastic containers for one, two and three months at either 25, 10, -3, or -20 °C. Each treatment consisted of 25 seeds replicated four times. Seed germination was tested after each duration of storage as in experiment 1.
The cumulative number of seeds that had germinated was expressed as a percentage of the total number of seeds germinated in each treatment and subjected to ANOVA. Least significant difference was used to separate
significant means. desiccation of these seeds suggests that the critical lower moisture threshold was about 20% below
which germination was reduced to less than 50% (Table 2). However, the initial SMC was lower than that suggested for this category of seeds (Hong & Ellis 1998). The relationship between SMC and viability of X. americana indicates that the seeds exhibit intermediate storage behaviour which concurs with the observation by Fletcher and Pritchard (2000) but contradicts that of Msanga (1995) in that X. americana seeds tolerate desiccation. Germination of V. madagascariensis was not affected by drying indicating that they are orthodox seeds
RESULTS AND DISCUSSION
Germination after pre-treatments and after
three months of storage
RESULTS AND DISCUSSION
Germination after pre-treatments and after three months of storage
The pre-treatments did not improve overall germination (Table 1). Simple treatments such as the control (TI) and soaking in tap water (T4) resulted in good germination (> 80%), which are in agreement with the findings of Prins and Maghembe (1994). Germination started on day 3 and was completed by day 8 in control treatments of C. edulis and V. madagascariensis. The satisfactory (> 90%) and quick germination indicates lack of seed dormancy. However,
germination in X. americana started on day 16 and continued till day 38, which suggests the existence of some form of physiological dormancy. Rapid germination may be an adaptation to produce seedlings to escape the adverse effects of unfavourable moisture conditions found in the area.
In nature, seeds ripen towards the beginning
of the rainy season, facilitating germination soon
after fruits have fallen from the tree. Washing C.
edulis seeds (T2 and T5) resulted in reduction in
germination by 21–26%. Seeds of this species are
small with a fairly exposed embryo. The embryo
may have been damaged during the pretreatments,
resulting in reduced germination.
This observation is consistent with that of Msanga
In nature, seeds ripen towards the beginning of the rainy season, facilitating germination soon after fruits have fallen from the tree. Washing C. edulis seeds (T2 and T5) resulted in reduction in germination by 21–26%. Seeds of this species are
small with a fairly exposed embryo. The embryo may have been damaged during the pretreatments, resulting in reduced germination. This observation is consistent with that of Msanga (1995).
Nipping (T3) seeds also resulted in reduced germination for all species. It appears that either the treatment caused embryo injury or that the seed’s internal structures might have been damaged by unimpeded swelling (Willan 1985). Exposure to boiling water was lethal to all seeds (T7, T8 and T9). This is in agreement with other observations on tropical seeds (Prins &
Maghembe 1994, Schmidt 2000). Similarly, seed ageing resulted in reduction of germination of all seeds but with greater
reduction on the germination of nipped C. edulis and X. americana seeds. Seeds of these species should be sown fresh at ripening
Effects of drying on seed germination
Drying seeds did not improve germination. The pattern of germination on C. edulis seeds was typical of that exhibited by intermediate seeds (Hong & Ellis 1996). The sensitivity to desiccation of these seeds suggests that the critical lower moisture threshold was about 20% below which germination was reduced to less than 50% (Table 2). However, the initial SMC was lower than that suggested for this category of seeds (Hong & Ellis 1998). The relationship between SMC and viability of X. americana indicates that the seeds exhibit intermediate storage behaviour which concurs with the observation by Fletcher and Pritchard (2000) but contradicts that of Msanga (1995) in that X. americana seeds tolerate
desiccation. Germination of V. madagascariensis was not affected by drying indicating that they are orthodox seeds.
Effects of seed moisture content and storage temperature on seed longevity
There were variable survival patterns under different seed storage conditions. Germination of C. edulis seeds with 20.129.7% SMC stored at 25 °C exceeded 80% during the three-month storage period (Table 3). However, a sharp
reduction in germination was observed for seeds with < 20.1% SMC irrespective of storage temperature and was lowest in seeds stored at -20 °C. This behaviour confirms the intermediate seed storage characteristics of C. edulis
The best consistent germination (≥70%) for