Seed quality and harvest time have a strong relationship with seed viability and vigor. Despite being a genetic characteristic, seed storability is strongly influenced by the pre-storage history of the seed, by seed maturation, and by environmental factors during the pre- and post-harvest stages. Physiological maturity occurs when a seed reaches its maximum dry weight with minimal seed moisture. Therefore, it is the most appropriate time to harvest the seeds to ensure their germination and viability. With regard to genotypes, ambient temperatures prevailing during harvest and other factors, seed quality between physiological maturity (PM) and harvest maturity (HM) varies significantly. Thus, it is vital to harvest seed crops at the optimal stage of seed maturation in order to produce the highest quality seeds.
Harvesting the seeds within the correct range of seed/cob moisture will maximize seed yield and quality and minimize mechanical injuries during processing. Swift changes occur in both the dry matter and moisture content of the maize cob during the period of seed maturation. Loss of moisture from the cob beyond physiological maturity determines seed moisture at harvest. Physiological maturity is always defined as the time at which seeds reach their maximum dry weight. The increase in dry weight during seed maturation more than compensates for the net loss of moisture from the cob. Cob moisture reduced from 50.76 % to 25.11% from 40 DAT to 60 DAT (Fig 1). Here the loss in moisture was 46.39% during seed maturation (40-50 DAT) as against 7.72% loss during physiological maturity to harvestable maturity (50-60 DAT). In the early stages of seed development, seed moisture remains high, and dehydration is a slow process. However, it accelerates when the seed reaches its maximum dry weight and continues to decrease until hygroscopic equilibrium is reached. That is the stage where the dry weight of seeds starts declining along with moisture and indicates the maturation of seeds. A similar pattern was observed in the development of maize seeds, where seed moisture decreased from 20.62 to 16.21 (40 - 45 DAT) and thereafter to 10.08% at 60 DAT (Fig 1). Prevailing temperature during seed maturation may also have its impact on loss of seed moisture. There is growing evidence supporting the claim that chickpea seeds harvested 29 days after anthesis (DAA) showed the highest moisture content percentage, while chickpea seeds harvested 45 days after anthesis (DAA) showed the lowest moisture content percentage (Mehta et al., 1993).
The moisture content is high during early harvesting, which has a direct impact on the dry weight, which was lower during early harvesting. The highest moisture content was found when seeds were collected at 30 days after flowering (DAF) and the lowest moisture content was found when seeds were collected at 40 days after flowering (DAF) (Mahesha et al., 2001).
Fig 1: Relationship between harvesting intervals on seed quality parameters.
Dehydration and seed filling occur simultaneously during seed maturation. It was found that seed moisture content decreased simultaneously with an increase in test weight, which was reflected in the germination percentage at 50 DAT (Shete et al., 1992).
In agreement with the previous reports the increase in maize seed moisture at 40 and 50 DAT concurrently resulted in accumulation of dry matter content which was evident from the hundred seed weight where the increase was 22.38% up to 50 DAT (Fig 1) indicating the progressing seed development and after 50 DAT the increase in dry mass was stagnant showing the attainment of seed maturity. The development of seeds begins with fertilization and continues until they reach physiological maturity. Seed maturation increases as time passes. As seed maturity approaches physiological maturity, its level of maturity increases (Purnomo D and Sitompul S M 2006
). As determined by the weight of 100 seeds, seed harvested from 50 DAT tends to be more vigorous than seed harvested at late harvest.
A seed’s dry weight is believed to be an accurate indicator of the seedling’s vigor since it gives an accurate estimate of the amount of stored reserves in the seed. Dry weights of seeds were significantly different between early harvests and delayed harvests. It may be due to low dry matter accumulation at early harvests and deterioration in seeds caused by physical factors at later harvests. The accumulation of dry matter content was evident from the hundred seed weight where the increase was 18.13 to 23.36 g (Fig 1) indicating the progressing seed development and after H3 the increase in dry mass was stagnant showing the attainment of seed maturity. A characteristic of mature seeds is their maximum dry weight, which identifies them as having reached physiological maturity.
The amount of dry matter accumulated in seeds is one physical indicator of seed maturity, while other non-physical or physiological indicators include the viability and vigor of the seed. Physiologically mature seeds are highly vigorous and can be stored for longer periods of time. Due to lack of development and maturation, lower germination rates of 52% and 72% were observed in H1 and H2. After physiological maturity i.e
., at 50 DAT higher germination of 96% was noticed in H3. The same trend was observed in speed of germination (Fig 2). During early harvest, the germination rate and speed of germination were significantly lower than those observed in later harvests. Germination percentages also tend to decline after physiological maturity is reached. The low germination rate of early harvested seeds may be due to the presence of a greater number of immature seeds. Despite the fact that the crop had been harvested earlier, the seed quality of the crop was poor due to immaturity (Jayaraj and Karivaratharaju, 1992
). Mehta et al., (1993)
reported that seeds harvested before 33 DAA exhibited a drastically reduced germination percentage and significant differences were also noted in germination due to harvesting. Generally, germination percentages were highest in the H2 stage, i.e
. seeds collected at 35 days after flowering (DAF), and the lowest in the H1 stage, i.e
. seeds collected at 30 days after flowering (DAF) (Mahesha et al., 2001).
In addition to decreasing seed moisture content, seed germination percentage increased with the advancement of harvesting dates (Shete et al., 1992).
Delaying of harvest after seed maturity may be the result of the onset of ageing biochemical processes after physiological maturation (Bewley and Black, 1985
; Powell, 1988
) and germination was only 90% at 60DAT as against 96% when harvested during 50DAT.
Fig 2: Effect of harvesting intervals on speed of germination and vigour index II.
The highest vigour index I and II was observed in H3 (3755 and 229) and the lowest vigour index is observed in H1 (1274 and 82) and H2 (2433 and 137) (Fig 2 and 3). It is possible that increased seed vigor index is related to seeds maturing in the H3 stage resulting in better germination percentages and seedling lengths.
Fig 3: Effect of harvesting intervals on germination and vigour index I.
Conductivity testing of seeds usually involves cellular membrane integrity or its manifestation determined by potentiometric measurements on hydrated seeds or on seed steeped in water and the association of such measurements with seed quality involving vigour and viability. A highly negative correlation was observed between EC and germination rate (Fig 3). Seed germination was negatively correlated with EC, which indicated that a greater amount of leachate was escaping from a low quality seed i.e
., immatured or developing seeds. High EC of seed (0.30) at 40DAT is assumed due to membrane deterioration during the imbibition period of lower quality seeds. Decrease in electrical conductivity was not significant after H3, which indicates the attainment of seed maturity. There is a pattern of variation in electrical conductivity values recorded during the maturation process of seeds, as reported by Powell (1986)
, which has been associated with the development of cellular membranes and their structural organization associated with the maturation process of seeds. As seed maturity increases, Styer and Cantliffe (1983)
found that seed leachate conductivity generally decreases. A high correlation has been found between leachate conductivity and field emergence (Waters and Blanchette, 1983)
. The higher membrane permeability and the broken pericarps may be responsible for the greater leakage of metabolites during imbibition of seeds, even though germinability may be normal. It has been reported that EC tests performed on soybean seeds have shown a high degree of accuracy to indicate the seed performance to establish a stand under a variety of field conditions (Vieira et al., 2004).
Tests of vigour, both AAT and cold germination, confirmed the results of the germination test. During the cold germination test, differences in the vigour of maize seeds were identified, which might have an impact on the emergence of the seeds in the field. Due to decreased metabolism and inactivation of enzymes that are essential for germination, seed harvests before maturity stages are of poor quality and cannot withstand a cold test. This was evident from this study where poor germination of 44% and 66% was seen in H1 and H2 during the cold germination test. In the cold germination test, seeds harvested when they are physiologically mature performed better, resulting in a germination rate of 92% (Fig 3). The seeds from the harvest after 50 DAT were better compared to earlier harvests. Cold germination testing not only measures the percentage of viable seeds in a sample, it also reflects the ability of those seeds to produce normal seedlings under less than optimum growing conditions like those that may occur in the field. Seeds harvested at physiological maturity register proper field emergence and produce normal and uniform seedlings (Yadav et al., 2020).
Accelerated aging, another vigour test, also showed the progressive decline in viability and vigor in seeds of maize, when the seeds were harvested early. AAT always mimics natural aging and hence reflects the fate of seeds during storage. Artificially aged seeds from different early dates of harvest registered not only poor germination but also decreased seedling vigor (Fig 4 and 5).
Fig 4: Accelerated aging test on germination and vigour index.
Fig 5: Accelerated aging test on speed of germination and vigour index II.
A possible explanation for this reduction is the lowering of biochemical activities in seeds. An aging process of immature or low-quality seeds can damage the enzymes that convert reserve food in the embryo into usable forms, resulting in substandard seed quality. The germination rate for seeds taken from H3 was 94% (Fig 4) on the first day of ageing; this rate gradually decreased and reached 90% (Fig 4) after 3 days of aging, the minimum required germination per cent for hybrid maize seed certification fixed as per Indian minimum seed certification standards (IMSCS). The seeds harvested from H3 maintained above 90% of germination upto three day of aging, but in four and five days of aging, we observed germination and vigour loss in H3. The reduction in germination might be due to the degradation of mitochondrial membrane leading to reduction in energy supply necessary for germination. The seeds that attain the physiological maturity (PM) were able to tolerate the exposed aging conditions of high temperature and high relative humidity, deteriorate at a slower rate and have high germination following aging, compared to earlier harvested seed lots.
Elias and Copeland (1997) also observed that seeds harvested earlier to PM resulted with germination of 19 and 25 % during accelerated aging test of two cultivars, whereas they were 64 and 72% for the standard germination test for the same cultivars. This may be because of seeds level of maturity, which makes them sensitive to any stress such as high temperature and relative humidity in accelerated aging test. Seeds that recorded high germination are considered as more vigorous and expected to maintain superior viability during storage.
In the multiple linear regression analysis, it indicates the influence of harvesting intervals on seed quality, where the R2
value ranges from 0.6 to 0.98 (Table 1) and had positive significant relationship between different days of harvest and seed quality parameters such as hundred seed weight, electrical conductivity, germination percentage, accelerated aging, seed and cob moisture content and cold germination test.
Table 1: Multiple linear regression equations fitted to explain the influence of harvesting intervals on seed quality parameters.
As per Table 1, the multiple linear regression equation resulted in negative variables with respect to cob moisture, seed moisture content and electrical conductivity (- 1.277, -0.5236 and-0.0064), this demonstrates how changes in electrical conductivity and moisture content have a direct impact on seed quality. As a result, the positive variables of 0.2654, 1.920, and 2.080 resulted in a positive factor in multiple linear regression, which indicates that seed quality has improved as a result of an increase in seed weight, germination %, and cold germination test results.
R square value in multiple linear regression analysis (Table 1), seed moisture content and 100 seed weight cause 93%and 88% change in seed quality, which shows that moisture content and 100 seed weight directly influence seed quality.