Conditions in the field

Imagine you had one lot of a rice cultivar growing in a cool climate (20-28°C), and another lot of the same cultivar growing in a warmer climate (24-32°C). What difference would the different climates and temperatures make to the seeds’ longevity in storage? A research team from the University of Reading did some experiments to find out.

Figure 6 (below) shows the behavior of the seeds of Japonica rice, a type of rice normally grown in temperate environments. The square readings are for seeds grown in a warmer climate, the round ones are for seeds grown in cooler conditions. You can see that at both temperatures, the moisture content drops and the mass initially increases, then stabilizes at mass maturity, as you’d expect from the generalized patterns you saw in Figure 3 on the page entitled ‘Pre-storage factors’.

The graph shows changes in dry weight and moisture content of developing seeds between twelve and seventy days after flowering. The x axis shows the harvesting date (days after flowering). There are two y axes, one shows seed dry weight, the other shows seed moisture content. The curves represent two sets of seeds, grown in a warmer and cooler climate. Until about thirty days after harvesting, the weight of the developing seed increases, then flattens off at “mass maturity”. The moisture content, by contrast, continues to drop beyond “mass maturity”. The shape of this graph resembles the idealised curve shown in Figure three, and climate does not appear to make a significant difference.
Figure 6: effect of temperature during development

However, something else must be going on: the researchers discovered differences in longevity associated with the two growing conditions after those seeds have been harvested, dried and stored. The seeds grown in the cooler climate achieved higher longevity than those grown under warmer conditions. In other words, for Japonica rice, the quality of seeds grown under certain climatic conditions is better than the quality of seeds grown under other conditions.

If temperature during development can affect how seeds behave in storage, you might wonder if the presence or absence of rain (or indeed irrigation) could be another factor. Another team from Reading set out to investigate, this time using Brassica seeds with different irrigation regimes, to simulate the effect of drought.

The graph shows the effect of irrigation, or lack of it, during seed development. The x axis shows the number of days from pollination to sixty days. The y axis shows the potential longevity that can be achieved by seeds. One graph plots seeds which were continuously irrigated – they reach peak longevity at about forty days. Two graphs plot seeds which had irrigation stopped at sixteen and twenty-four days, simulating a severe and mild drought respectively. The seeds that experienced the most severe drought reached peak potential longevity earliest, at about thirty days; after that, their potential longevity drops off. The seeds experiencing a mild drought reached peak potential longevity a little later, at about thirty-six days; then they too showed a dropping off of potential longevity. The seeds that were uniformly irrigated reach their peak potential longevity even later, at about forty-five days, and they maintained their peak level of longevity for ten more days.
Figure 7: effect of irrigation during seed development

Figure 7 (above) shows three samples of Brassica. The red triangles at the base show the time of mass maturity. The graph with round points shows what happens when the parent plant was continuously irrigated. For the graph with triangular points, irrigation was stopped 16 days after pollination, and for the graph with square points, irrigation was stopped 24 days after pollination, imitating what might happen during a severe drought.

Activity 2

Allow 5 minutes for this activity

Take a look at Figure 7 and decide what changes you can see, as a result of different lengths of drought at the end of seed development. Use the text box to write your thoughts.

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Discussion

Stopping irrigation before harvest had an impact on how quickly the seeds developed: the earlier the irrigation stopped, the quicker they reached mass maturity. In the continuous irrigation group, there is a small dropping-off of potential longevity the later you harvest after mass maturity, whereas in the two experimental conditions, where an artificial drought was imposed, this decline in potential longevity is steeper.

Deciding when to harvest

Genebankers’ perspectives