For a reasonable understanding of the difference of weed ecological distribution caused by soil moisture conditions in non-cultivated paddy fields, a growth analysis experiment was carried out on several Gramineous summer species under three different soil moisture levels. Soil moisture treatments were started at the early growth stage of plants with five to seven leaves, by means of keeping the soil submerged under water, wet, and dry conditions. The wet and dry conditions of the soil were controlled by means of constant capillary tention method. Results obtained are as follows: 1. It was found that each of the species in this experiment exhibited a tendency to flourish better in an optimum soil moisture, required for their growth. They were grouped under three categories according to their growth response to the different soil moisture conditions. Some species showed the tendency of declining relative growth rate (RGR) as the soil moisture decreased (type-I), whereas some other species showed the highest RGR under wet conditions (type-II) and the remaining species scarcely reduced the RGR even under drought conditions (type-III) (Fig.3). 2. Change in the two growth parameters, the net assimilation rate (NAR) and the leaf-area ratio (LAR), depending on soil moisture conditions were different for each of the three types. LAR of the species in type-III and NAR of the species in type-II were more affected by soil moisture conditions in these two types, whereas type-I species were little affected by soil moisture conditions in both the parameters (Fig. 4). 3. Comparing the growth response to soil moisture of C3 with C4 plants, the results showed that C4 species had a tendcncy of higher RGR and LAR under relatively dry conditions, while C3 species grew better under relatively wet conditions. 4. Results showed that the ecological distribution of the weeds in non-cultivated paddy fields, as for their growth response was concerned, seemed to be attributed to the soil moisture.
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