The effects of soil physical conditions on the growth of rice seedlings
This study was carried out to see the effects of mechanical impedance and temperature on the growth of rice seedlings and to examine genotypic responses under different mechanical impedances in soil and media. Procedures for packing soil or sand from which roots can experience uniform penetration resistance for a range of mechanical impedance treatments ( 0.05, 0.9, 1.2 MPa in soil and 1.6, 2.3, 3.2 MPa in sand) were developed using a needle penetrometer to check when uniform mechanical impedance had been achieved. Increasing the penetration resistance of the medium (soil or sand) progressively reduced the seminal root axis length, number of first order laterals, total root length and shoot length for rice varieties (BR-14 and BR-29) 9 days after transplanting of pre-germinated seeds. On this basis, similar experiments were carried out on more varieties of rice, Azucena, a long rooted tropical japonica variety. Bala, an upland indica variety with shorted roots than Azucena, and four recombinant inbred lines (F6) two of which contained Bala alleles at a root thickness quantitative train loci (QTL) on chromosome 2, and two of which contained Azucena alleles at this region. Both root and shoot growth of Azucena and Bala parents and their lines decreased with an increase in penetration resistance, but differences between Azucena and Bala varieties were apparent. Azucena and its two lines had significantly longer seminal roots, greater root diameter and larger lateral spacing than Bala and its two lines in higher penetration resistances (1.2, 2.3 and 3.2 MPa). In contrast, Bala and its two lines had a greater number of first order laterals than Azucena and its two lines in higher penetration resistances. As a result, Bala lines had significantly greater total root length then Azucena lines. Bala had a tendency of higher ratios of dry root to shoot weight with penetration resistances.