Responses of Rice to Low P Supply in Alternate Aerated and Stagnant Solution Culture

Abstract

Rainfed lowland rice usually encounters intermittent aerobic and anaerobic conditions. Water supply for rainfed lowland rice comes from rainfall, which can not be controlled in the amount and timing during the crop season, and resulting in variation in the supply of oxygen and phosphorus. Rice grown under such condition has to adapt to environment with varying oxygen and P availability. These adaptations of rice may affect growth and nutrient uptake. The objective of this study was to evaluate the growth, morphological and physiological responses in rice as affected by oxygen regimes and P supply. Pregerminated seeds of a high yielding Thai rice cultivar, Chainat 1, were grown in aerated full strength nutrient solution for 10 days, then transferred to treatments with added low P (2 ppm) or high P (8 ppm) in aerated (A) or stagnant nutrient solution (S, with 0.1% agar) for 12 days. After that, plants from each P level were split into two groups. One group continued in aerated (AA) or stagnant (SS) solution as before.  The other group of aerated plants were transferred to stagnant solution (AS) and stagnant plants transferred to aerated (SA) condition. Before the transfer and 8 days after the transfer plants were assessed for tiller number, root and shoot dry weight, length of the longest roots, number of adventitious roots, root porosity and adventitious root examined for aerenchyma formation (5 cm from tip). The rice plants tillered less in stagnant than in aerated condition throughout the entire experimental period. This difference was greater in high P than in low P. The effect of aeration on plant dry weight became significant in low P at 20 days, although it was not evident at 12 days. Roots of aerated plants were almost twice as long as roots in stagnant solution, aerated plants in low P also had longer roots than in high P. Aeration increased root:shoot ratio (R:S) in low P, although it had no effect in high P. However, plants in stagnant solution had more adventitious roots, especially with high P.  Roots in stagnant solution had higher porosity (% gas space) than

those aerated. This difference also associated with greater extent of aerenchyma formation in stagnant than in aerated solution. Eight days after the transfer from A to S (AS), the tillering, plant dry weight and root  elongation were slowed down compared with those kept in aerated all the time (AA). In both low P and high P, roots elongated more slowly when transferred from A to S compared with plants kept in AA. Transferring from A to S, the R:S slowed down compared with plants kept in AA. However, when transferred from A to S adventitious root number and root porosity increased compared with plants kept in AA. The effects of low P and high P on these were not seen during 20 days when plant kept in A or AA, but after transfer from A to S the adventitious root number and root porosity were greater in high P than in low P. In contrast, after transfer from S to A, the tiller number, dry weight, root length and R:S increased compared with plants kept in stagnant all the time (SS). When transferred from S to A, adventitious root number increased slowly compared with that kept in SS.

                The rice plant responded to P in both aerated and stagnant conditions, but the response to P stronger in stagnant than in aerated condition. The effects of transferring between aerated and stagnant solution were measurable after 8 days.