RICE RATOONING - IRRI books - International Rice Research Institute

VARIETAL IMPROVEMENT FOR RICE RATOONING 249Nutrient reservesGenotypic. Ratooning ability of Mingolo variety has been related to highcarbohydrate concentration (11). Other varieties were found to differ in timing ofdeposit and in final quantity of reserves in the stems (21).Environmental. In studies where nutrient (carbohydrate) supply in the stemswas manipulated, ratoon growth was seen as an indicator of the physiologicalsituation of the crop (20, 29). Percentage of ratoon tillers, ratoon height, and ratoonweight depend on total available carbohydrate and weight of the stem base. Insource-sink manipulations, subsequent ratooning was stimulated by prior removalof competitive sink and hampered by leaf pruning (29).Agronomic practices, such as delay in cutting, cutting height, etc, have beenrelated to ratooning ability (21). Conservation, or proper translocation, of nutrientsresults in better ratooning ability. Main crop sterility encouraged earlier ratoon cropdevelopment because it removed competitive sink. The post-grainfill accumulationof carbohydrates is in the lower 20% of stems; successful ratooning from a lowcutting height depends on late cutting, to give time for reserves to move down.Main-crop photosynthetic tissue and rootsGenotypic. Slow senescence has been mentioned as a varietal trait that facilitatesvigorous ratoon growth because the new growth uses the remaining main-cropphotosynthetic surface (11).Environmental. Early ratoon growth (up to 2 wk after main crop harvest) wasfound to depend chiefly on the main-crop root system (8). Lower cutting heightsslow early ratoon growth and lengthen ratoon crop duration (26).Total maturity durationGenotypic. This crucial factor in the success of ratooning technology is stronglygenotypically controlled. Many authors have investigated the need for medium- toshort-duration main crop varieties. In one study, panicles emerged from stemcuttings of variety PTB10 1 mo after transplanting; in variety PH37, this took 2.5 mo(23). Early-maturing varieties Dhairal and CH-4-10 were suitable for ratooning;late-maturing Charmock was not (30). Earliness was also favored for ratooningability (15). A photoperiod-sensitive Mahsuri mutant, CRM 10-3628, useful as aratoon crop, enabled the first ratoon crop to start in mid-October (1). The ratooncrop outyielded the main crop (3.3 t/ha vs 2.7 t/ha). Lateness in ratooned rices wasassociated with lateness in the main crop (25). Early flowering of the ratoon wasrelated to high carbohydrate concentration in the stems. But late-flowering F 2segregants were disproportionally good ratooners (11).Environmental. Maturity took longer with taller cuts (25). But a lower 5-cmcutting height delayed maturity 9 d beyond a l5-cm cutting height (26). Cutting at5-25 cm high gave ratoon crops 2-3 wk later than cutting at 45 cm high (15). Longgrowth duration induced by long photoperiods correlated with low ability toregenerate ratoon tillers, probably because of low N content in the late-floweringplants (16).