Given the high germination rates for all cultivars, the consistently greater index for M-209 reflects a greater degree of apparent vigor for that cultivar. Greater than 75% of seedlings emerged from 2.5 cm, 5.1 cm, and 7.6 cm planting depths after 8 DAP, 10 DAP, and 14 DAP, respectively. Consequently, below-soil elongation data reported are for up to those dates only. Total below-soil elongation over time of each cultivar followed a similar pattern for each seeding depth: M-209 tended to the most rapid elongation, followed by M-205, M-206, and M-105, respectively, in decreasing relative elongation rates. For each cultivar, total elongation by 8-10 DAP was no different regardless of planting depth , suggesting that total cultivar elongation ability was independent of effects of planting depth under the study conditions. Turner et al. similarly found that depth did not play a role inelongation rates of either coleoptiles or mesocotyls of semidwarf or conventional rice in well watered soils. M-205 and M-209 total elongation rates were unchanged from 10-14 DAP at 7.6 cm depth, while M-105 and M-206 total elongation rates slowed by 69% and 71%, respectively. Coleoptile elongation was also similar for each cultivar at 8 DAP for all planting depths, yet by 10 DAP at 7.6 cm depth, differences between cultivars became more apparent . Interestingly, industrial drying rack coleoptile elongation slowed for all cultivars after 10 DAP at 7.6 cm depth, regardless of coleoptile length or the relative soil depth of coleoptile tips.
M-205 coleoptile length at 7.6 cm planting depth only increased 2% from 10 to 14 DAP, yet its mesocotyl length increased more than threefold over the same period, resulting in a nearly linear total elongation. Mesocotyl elongation rates increased for all cultivars after 8 DAP at the 5.1 cm and 7.6 cm planting depths . Irrigation occurred at 8 DAP, therefore it is possible that increased plantavailable water enhanced cellular expansion after that date, however it is also possible that a decrease in available O2 subsequent to soil saturation may also have contributed to the rapid increase in mesocotyl elongation. Rice coleoptile elongation is well known to be enhanced by hypoxic or anoxic environments due to heightened α-amylase activity and fermentative metabolism , yet research detailing mesocotyl elongation in response to hypoxia is less common. Raskin and Kende found that hypoxia increased elongation of both coleoptiles and mesocotyls, though Huang, Greenway, and Colmer only observed mesocotyl elongation enhancement under anoxia that was present at imbibition, as opposed to anoxia imposed several days afterward. In this study, mesocotyl lengths were far more variable than coleoptiles between cultivars at any depth, and between depths for a given cultivar and DAP. For example, observed mesocotyls were far shorter at 8 DAP for all cultivars at 5.1 cm and 7.6 cm depths, compared to the same DAP at 2.5 cm planting depth . However at 10 DAP mesocotyl elongation was similar for all cultivars at both 5.1 cm and 7.6 cm depths. Beyond 10 DAP, mesocotyl elongation for all cultivars at 7.6 cm depth proceeded relatively linearly, with M- 205 having the most rapid elongation at 0.31 cm / day, and M-206 having the slowest at 0.05 cm / day.
M-209 had significantly greater total and coleoptile lengths than the other cultivars, , yet M-205 and M-209 had no differences in final mesocotyl lengths , though both of those were significantly longer than the other cultivars’ mesocotyls. It has been previously reported that semidwarf rice cultivars do not establish well under deeper seeding, compared to conventional-stature cultivars. Earlier research demonstrated a correlation between mesocotyl length and vigor in semidwarf and taller cultivars . Semidwarf cultivars tended to have shorter mesocotyl length, as well as reduced stand establishment and reduced early-season growth rate when seeded at greater depths. However, McKenzie et al. found that crosses of a semidwarf line with a conventional line showed no difference in seedling length between semidwarf and taller descendants, but all seedlings were significantly longer than the semidwarf parent. More recently, Ju et al. found that several semidwarf cultivars had higher rates of mesocotyl elongation than conventional lines, and two long-mesocotyl semidwarf lines had the highest emergence percentages from up to 6 cm soil depth. In the present study the final mesocotyl lengths for all cultivars at the deepest planting were 1 cm or greater. The relative proportion of mesocotyl length to total length by 14 DAP at 7.6 cm depth was 0.19 – 0.2 for all cultivars except M-205, which hada mesocotyl – to – total length ratio of 0.25. It is unclear why M-205 had a greater proportional mesocotyl length, though the present results suggest that the increased mesocotyl elongation rate may provide some vigor advantage over M-105 and M-206.Rice seedlings began to emerge by 4 DAP at 1.3 and 2.5 cm depth plantings, and by 5 DAP at all deeper plantings. However, at 7.6 cm depth only M-209 had any emergence until 8 DAP. M-205 and M-209 tended to emerge earlier and more rapidly than the other cultivars, across planting depths . In 2.5 cm plantings, M-105 and M-206 had more rapid emergence rates around T50, although at deeper plantings, M-205 and M-209 generally had larger b and smaller T50 values, indicating early and rapid emergence from 3 cm or greater planting depth for those cultivars. Overall, M-205 and M-209 had the lowest time to 50% emergence for all depths except surface plantings.
Plotting the reciprocal of e against planting depth revealed consistently greater emergence rates for M-205 and M-209. These values decreased linearly with depth for all cultivars, however the slopes of M-205 and M- 209 were less steep than for M-105 and M-206 . Traits conveying higher seedling vigor would be expected to contribute to earlier and more rapid crop emergence , thus contributing to synchronous stand emergence and development. By these measures, high vigor traits seen here in M-205 and M-209 should facilitate early-season crop and weed management . There were no significant differences in total emergence between cultivars seeded on the soil surface, but differences in total emergence became apparent between cultivars when seeded atany below-surface depth . Total emergence was greater than 95% for all cultivars at 1.3 cm and 2.5 cm planting depths, and all cultivars showed decreasing total emergence as burial depths increased . Reductions in total emergence from 1.3 cm to 7.6 cm depths were greatest for M-105 and M-206 at 79% and 76% respectively, followed by M-205 and M-209 at 56% and 38%, respectively. Relative growth rates of emerged seedlings were similar over time, however M-205 and M-209 were taller than the other cultivars at 0 cm, 6.4 cm, and 7.6 cm depths from 2 WAP onward . Total 8 WAP height responses to seeding depth were varied. Between 5.1 cm and 7.6 cm M- 105 and M-206 heights decreased 8.8%, and 4.9%, respectively. In contrast, M-205 and M-209 plant heights increased 5.2% and 5.7%, respectively, drying rack cannabis between 5.1 cm and 7.6 cm depths. Seeding depth had no effects on the length of time to first tiller appearance for any cultivar, though M-205 and M-209 had somewhat higher tiller numbers at each depth over time. At all depths M- 205 and M-209 increased tillers at greater rates than the other cultivars, yet tillering at 8 WAP was only significantly different at 6.4 cm and 7.6 cm depths . It is interesting that tiller initiation and total tillers were largely unchanged across depths, and that M-205 and M-209 saw tillering increases at the greatest depths. Reductions in tillering and plant height with increased crown soil depth are well documented in other cereals. Hucl and Baker found slight but non-significant reductions in rate of tiller emergence and total tillers plant-1 with increasing crown depth from 3 cm to 6 cm in Canadian spring wheat, and studies on European wheat and barley cultivars saw decreases in full-season tillering and height at depths greater than 5 cm . In contrast, other studies found slight increases in wheat tiller number from 3 cm to 6 cm planting depths , and from 5 cm to 10 cm planting depths , although tillering decreased with greater planting depths in both cited studies. It is possible that the observed tillering and height accumulation advantages of M-205 and M-209 would be transient if this study progressed to heading or grain harvest. By way of example, cultivar-specific advantages in above ground growth rate early in the season would be expected to be overcome by the time of heading, as final plant height is generally a fixed characteristic of a cultivar .
However, in this study we saw that M-205 and M-209 had greater rates of below ground elongation overall, as well as greater above ground height and tiller number over time at the deepest plantings. This suggests that these cultivars, though able to mobilize seed reserves to elongate and emerge more quickly, did not suffer for their early vigor in comparison to the less vigorous cultivars in the study. The data on height and tillering responses observed in this study are insufficient to predict whether these cultivars would respond similarly in the field; however, further studies on the effects of increasing crown soil depth on these cultivars’ growth and development at a field scale should shed light on these characteristics, and their potential effects on yield. Leaves per plant only differed where tiller number was significantly different. No differences were found in leaves per tiller across all cultivars and depths . Dry weights at each depth were only significantly different between cultivars at 0 cm, 6.4 cm, and 7.6 cm depths , and likely reflected the combined effects of final height and tiller number. Cultivar dry weights across were no different between 1.3 cm and 5.1 cm plantings. Between 5.1 cm and 7.6 cm planting depths, M-105 and M-206 dry weights were reduced by 45% and 50%, respectively, whereas M-205 and M-209 dry weights increased by 31% and 58%, respectively.Direct comparisons between below-soil elongation and parameters of emergence and development were not possible in these studies. However, our observations of differences between cultivars in seedling organ elongation and early-season growth parameters generally agreed with previous research linking elongation to rates of emergence, height, or tillering . As new cultivars are developed, seedling organ elongation rates might be used to by breeders to identify seedlines that are well-suited to the stresses of deep dry seeding. Our combined findings suggest that the observed vigorous characteristics of M-205 and M-209 seedlings should allow them to emerge rapidly and evenly several days after planting, if drilled to depths greater than 3 cm the field. This in turn would facilitate the usage of a stale seedbed technique without a delay of planting, and allow the application of non-selective herbicides to control early-emerging and resistant weeds without damaging emerging rice. In addition, if M-205 and M-209 demonstrate vigorous early growth and tillering in the field, that may hasten canopy closure, potentially conferring an additional competitive advantage over later-emerging weeds.Mechanized direct-seeded cultivation of rice is growing as a proportion of total area planted to rice around the world . Direct-seeded rice systems generally fall into the broad categories of dry-seeding or water-seeding. Dry-seeded rice can be broadcast onto dry soil by seed spreader or sown directly into soil by mechanical drill, whereas water-seeded rice can be broadcast into flooded fields by seed spreader or aircraft . Although direct-seeding of rice can significantly reduce time and labor investment in the planting stage, the various establishment methods described generally ensure that rice and weeds germinate and emerge synchronously , and weed management thus becomes largely dependent on chemical interventions . Herbicides can further reduce time and labor costs for weed management, however injudicious application techniques or over reliance on very few key active ingredients often result in the development of herbicide-resistant weed biotypes . As mechanization continues to be adopted in the rice sphere, integrated approaches to weed management are becoming more and more crucial. Rice grown in the Central Valley of California, USA comprises approximately 200 000 ha of irrigated fields. The region, which is among the highest-yielding in the world for rice production , is characterized by hot, dry, summers with abundant sunshine, and supports a single crop per year . California rice growers predominantly use a WS system, wherein pre-germinated seed is sown by aircraft into flooded fields. Rice seed sink to the soil surface and root down, emerging from the water after several days.