As a major world crop, winter wheat has shown increased yields following advances
in breeding and crop management. On the semi-arid Loess Plateau of China, water stress is
one of the major limiting factors of yield for winter wheat. The improvements in grain
yield and water use efficiency (WUE) are two equally important goals in arid or semi-arid
areas. ChangWu 135, which developed for the semi-arid dryland agriculture area of the
Loess Plateau, was studied in the experiments. Pot experiments and field experiments were
conducted to assess whether seeding density in winter wheat affects grain yield and water
use efficiency when combined with root pruning by changes in the accumulation,
remobilization of photosynthate and water consumption.
Two experiments were conducted in our research:
(1) Pot experiment and field experiment were conducted in 2007/08. Pot experiment
comprised four treatments: (1) LN, low density (12 seedlings to a pot, 8 cm in diameter ×
50 cm high) and no root pruning; (2) LP, low density with root pruning; (3) HN, high
density (15 seedlings to a pot, 25% higher than low density) and no root pruning; and (4)
HP, high density with root pruning. Two water regimes were employed from stem
elongation: (i) plants maintained at 85% field capacity, and (ii) plants maintained at 55%
field capacity by regular watering. At stem elongation roots were pruned 40~42% on 2
March. And roots were cut back to keep their length to 13 cm and to limit their lateral
spread to 3 cm from the plant.
(2) Field experiment comprised four treatments: (1) LN: plants at low density (225 seeds
m-2, the recommended density in the region), roots not pruned; (2) LP: plants at low
density, roots pruned; (3) HN: plants at high density (280 seeds m-2, 25% higher than low
density), roots not pruned; and (4) HP: plants at high density, roots pruned. At stem
elongation roots were pruned by 40~42% on 15 March. And roots were cut back to keep their length to 13 cm and to limit their lateral spread to 3 cm from the plant.
Experiment 2: Field experiments conducted in 2007/08 and 2008/09 comprised. The
seeding rates were: SR1 (225 seeds m-2 the optimal seeding rate for the region),; SR2, 280
seeds m-2; and SR3, 340 seeds m-2. Two root pruning treatments were: W (roots pruned on
15 November just at the beginning of over-wintering period) and S (root pruned on 15
March at the beginning of the spring-growth stage); un-pruned plants (CK) served as the
control. Roots were pruned by 40~42%.
The main results are as follows:
1. The effects of grain yield by seeding rate and root pruning. In the pot study, the yield
reduction with water shortage. Grain yield increased significant with seeding rate
increased under the well watered condition. Normal seeding rate had no effect on grain
yield, while higher seeding rate increased grain yield significantly when root pruning.
Field Experiment observed that grain yield decreased with the seeding rate increased in
severe drought years. However, the reverse result was observed favorable weather.
Similar results found in pot experiment.
2. The effects of water use efficiency by seeding rate and root pruning. In pot experiment,
availability of water significantly increased overall water use as did increasing the
planting density (P<0.01), but root pruning increased water use efficiency significantly.
In experiment 2, root pruning in winter (W) primarily led to a significant decrease in
water use from winter to stem elongation stage. While root pruning in spring
significant reduced water use after stem elongation stage, used less soil water than
winter pruned roots and the un-pruned controls, and had highest water use efficiency.
3. The effect of root biomass by seeding rate and root pruning. In experiment 1 ， root
biomass and R:S ratio significant increased with increasing seeding rate. The reserves
results found in root pruning treatment. In field experiment, root biomass in each layer
of soil were higher at the higher planting density at anthesis, while root pruning
significantly (P<0.05) reduced the root biomass in the upper 40 cm of soil, but
increased the root biomass and the proportion of root biomass in the 80-120 cm soil
4. The effect of population structure by seeding rate and root pruning. In experiment 2,
higher seeding rates led to higher LAI. However in 2008 seeding rate had no significant effect on LAI. The total number of tillers (fertile tillers: those with a head
and infertile tillers: those without a head) per unit area increased linearly with seeding
rate whereas the number of tillers per plant decreased, and the spiker density did not
affect by seeding rate. Two root pruning treatments increased the values of LAI in both
seasons and decreased tiller density and the number of tillers per plant (P<0.05), but
had little effect on the number of fertile tillers (spikes).
5. The effect of the exchange of gases by seeding rate and root pruning. Water shortage
and increasing the planting density significantly decreased flag leaf photosynthesis and
photosynthetic activity, while root pruning significantly increased the photosynthetic
capacity of the flag leaf in pot experiment. In experiment 2, higher seeding rates
decreased the rate of flag leaf photosynthesis (Pn) and photosynthetic activity ，
increased the rate of root respiration (Rr), leading to a significant decrease in root
efficiency (Pn : Rr). In contrast, leaf photosynthetic rate increased and root respiration
rate decreased in the root pruning treatments, leading to a higher Pn : Rr ratio. It
indicated that the reducing of root biomass decreased the carbon consumed, leading to
more photosynthetic product was used to increase grain yield.
6. The effect of seeding rate and root pruning on competition ability of winter wheat was
invested using de Wit replacement series. In experiment 1, the relative yield of
unpruned plant was significantly higher than 1 while root pruning wheat was lower
than 1 in mixture. The relative total yield of normal density was 1.06 when water was
plentiful, which was significant greater than 1, indicated that unpruned plant was a
superior competitor, the mixture of unpruned plant and root pruning in normal density
could increase the grain yield significantly under the well watered condition.
7. Nitrogen and phosphorus use efficiency were researched in field experiment. The
nitrogen and phosphorus in leaves decreased with the seeding rate increased. Two root
pruning treatments had higher nitrogen and phosphorus content in leaves than that of
unpruned treatment. It suggested that the higher photosynthetic rate of root pruned
plant may associate with the higher nitrogen and phosphorus content in leaves.
8. In experiment 2, both post-heading dry matter accumulation and translocation
significantly increased with seeding rate. In W and S, post-heading dry matter
accumulation significant higher than unprunned plants. Post-heading dry matter accumulation was very important for increasing grain yield, it suggested that the
greater grain yield of root pruning plants may associate with the higher post-heading
dry matter accumulation.
In presednt study, increasing seeding rate and root pruning treatments can increase
grain yield and water use efficiency in average-rainfall years on the semi-arid Loess
Plateau. The greater grain yield in higher seeding rate derived from higher spike density
and grain number per unit. While root pruning reduced the water consumption before
anthesis, more soil water used for grain filling, and increased water use efficiency.
Meanwhile, root pruning reduced the infertile tillers, improved the canopy structure,
decreased the useless carbon consumed, and increased the post-heading dry matter
accumulation, and resulted in higher grain yield. However, the competition of soil water in
winter wheat increased due to increasing seeding rate and led to decreased grain yield in
severe drought years. In conclusion, it is hopeful that improved grain yield and water use
efficiency by increasing seeding rate and root pruning in arid or semi-arid areas.
Key words: Winter wheat (Triticum aestivum), Grain yield, Water use efficiency,
Semi-arid Loess Plateau, Competition ability, Root efficiency