Quality And Quantity Of Rice Yield And Their Physiological Mechanism Under Non-flooded Mulching Cultivation

Global agriculture in the 21st century faces two major challenges. Total food production needs to increase to feed a still growing world population, and this increase needs to be accomplished under increasing scarcity of water resources. Plastic film or straw mulching cultivation under non-flooded condition has been considered as a new water-saving technique in rice (Oryza sativa L.) production. This study aimed to investigate the yield performance in terms of quality and quantity, water use efficiency (WUE) and their physiological mechanism under such practices. A field experiment was conducted with three high-yielding rice cultivars, Zhendao 88 ( a japonica cultivar), Yangdao 6 (an indica cultivar) and Shanyou 63 (an indica hybrid cultivar) and four cultivation treatments imposed from transplanting to maturity: traditional flooding as control (TF), non-flooded plastic film mulching (PM), non-flooded wheat straw mulching (SM), and non-flooded no mulching (NM). Grain yield and quality, WUE and physiological parameters, such as plant hormones, activities of key enzymes involved in sucrose-starch conversion, were determined. The main results are as follows:1. Compared with that under TF, grain yield showed some reduction under all the non-flooded cultivations, but differed largely among the treatments. The reduction was 37%-49%, 8.5%-17%, and 0.3%-0.7%, respectively, for NM, PM, and SM. The difference in grain yield was significant between NM and TF or between PM and TF, and was not significant between SM and TF. The yield reduction under NM was attributed to decreases in panicle number, spikelets per panicle, percentage of filled grains, and grain weight. The reduction in the total number of spikelets and grain weight contributed to the decrease in grain yield under PM. SM significantly reduced number of spikelets per panicle but significantly increased percentage of filled grains and grain weight. Both NM and PM significantly reduced, whereas SM significantly increased the maximum and mean grain filling rate, indicating that an increase in grain weight under SM is associated with the enhancement in grain filling rate, and decreases in grain filling rates under NM and PM result in the reduction in grain weight.2. Water application during the growing season across the study years, on average, was 206 mm to the PM, 233 mm to the SM, and 258 mm to the NM, which was only 22%, 24%, and 27%, respectively, of that (953 mm) applied to the TF. Compared with that under the TF, irrigation water use efficiency (grain yield per unit irrigation water) was markedly increased under all the non-flooded treatments, and by 267%-367% under the PM, 278%-321% under the SM, and 98%-138% under the NM treatments. The results suggest that SM could maintain a high grain yield and markedly increase WUE.3. SM significantly improved milling, appearance, and cooking qualities, whereas PM or NM decreased these qualities. SM also significantly increased the peak viscosity and breakdown value, and reduced hot viscosity, final viscosity, and setback value, and PM or NM had the opposite effect. Such results suggest that SM could improve the quality of rice.4. Compared with TF, SM significantly increased root oxidation activity, photosynthetic rate, and activities of sucrose synthase (SuSase), adenosine diphosphate glucose pyrophosphorylase (AGPase), starch synthase (StSase) and starch branching enzyme (SBE) in grains during the grain filling period, whereas NM and PM significantly reduced these parameters.5. Contents of indole-3-acetic acid (IAA), gibberellins (GA1 + GA4) and zeatin (Z) + zeatin riboside (ZR) in grains differed little among the treatments at the early grain filing stage, and showed SM > TF > PM > NM at the mid and late filling stages. Ethylene evolution rate of grains exhibited NM > PM > TF > SM. Abscisic acid (ABA) contences in grains were increased under both PM and SM, but showed no significant differences among TF, PM and SM treatments. NM significantly increased ABA in grains. Contents of IAA and GA1 + GA4 at the mid and late filling stages, ABA contents at the early and mid filling stage, and Z + ZR contents during the whole grain filling period significantly or very significantly and positively correlated with grain weight, brown rice, milled rice and breakdown value (r = 0.71* to 0.96**), and negatively correlated with setback value (r = -0.76* to -0.91**). Contents of IAA and Z + ZR at the late filling stage significantly or very significantly and negatively correlated with the percentage of chalky kernels and chalkiness (r = -0.73* to -0.85**). The ethylene evolution rates at each filling stage significantly or very significantly and negatively correlated with grain weight, brown rice, milled rice, alkali spreading value and breakdown value (r = -0.71* to -0.96**), and positively correlated with the percentage of chalky kernels, chalkiness and setback value (r = 0.73* to 0.93**).6. SM increased the ratio of ABA to 1-aminocylopropane -1-carboxylic acid (ACC) in the grains, and both NM and PM reduced it (ABA/ACC), implying that there is a greater enhancement of ABA concentration than ethylene production in the SM treatment, whereas the increase in ethylene outweighs the increase in ABA under PM and NM treatments. The content of ABA correlated with grain filling rate with a hyperbolic curve, whereas the ethylene evolution rate correlated with grain filling rate with an exponential decay equation. The ratio of ABA to ACC significantly correlated with grain filling rate with a linear relationship. Application of amino-ethoxyvinylglycine (AVG, inhibitor of ethylene synthesis by inhibiting ACC synthase) or ABA to panicles under TF and PM at the early grain filling stage significantly increased activities of the key enzymes involved in sucrose to starch conversion in the grains, SuSase, AGPase and soluble StSase, grain filling rate and grain weight. Application of ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of non-flooded mulching cultivation on grain filling.Collectively, both non-flooded plastic mulching cultivation (PM) and non-flooded wheat straw mulching cultivation (SM) could significantly increase WUE. SM not only maintained a high grain yield, but also improved quality of rice. Under the PM, quality of rice was decreased and grain yield differed with cultivars. Increases in photosynthetic rate, root activity, and activities of the key enzymes involved in the sucrose-starch metabolic pathway in grains, and the balance between hormones especially increase in the ratio of ABA to ethylene (ABA/ACC) under SM contributed to the high grain yield and the improvement in rice quality. SM would be a better practice than PM in areas where water is scarce but temperature is favorable to rice growth, such as the case in Southeast China.