Phenotypic Differentiation between Weedy Rice Populations Associated with their Local Adaptation in Early-and Late-Season Rice Fields

Background: Temperatures and photoperiods can profoundly affect plant growth and development and play vital roles in the local adaptation of plant species. Weedy rice (Oryza sativa f. spontanea) is a conspecific weed of cultivated rice, and it was found in the same rice fields (sympatry) of early and late rice-cultivation seasons in Leizhou, Guangdong Province of China. Generally, the phenological conditions, such as temperature and photoperiod, are different in the two seasons. Therefore, the early- and late-season weedy rice populations in the same rice fields provide a perfect system for estimating sympatric divergence in plant species. The previous study had demonstrated considerable genetic divergence between the early- and late-season weedy rice populations. Here, we designed in situ common garden experiments to estimate the phenotypical differences between the two-season weedy rice populations and disclose the local adaptation in weedy rice populations associated with their ambient temperature and photoperiod.Results: Distinct air temperature and day length variation patterns were recognized between the early and late rice-cultivation seasons, based on the 10-year historical climate data. More stressful conditions, indicated by low air temperature and long day length, were found for weedy rice growth in the early seasons. Noticeably, significant differences in plant heights, the number of tillers, flowering time, and reproductive traits were detected between the two-season weedy rice populations according to the early-season common garden experiment. The early-season populations showed evident higher plant heights, more tillers, and earlier flowering time than the late-season populations. However, such differences were not detected in the late-season common garden experiment. In addition, evident local adaptation represented by the traits such as plant heights, flowering time, and reproductive traits was only detected in the early-season weedy rice populations. The principal component analysis also showed clear population clusters between the two-season populations using the phenotypical data.Conclusions: This study provided clear evidence of phenotypic differentiation between the sympatric early- and late-season weedy rice populations, probably associated with the local adaptation to their ambient temperature and photoperiod. Our findings also have potential roles in facilitating the design of strategies for effective weedy rice control practices.

Asymmetric Differences in the Effects of Average Air Temperature and Solar Radiation on Early Rice and Late Rice Yield

China is the world’s largest rice producer. Thus, the stability of rice production plays a decisive role in food security. Among the types of rice, double rice (including early rice and late rice) accounts for the largest proportion of rice in China. Climate change is widely expected to affect rice yields. Studying the response of double rice yield to climate change will benefit strategic decisions related to future crop adaptation. In this paper, the relationship between climate factors and the yield of double rice during 1992–2013 in south China was analysed to determine the responses of double rice yield to climate change. The results showed that the daily average air temperature during the early rice and late rice growing seasons increased by 0.34 °C and 0.68 °C, 0.29 °C and 0.67 °C, and 0.11 °C and 0.31 °C per 10-year period in the northern subtropical zone (NST), middle subtropical zone (MST) and south subtropical zone (SST), respectively, in the last 20 years. The change trend in solar radiation was not obvious, but it fluctuated greatly. A 1 °C increase in average air temperatures decreased early rice yield by 5.36% and 2.16% in SST and MST, respectively; decreased late rice yield by 0.75% and 1.43% in MST and NST, respectively; and increased late rice yield by 3.93% in SST. A solar radiation increases of 100 MJ m−2 increased early rice yield by 1.02%, 1.54% and 1.71% in SST, MST and NST, respectively, and decreased late rice yield by 0.89% in SST. We found that annual average temperatures of 17.3 °C and 18.6 °C were the early rice and late rice yield variation thresholds, respectively; in addition, above the background temperature in south China, the early rice yield will decrease and the late rice yield will increase.

Simulation of the critical nitrogen dilution curve in Jiangxi double-cropped rice region based on leaf dry matter weight

In order to investigate the feasibility of using rice critical nitrogen concentration as a nitrogen nutrition diagnosis index, a two-year positioning field gradient experiment using four rice varieties and four nitrogen levels (0, 75, 150, 225 kg·ha–1 for early rice; 0, 90, 180, 270 kg·ha–1 for late rice) was conducted for early and late rice. The critical dilution curves (Nc%) of the double-cropped rice based on leaf dry matter (LDM) were constructed and verified using the field data. Two critical nitrogen dilution curves and nitrogen nutrition indexes (NNI) of rice LDM were constructed for early rice [Nc% = 2.66LDM−0.79, R2 = 0.88, NNI ranged between 0.29–1.74, and the average normalized root mean square error (n-RMSE = 19.35%)] and late rice [Nc% = 7.46LDM−1.42, R2 = 0.91, NNI was between 0.55–1.53, and the average (n-RMSE = 15.14%)]. The relationship between NNI and relative yield was a quadratic polynomial equation and suggested that the optimum nitrogen application rate for early rice was sightly smaller than 150 kg·ha–1, and that for late rice was about 180 kg·ha-1. The developed critical nitrogen concentration dilution curves, based on leaf dry matter, were able to diagnose nitrogen nutrition in the double-cropped rice region.

The Effects of Returning Straw and Milk Vetch on Rice Growth and Greenhouse Gas Emissions

The effects of different nitrogen application levels on rice yield and greenhouse gas (NO2, CH4) emissions from rice fields under the combined action of straw and milk vetch. Two treatments were set up in the main area of this experiment: R0 (no straw returned to the field, 0); R1 (amount of straw returned to the field under normal conditions, 6000kg·hm-2). Before turning the straw back into the field, use a circular knife to cut the straw into 10~13cm.And 3 kinds of nitrogen application treatments in the sub-district: N1 (no nitrogen application, 0), N2 (nitrogen application, 15 kg·hm-2), N3 (nitrogen application, 30kg·hm-2), two-factor cross-combination,and a non-nitrogen control CK, total of 7 treatments. The results showed that in 2017, early rice R1N2 treatment increased the most obvious yield, which was 32.44% higher than CK, and late rice R1N1 treatment increased the most significantly, which was 17.91% higher than CK. CH4 emissions is positively correlated with the amount of straw returned to the field, while the amount of N2O is the opposite.The N2O emission flux was highest in the treatment of R1N3, and the CH4 emission flux was the highest in the treatment of R1N2.

Azolla incorporation under flooding reduces grain cadmium accumulation by decreasing soil redox potential

Cadmium (Cd) presents severe risks to human health and environments. The present study proposed a green option to reduce bioavailable Cd. Rice pot experiments were conducted under continuous flooding with three treatments (T1: intercropping azolla with rice; T2: incorporating azolla into soil before rice transplantation; CK: no azolla). The results showed that azolla incorporation reduced soluble Cd by 37% compared with the CK treatment, which may be explained by the decreased soil redox potential (Eh) (r = 0.867, P < 0.01). The higher relative abundance of Methylobacter observed in azolla incorporation treatment may account for dissolved organic carbon increase (r = 0.694; P < 0.05), and hence decreased the Cd availability for rice. Azolla incorporation increased the abundance of Nitrospira, indicating the potentially prominent role of nitrogen mineralization in increasing rice yields. Further, lower soluble Cd decreased the expression of OsNramp5, but increased OsHMA3 levels in rice roots, which decreased Cd accumulation in grains. Through these effects, azolla incorporation decreased Cd concentrations in rice grains by 80.3% and increased the production by 13.4%. The negligible amount of Cd absorbed by azolla would not increase the risk of long-term application. Thus, intercropping azolla with early rice and incorporating azolla into soil before late rice transplantation can contribute to safe production at large scales of double rice cultivation.

Cataloging of Cd Allocation in Late Rice Cultivars Grown in Polluted Gleysol: Implications for Selection of Cultivars with Minimal Risk to Human Health

Cadmium (Cd) is a toxic trace metal that has polluted 20% of agricultural land in China where its concentration exceeds the standards for Chinese farmland. Plants are capable of accumulating Cd and other trace metals, but this capacity varies with species and cultivars within a species. Rice is a staple food consumed by half of the global population. In order to select safe late rice cultivars that are suitable late rice cultivars that can be cultivated in for growing in slightly contaminated soil, a two-year field experiment was conducted with 27 in the first year and 9 late rice cultivars in the second year. The results showed that plant Cd concentrations varied among the cultivars, with high magnitudes of variation occurred in straw and grains. Five genotypes including LR-12, LR-17, LR-24, LR-25 and LR-26 were identified as low accumulators for the first year while LR-15 and LR-17 were identified as promising cultivars based on Cd concentration in the polished rice grains (<0.02 mg kg−1 DW). In addition, these cultivars had favorable traits, including mineral nutrition and grain yield. Therefore, these genotypes should be considered for cultivation in slightly or moderately Cd contaminated soils.