Rice, the main food crop in China, has been sporadically reported to suffer from weedy rice infestation.  However, the overall occurrence and distribution pattern of Chinese weedy rice remains unclear because a systematic survey has not been conducted.  In order to reveal the infestation of Chinese weedy rice, a field survey was conducted in 999 sampling sites all over the rice-growing regions in China from 2009 to 2016 using seven-scale visual scoring of the level of weed infestation.  Weedy rice was found 39% occurrence incidence in a total of 387 sites.  The sampling sites with 50% or higher overall weedy rice infestation index mainly radiated from Jiangsu, Heilongjiang, Ningxia and Guangdong to the whole East China, Northeast China, Northwest China and South China.  A total of 45 morphological characters from 287 populations (collected simultaneously with the field survey) out of those occurred sites were observed and analyzed using multivariate analysis in common gardens with the same cultivation conditions in 2017 and 2019.  Canonical correlation analysis showed that 45 morphological characters were significantly related to the latitude, mean temperature, minimum temperature, precipitation and mean diurnal range factors.  The 287 weedy rice populations were divided into three morphological groups with climate-dependent geographical differentiation: strong tiller type only in Jiangsu, large leaf type in South China and Central China and large grain type mainly in North China.  Weedy rice seriously infested rice fields and had a geography, climate and cultivated rice type-dependent morphological and biotype differentiation in China.  It is suggested to pay attention to the harmfulness of weedy rice and adopt comprehensive control strategies.

Nitrogen (N), the major forms of which are nitrate (NO₃^–) and ammonium (NH₄⁺), plays an important role in plant growth and mediation of root development.  However, the role of auxin in root growth in response to different NH₄⁺/NO₃^– ratios remains unclear.  Two tobacco cultivars (Nicotiana tabacum L.) were adopted in this study, which displayed variant growth features under the situations with sole NO₃^– nutrition ratio (NH₄⁺/NO₃^– ratio: 0/100), low NO₃^– nutrition ratio (NH₄⁺/NO₃^– ratio: 97/3), and optimal NH₄⁺/NO₃^– ratio (50/50).  We investigated the effects of the different NH₄⁺/NO₃^– ratios on the formation and elongation of lateral roots (LRs), auxin concentration, DR5::GUS expression, 3H-labeled indole acetic acid ([3H]IAA) transport, and the expression of six PIN genes in tobacco roots.  We also examined the effects of exogenous auxin and a transport inhibitor on LRs growth.  The results are shown as follows, compared to optimal N nutrition conditions, the biomass and nitrogen (N) accumulation were largely reduced by sole and low NO₃^– nutrition treatment in NC89, but no difference was observed in Zhongyan 100.  In most cases, sole and low NO₃^– nutrition impaired the elongation and formation of first-order lateral roots (1° LRs), only in NC89, thus reducing the root growth.  IAA concentration and DR5::GUS expression levels decreased in roots when NC89 was subjected to sole and low NO₃^– nutrition media, suggesting that different NH₄⁺/NO₃^– ratios affect the transport of auxin from leaves to roots.  Results were similar following exogenous NAA application to low NO₃^– nutrition treated seedlings.  Based on direct [3H]IAA transport measurement, the transport of polar auxin from shoots to roots decreased due to low NO₃^– nutrition.  PIN4 expression levels were markedly decreased in roots of NC89 by sole and low NO₃^– nutrition, while they were unaffected in Zhongyan 100 roots.  Overall, our findings suggest that LRs formation in tobacco seedlings is regulated by NH₄⁺/NO₃^– ratios via modifying polar transport of auxin.