Consistent alleviation of abiotic stress with silicon addition: a meta-analysis

17  Hundreds of single species studies have demonstrated the facility of silicon (Si) to alleviate diverse 18 abiotic stresses in plants. Understanding of the mechanisms of Si mediated stress alleviation is 19 progressing, and several reviews have brought information together. A quantitative assessment of the 20 alleviative capacity of Si, however, which could elucidate plant Si function more broadly, was lacking. 21  We combined the results of 145 experiments, predominantly on agricultural species, in a meta22 analysis to statistically assess the responses of stressed plants to Si supply across multiple plant 23 families and abiotic stresses. We interrogated our database to determine if: stressed plants increased 24 in dry mass and net assimilation rate, oxidative stress markers were reduced, anti-oxidant responses 25 were increased and if element uptake showed consistent changes when supplied with Si. 26  We demonstrated that across plant families and stress types, Si increases dry weight, assimilation rate 27 and chlorophyll biosynthesis and alleviates oxidative damage in stressed plants. In general, results 28 indicated that plant family (as a proxy for accumulator type) and stress type had significant 29 explanatory power for variation in responses. The consistent reduction in oxidative damage was not 30 mirrored by consistent increases in anti-oxidant production, indicative of the several different stress 31 alleviation mechanisms in which Si is involved. Silicon addition increased K in shoots, decreased As and 32 Cd in roots and Na and Cd in shoots. Silicon addition did not affect Al, Ca or Mn concentration in 33 shoots and roots of stress plants. Plants had significantly lower concentrations of Si accumulated in 34 shoots but not in roots when stressed. 35  Meta-analyses showed consistent alleviation by Si of oxidative damage caused by a range of abiotic 36 stresses across diverse species. Our findings indicate that Si is likely to be a useful fertilizer for many 37 crops facing a spectrum of abiotic stresses. Similarities in responses across families provides strong 38 support for a role of Si in the alleviation of abiotic stress in natural systems, where it has barely been 39 explored. We suggest this role may become more important under a changing climate and more 40 experiments using non-agricultural species are now needed. 41 42