Biotic degradation at night, abiotic degradation at day: positive feedbacks on litter decomposition in drylands.

The arid and semi-arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO2 ) uptake, which depends largely on plant litter decomposition and the subsequent release of CO2 back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO2 efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO2 efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long-term trend of global carbon sequestration.

The role of tobacco Aquaporin1 in improving water use efficiency, hydraulic conductivity, and yield production under salt stress.

Tobacco (Nicotiana tabacum; C3) plants increase their water use efficiency (WUE) under abiotic stress and are suggested to show characteristics of C4 photosynthesis in stems, petioles, and transmitting tract cells. The tobacco stress-induced Aquaporin1 (NtAQP1) functions as both water and CO(2) channel. In tobacco plants, overexpression of NtAQP1 increases leaf net photosynthesis (A(N)), mesophyll CO(2) conductance, and stomatal conductance, whereas its silencing reduces root hydraulic conductivity (L(p)). Nevertheless, interaction between NtAQP1 leaf and root activities and its impact on plant WUE and productivity under normal and stress conditions have never been suggested. Thus, the aim of this study was to suggest a role for NtAQP1 in plant WUE, stress resistance, and productivity. Expressing NtAQP1 in tomato (Solanum lycopersicum) plants (TOM-NtAQP1) resulted in higher stomatal conductance, whole-plant transpiration, and A(N) under all conditions tested. In contrast to controls, where, under salt stress, L(p) decreased more than 3-fold, TOM-NtAQP1 plants, similar to maize (Zea mays; C4) plants, did not reduce L(p) dramatically (only by approximately 40%). Reciprocal grafting provided novel evidence for NtAQP1's role in preventing hydraulic failure and maintaining the whole-plant transpiration rate. Our results revealed independent, albeit closely related, NtAQP1 activities in roots and leaves. This dual activity, which increases the plant's water use and A(N) under optimal and stress conditions, resulted in improved WUE. Consequently, it contributed to the plant's stress resistance in terms of yield production under all tested conditions, as demonstrated in both tomato and Arabidopsis (Arabidopsis thaliana) plants constitutively expressing NtAQP1. The putative involvement of NtAQP1 in tobacco's C4-like photosynthesis characteristics is discussed.