Summary Photosynthesis is currently a focus for crop improvement. The majority of this work has taken place and been assessed in leaves, and limited consideration has been given to the contribution that other green tissues make to whole‐plant carbon assimilation. The major focus of this review is to evaluate the impact of non‐foliar photosynthesis on carbon‐use efficiency and total assimilation. Here we appraise and summarize past and current literature on the substantial contribution of different photosynthetically active organs and tissues to productivity in a variety of different plant types, with an emphasis on fruit and cereal crops. Previous studies provide evidence that non‐leaf photosynthesis could be an unexploited potential target for crop improvement. We also briefly examine the role of stomata in non‐foliar tissues, gas exchange, maintenance of optimal temperatures and thus photosynthesis. In the final section, we discuss possible opportunities to manipulate these processes and provide evidence that Triticum aestivum (wheat) plants genetically manipulated to increase leaf photosynthesis also displayed higher rates of ear assimilation, which translated to increased grain yield. By understanding these processes, we can start to provide insights into manipulating non‐foliar photosynthesis and stomatal behaviour to identify novel targets for exploitation in continuing breeding programmes.

Highlights Stomatal conductance is a major determinant of crop yield. The speed of stomatal response to changing environmental conditions greatly impacts photosynthesis and water use. Existing natural variation in the magnitude and rapidity of stomatal conductance is a potential target for future breeding. Genetic manipulation of stomatal conductance has the potential to improve crop performance.

Raising crop yield potential is a major goal to ensure food security for the growing global population. Photosynthesis is the primary determinant of crop productivity and any gain in photosynthetic CO 2 assimilation per unit of leaf area (A) has the potential to increase yield. Significant intraspecific variation in A is known to exist in various autotrophic organs that represent an unexploited target for crop improvement. However, the large number of factors that influence photosynthetic rates often makes it difficult to measure or estimate A under dynamic field conditions (i.e. fluctuating light intensities or temperatures). This complexity often results in photosynthetic capacity, rather than realized photosynthetic rates being used to assess natural variation in photosynthesis. Here we review the work on natural variation in A, the different factors determining A and their interaction in yield formation. A series of drawbacks and perspectives are presented for the most common analyses generally used to estimate A. The different yield components and their determination based on different photosynthetic organs are discussed with a major focus on potential exploitation of various traits for crop improvement. To conclude, an example of different possibilities to increase yield in wheat through enhancing A is illustrated. -Greenhouse conditions Ouyang et al. (2017) Rice 0.15-0.31 0.05-0.21 Pot experiment

Oilseed rape (Brassica napus L.) yield is strongly decreased by water deficit, and crop-management solutions are urgently required considering the emerging difficulties in breeding for drought-tolerant varieties. Film-forming antitranspirants (polymers) are agrochemicals that, applied to the crop canopy, mechanically block the stomata and decrease canopy transpiration. In this study, the drought-protection efficacy of an adaxial-surface application at the flowering stage of two film-forming treatments (poly-1-p-menthene and di-1-p-menthene) was investigated in pot-grown, droughted oilseed rape over two glasshouse experiments. Over the drought period, the two compounds reduced leaf stomatal conductance (P < 0.001), and as the soil moisture deficit increased, they sustained carbon assimilation and improved water-use efficiency with differing efficacy. Following the antitranspirant treatments, ABA concentration in leaves and reproductive organs was severely reduced and this was accompanied by significant improvements in leaf and flower–pod water potential. Drought significantly decreased the seed dry matter production of oilseed rape plants, by 39% on average. The treatments significantly increased seed dry matter by 13% (poly-1-p-menthene) and 17% (di-1-p-menthene), on average, compared with the unsprayed droughted plants, as a result of a significant increase in number of pods per plant, by 11% and 13%, respectively. The results suggest that film-forming compounds may be a useful crop-management tool to avoid severe drought-induced yield losses in oilseed rape by improving water-use efficiency and plant water status, thus alleviating ABA signalling under water deficit.

Stomata are the primary gatekeepers for CO 2 uptake for photosynthesis and water loss via transpiration and therefore play a central role in crop performance. Although stomatal conductance ( g s ) and assimilation rate ( A ) are often highly correlated, studies have demonstrated an uncoupling between A and g s that can result in sub-optimal physiological processes in dynamic light environments. Wheat ( Triticum aestivum L.) is exposed to changes in irradiance due to leaf self-shading, moving clouds and shifting sun angle to which both A and g s respond. However, stomatal responses are generally an order of magnitude slower than photosynthetic responses, leading to non-synchronized A and g s responses that impact CO 2 uptake and water use efficiency ( i WUE ). Here we phenotyped a panel of eight wheat cultivars (estimated to capture 80% of the single nucleotide polymorphism variation in North–West European bread wheat) for differences in the speed of stomatal responses (to changes in light intensity) and photosynthetic performance at different stages of development. The impact of water stress and elevated [CO 2 ] on stomatal kinetics was also examined in a selected cultivar. Significant genotypic variation was reported for the time constant for stomatal opening ( K i , P = 0.038) and the time to reach 95% steady state A ( P = 0.045). Slow g s opening responses limited A by ∼10% and slow closure reduced i WUE , with these impacts found to be greatest in cultivars Soissons, Alchemy and Xi19. A decrease in stomatal rapidity (and thus an increase in the limitation of photosynthesis) ( P < 0.001) was found during the post-anthesis stage compared to the early booting stage. Reduced water availability triggered stomatal closure and asymmetric stomatal opening and closing responses, while elevated atmospheric [CO 2 ] conditions reduced the time for stomatal opening during a low to high light transition, thus suggesting a major environmental effect on dynamic stomatal kinetics. We discuss these findings in terms of exploiting various traits to develop ideotypes for specific environments, and suggest that intraspecific variation in the rapidity of stomatal responses could provide a potential unexploited breeding target to optimize the physiological responses of wheat to dynamic field conditions.

The effects of a short (30 min) heat shock (HS) on plants subsequently grown under a salinity stress (SS, 200 mM NaCl) for 10 d were investigated in barley (Hordeum vulgare L.) cv. Tokak 157/37. The maximum temperature for HS allowing plant survival was 45 °C. The root length was significantly decreased by SS, whereas HS alone did not affect root growth. Interestingly, HS stimulated root elongation under SS. An osmotic adjustment was promoted in leaves by SS. On the contrary, HS increased the osmotic potential in leaves in the absence of SS, and partly counteracted the effect of SS in the HS+SS treatment. Cu/Zn-SOD, HvAPX, HvCAT2, HSP17, HSP18, and HSP90 were transcribed in leaves of HS-treated plants, but not in control plants. The HSP70 was constitutively transcribed in both the SS and control plants, but after HS, a shorter amplicon was also observed. The genes coding antioxidants, Cu/Zn-SOD, HvCAT2 and HvAPX, were differentially influenced by SS or HS+SS in the roots and leaves. In the roots, the mRNA content of BAS1, HvDRF1, HvMT2, and HvNHX1 increased after the HS treatment. In a recovery experiment in which plants were grown to maturity after HS and HS+SS stress exposure, the plant height increased and the time to maturity was reduced in comparison with SS. Our results show that HS could stimulate plant growth and reduce some of the negative effects of SS, and that it affected the transcription of several stress-related genes.

The productivity of many important crops is significantly threatened by water shortage, and the elevated atmospheric CO can significantly interact with physiological processes and crop responses to drought. We examined the effects of three different CO concentrations (historical ~300 ppm, ambient ~400 ppm and elevated ~700 ppm) on physiological traits of oilseed rape (Brassica napus L.) seedlings subjected to well-watered and reduced water availability. Our data show (1) that, as expected, increasing CO level positively modulates leaf photosynthetic traits, leaf water-use efficiency and growth under non-stressed conditions, although a pronounced acclimation of photosynthesis to elevated CO occurred; (2) that the predicted elevated CO concentration does not reduce total evapotranspiration under drought when compared with present (400 ppm) and historical (300 ppm) concentrations because of a larger leaf area that does not buffer transpiration; and (3) that accordingly, the physiological traits analysed decreased similarly under stress for all CO concentrations. Our data support the hypothesis that increasing CO concentrations may not significantly counteract the negative effect of increasing drought intensity on Brassica napus performance.

Stomata control CO2 uptake for photosynthesis and water loss through transpiration, thus playing a key role in leaf thermoregulation, water-use efficiency (iWUE) and plant productivity. In this work, we investigated the relationship between several leaf traits and hypothesized that stomatal behavior to fast (i.e. minutes) environmental changes co-determines along with steady-state traits the physiological response of grapevine to the surrounding fluctuating environment over the growing season. No relationship between iWUE, heat stress (HS) tolerance and stomatal traits was observed in field grown grapevine, suggesting that other physiological mechanisms are involved in determining leaf evaporative cooling capacity and the seasonal ratio of CO2 uptake (A) to stomatal conductance (gs). Indeed, cultivars that in the field had an unexpected combination of high iWUE but low sensitivity to thermal stress, displayed a quick stomatal closure to light, but a sluggish closure to increased vapor pressure deficit (VPD) levels. This strategy aiming both at conserving water under a high-to-low light transition and in prioritizing evaporative cooling under a low-to-high VPD transition, was mainly observed in Regina and Syrah. Moreover, cultivars with different known responses to soil moisture deficit or high air VPD (isohydric vs anisohydric) had opposite behavior under fluctuating environments, with the isohydric cultivar showing slow stomatal closure to reduced light intensity but quick temporal responses to VPD manipulation. We propose that stomatal behavior to fast environmental fluctuations can play a critical role on leaf thermoregulation and water conservation under natural field conditions in grapevine.