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£loo per tonne to as much as£3000 per tonne[87,88].Therefore,future options that maximize the ability of the crop to take full benefit of the
<br /> e[CO2],or maintaining higher yields when CO2 costs are unmanageable become more important.
<br /> Araus et al[89],noted that canopy photosynthesis holds a crucial place in a context of yield gains through photosynthetic improvement,which
<br /> requires additional factors including the availability and uptake of nutrients,such as nitrogen,irrigation,the transport of photoassimilates and
<br /> sink-source balance.As such,in addition to improving photosynthetic rates via CO2 supplementation,the improvement of other plant processes
<br /> such as N uptake,non-foliar photosynthesis,stomatal function,and rubisco(activase)thermotolerance so that crops are better adapted for growth
<br /> in[CO2]enriched environments such as greenhouses are discussed below(Figure 3).These works will also need to account for changes to the
<br /> landscape of greenhouse crop cultivation,such as a move to vertical farming,changes in growth medium from soil to substrates such as coir
<br /> (derived from coconut husks)or rockwool[9o].It is estimated that more than 5o%of strawberry production occurs in substrate rather than soil
<br /> [91].Coir is often used as it has been shown to retains water more efficiently than soil,so strawberry plants require less frequent watering
<br /> improving water use efficiency.Coir also has a high level of aeration,which is ideal for strawberries'whose root systems require a lot of oxygen.
<br /> More recent developments in hydroponics[92]and aeroponics[93],will impact on irrigation,fertiliser regimes and N uptake.
<br /> Manipulation of electron
<br /> Elevation in[CO>] tronspon/�
<br /> Ambient
<br /> ICOM 420 +
<br /> ppm ' Manipulation of stomata
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<br /> Increased nitrogen
<br /> assimilation
<br /> Photosynthetic pigment
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<br /> As ICO,)surpasses 550
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<br /> enhanced RuBP regeneration
<br /> i Is increased
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<br /> Increased carbon assimaillionyield possible?
<br /> and biomass and fruit yield Genetic engineering approaches I}
<br /> observed in trading pops to reprogram plants for growth in
<br /> elevated(co(
<br /> Increased Increased fertilizer uptake. I
<br /> fertilizer Optimisation of fertilisation
<br /> Figure 3 Effects of elevated[CO2]on yield of fruiting crops and a representation of the potential for the manipulation of plant material for further yield increases.Created with
<br /> BioRender.com
<br /> Nitrogen use efficiency(NUE)
<br /> With regards to fruit quality,this is a complex trait that may not be simply attributed to enhanced carbon assimilation.More research is needed to
<br /> link increased assimilate,with assimilate distribution and transport,NUE to better understand the sink-source relationship in any given crop,
<br /> which can vary significantly across varieties and crop types.NUE is determined by yield per unit of available N in the growth medium(i.e often coir
<br /> in greenhouse grown crops).Plants with higher NUE may allocate N toward both the photosynthetic complexes(i.e N is major component of
<br /> chlorophyll;total N allocated to Rubisco 18.2±6.2%[94];)and/or toward the development of additional sinks.The second definition of NUE could
<br /> be described as the efficiency with which N is applied to soils,(through artificial means in greenhouse crops),is taken up by plants and converted to
<br /> usable products(i.e.biomass,grain yield).This can be manipulated through breeding to identify new varieties with high NUE uptake from selected
<br /> growing mediums or through engineering nitrogen symbiosis(Figure 3).Recently,scientists reported the engineering nitrogen-fixation into non-
<br /> legume cereal crops by enabling them to interact with soil bacteria to convert N from the air into ammonia fertiliser[95].These works could firstly
<br /> reduce the reliance on commercial synthetic fertilisers and secondly provide alternate sources of N that along with improvements to carbon
<br /> assimilation,foliar or non-foliar,co-contribute to improving photosynthesis and yields in crops(Figure 3).
<br /> A recent review has identified a number of targets in the literature to improve N uptake,assimilation and remobilisation through genetic
<br /> manipulation(see[96]for review).One of these,the over-expression of the nitrate transporter(NRT2.3)was shown to increase nitrate
<br /> concentrations in tomato increasing biomass and fruit weight[97].More recently,the transcription factor DREBIC has been identified as a
<br /> regulator of NUE by controlling the expression of several important growth-related genes including the rubisco small subunit 3(RBCS3),nitrate
<br /> transporters(NRT1.1B,NRT2.4),nitrate reductase(NR2)and the flowering regulator(FTL3).Once over-expressed(OE),OsDREB1C increased the
<br /> abundance of photosynthetic pigments,plants were shown to have about one-third more chloroplasts,38%more rubisco and improved
<br /> photosynthesis and N uptake.The OE of OsDREB1C resulted in a>40%increase in grain yield in elite rice varieties and an_20%increase in wheat
<br /> yields,while in Arabidopsis,a significant increase in biomass[98].Many of these identified genes have potential for improving NUE in fruiting
<br /> crops grown in e[CO2].A recent report of a large grain rice cultivar,Akita 63,having a high yield due to an enlarged sink capacity without and
<br /> photosynthesis improvement.However,this work demonstrated that source capacity was strongly limiting the yield potential under high N
<br /> fertilization.These authors suggested that enhancing photosynthesis is an important step to further increase yield of current high-yielding
<br /> cultivars[99].This work can be extrapolated that engineering NUE and photosynthesis in plants grown at e[CO2]could provide a step-change in
<br /> yields in greenhouse cultivated crops.
<br /> Genetic variation in photosynthetic traits in crops and wild relatives
<br /> PDF
<br /> Methods of improving these traits including breeding,by exploiting the potential of crop wild relatives as a source of new traits,and/or the genetic
<br /> manipulation/genome editing of specific traits.There is already evidence that substantial genetic variation exists within wild relatives of fruiting "Bind
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