Up or down changes in carbohydrate and lipid metabolism and protein handling pathways are also evident, although these are more difficult to interpret. Aquaporins, which affect membrane water permeability, were found to be upregulated in two studies . When drought-stressed seedlings are re-watered, most gene expression quickly returns to normal. In Pinus taeda, only 76 of the 2445 genes with altered expression during drought were still different after 48 h of recovery . Lorenz et al. found 11 genes upregulated in ‘recovered’ P. taeda seedlings relative to either drought-stressed or well-watered seedlings, including probable cell wall proteins, an aquaporin and a gene involved in vacuole function. These may reflect recovery or repair processes that occur once drought stress is removed.Pinus pinaster has at least eight dehydrin genes, based on expressed sequence tag analyses . Three of five were downregulated during drought, whereas the other two were upregulated . Most dehydrin induction occurred after 20 d of drought , which may be why a similar but shorter study did not reveal the upregulation of dehydrins. Expression can also vary by tissue. Of seven dehydrins examined in P. abies, drought stress upregulated four in needles, but only two in bark, with one being down regulated in bark . To investigate the link between drought and defense gene expression, Fossdal et al. exposed P. abies seedlings to a pathogen , drought stress or both,grow strawberry in containers and examined the transcription of 14 candidate defense genes. Genes were upregulated more slowly in drought-stressed seedlings than in pathogeninoculated seedlings.
The combined treatment led to more rapid and/or higher expression of many defense genes than either alone, which may be related to the synergistic mortality risks posed by biotic and abiotic stressors. Pleiotropic effects for some drought/ defense-related genes are also possible, but none have been identified to date.Multiple provenance studies have identified patterns consistent with local adaptation to drought. Trees from drier climates often exhibit conservative growth strategies , such as slower height or needle growth , less above ground biomass or a shorter growing season . Seedlings from dry environments often also exhibit more root growth and higher drought survival . Provenance trials of Pinus halepensis have shown mixed responses, with low growth and high water use efficiency in dry-sourced populations , but high growth in populations from intermediate-aridity areas , which may be related to growth plasticity. Because of the importance of carbon resources for plants, WUE – the ratio of carbon fixed to water lost – has long been considered to be closely tied to drought tolerance . Measures that integrate over longer time periods, such as the carbon isotope ratio d13C , are most frequently used to represent changes in WUE in trees. However, although different measures of WUE are often correlated , they are not interchangeable. For example, carbon discrimination is sensitive to chloroplast carbon concentrations and mesophyll conductance, whereas WUE itself is heavily influenced by evaporative demand, which does not directly affect D . In addition, nitrogen fertilization can increase WUE and decrease D, but does not affect gs or transpiration . Thus, WUE and D do not always co-vary, and caution is needed in the interpretation of d13C as a measure of WUE. Additional caution is warranted when using WUE as an indication of drought tolerance. High WUE may not be adaptive in some dry environments if the use of less water per unit carbon fixed does not result in slower depletion of soil water , or if plants with higher WUE grow faster and thus use more total water. Although a few studies have shown higher d13C for populations from dry sites , others have shown the opposite .
There was no difference between three populations of Pinus ponderosa seedlings from varying climates in d13C or instantaneousWUE; the drought adapted populations exhibited greater plasticity in water use . In P. halepensis, however, individuals from more mesic sources showed higher plasticity of WUE than those from drier sources , but dry sources may show higher average WUE . Highly plastic growth and water usage reduce apparent WUE over the whole season compared with consistently moderate to low water usage . Instantaneous measures of WUE can change over a day, whereas integrated measures can differ significantly for a source population grown under different conditions or for the same tree across years . Changes in WUE may thus be a useful indication of drought stress, but, in conifers, radial growth and WUE are often weakly or negatively correlated . In pines, higher WUE usually results from reduced gs and/or reduced leaf area , which can limit photosynthesis and growth . Low gs can also result in higher tissue temperatures, which can be damaging, particularly in seedlings . Drought length and severity can influence measures of relative drought tolerance between populations. In P. ponderosa seedlings,the relative growth rate under moist conditions was positively correlated with previously measured tolerance to severe drought, whereas, under 4-wk drought, the intermediate-drought-tolerant population grew faster . When Silim et al. examined Picea sitchensis, P. glauca and their hybrids, they found that P. sitchensis and the hybrids had the highest WUE and growth in well-watered conditions, but P. glauca and the hybrids had higher WUE and growth in drought conditions. Similarly, the relative transpiration and photosynthetic rates, WUE and growth of P. halepensis tree provenances differed between near-desert and Mediterranean planting sites . Such shifts in ranking are often a result of plasticity differences between populations. Provenances of P. pinaster from across the species’ range did not vary in cavitation resistance, suggesting uniform selection or lack of genetic variation . In P. halepensis, however, the percentage loss of conductivity differed significantly between provenances, but not between environments .
Although plasticity has been observed in xylem wall thickening, time to thickening and number of cells in Picea mariana in drought experiments , cell anatomy studies often focus on only one population, so that the extent of local adaptation is unknown.Genome scans have identified loci in conifers that may be under differential selection across environments . Of 13 candidate genes for drought response in P. pinaster, two showed signs of divergent selection, although only one exhibited a pattern correlated with a climatic gradient; three, including two dehydrins, showed evidence of balancing selection . Prunier et al. examined SNPs from 313 candidate genes in P. mariana and found 16 that exhibited differentiation correlated with precipitation, including a LEA protein and genes in the ubiquitin protein handling pathway. However, differentiation between populations can be driven by processes unrelated to climatic gradients. Conifer populations are likely to violate the assumptions of such tests because they rarely form discrete isolated populations and are often far from demographic equilibrium; mis-specification of population hierarchical structure can lead to high false positive rates . However, newer methods are being developed that avoid frequently violated assumptions and reduce false positives . The approach preferred by recent studies is to directly assess the association of loci with environmental gradients ,hydroponic nft channel whilst controlling for population structure . JaramilloCorrea et al. examined the correlation of P. pinaster candidate gene SNP allele frequencies with climate principal component axes, using transcriptome-wide SNPs to control for population structure and demographic history. They identified 18 environmentally associated SNPs, many of which were in genes relating to carbohydrate transport, cell wall construction and photosynthesis. Two surveys of P. taeda examined associations between candidate gene SNPs and environmental gradients. One examined the association of these loci with five climatic PC axes , whereas the other used an aridity index for each county . There was some overlap in function between the loci identified. However, the studies disagreed on whether SNPs associated with climate also tended to be Fst outliers. G2E associations have been detected even over short geographical distances, suggesting that selection can drive local adaptation in the presence of high gene flow. Eckert et al. examined Pinus lambertiana populations around Lake Tahoe, an area of c. 35 9 65 km2 , and found 11 genes associated with environmental PCs reflecting differences in water availability. These included genes involved in carbohydrate metabolism and transport and response to biotic stress . A cross-species comparison of environmental associations suggests some similarities in the genetic mechanisms involved in climatic tolerances across conifer genera. For each of four European conifer species in the Italian Alps, 6–18 SNPs were associated with precipitation/temperature PC axes . There was some overlap between species in the genes represented, including heat shock proteins, and cell wall construction and carbohydrate metabolism genes .Gene expression studies have identified a range of genes that may be involved in drought responses, but these results are not easily connected to the results of physiological or provenance response studies.
First, RNA transcripts reflect the genes being expressed at a particular instant, whereas morphological or physiological traits are the result of processes acting over a longer time. Second, most gene expression studies do not examine differences between populations. Although some evidence suggests that stronger gene expression changes during stress are associated with greater growth or survival, different genotypes and demographic stages can show significant differences in gene expression changes . A few studies have begun to address this. Provenances of P. pinaster differed in the expression response of two dehydrin genes, as well as in physiology and mortality rates . Similarly, three genotypes of P. taeda differed in their gene expression responses to drought and re-watering . More such studies are needed, but care must be taken to distinguish between drivers of expression differences. For instance, a more drought-sensitive tree might express higher levels of dehydrins at a given drought stage because the leaf water potential has dropped faster than in a drought-resistant tree, whereas the resistant tree might express higher levels of dehydrins than the sensitive tree at a given leaf water potential. Genome scan and G2E association studies can be useful tools in the search for genes responsible for local adaptation. Although such studies can identify loci at which allele frequencies differ between environments, it is not always clear how these differences are connected to phenotypic differences, and thus what traits are under selection in a given environment. This is where QTL and G2P association studies are useful.Most conifer QTL studies have focused on wood traits, growth or yield. Of the two that have examined drought tolerance, the first identified four significant and four suggestive QTLs for d13C in P. pinaster, none of which co-located with QTLs for ring width . The second examined a wider range of traits – photosynthesis , chlorophyll fluorescence, gs, d13C, intrinsic WUE and specific leaf area – in F1 cross seedlings of P. pinaster when well watered or after 1 or 2 wk without water, and identified 28 significant and 27 suggestive QTLs . Locations of the QTLs for each trait varied by time point. Candidate genes within the QTLs were identified : those for gs and WUEi included stomatal regulation, ABA signaling and cell wall construction genes; those for d13C included an aquaporin; and those for chlorophyll fluorescence included transcription factors and a histone chaperone. G2P studies focusing on quantitative traits have generally been successful in identifying associated loci . However, only a few studies have investigated drought tolerance in conifers , with less success. All such studies used d13C as the focal trait. As we argue in Section VI, other traits would probably yield results that are more helpful for the understanding of drought responses. Gonzalez-Martinez et al. examined 41 candidate stress response genes of P. taeda, using 61 tree families planted at two sites. However, drought stress was probably mild, and they only identified one strongly associated gene and one weakly associated gene at each site. A later study on the same species examining 3938 SNPs identified seven new associations with d13C . Four of the associations were with unknown proteins, with only a transcription factor probably involved in the ABA-mediated stress response having an obvious connection to drought responses. G2P and G2E association studies complement one another, with the first identifying loci linked to targeted traits, but not whether these loci are under selection in nature, and the second doing the opposite.