| Young cotton leaves respond differently to the environment than more mature leaves. It has been shown that young leaves (1-5 days old) when measured early in the growing season (but not in the late season) were as much as 15°C warmer (during periods of peak irradiance mid day) than more mature leaves (∼13 days old) as a consequence of lower stomatal conductance and less developed stomata. Warmer, young leaves also had higher photosynthetic thermal optima than the cooler, more mature leaves as determined by gas exchange and chlorophyll a fluorescence. We hypothesized that changes in thermal optima during leaf expansion were caused by alterations in fatty acid saturation of total lipids plus the key thylakoid glycolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) along with the phospholipids, phosphatidylglycerol (PG) and phosphatidylcholine (PC). The results generally supported the hypothesis: total leaf lipids showed decreases in saturated 16:0 and increases in trienoic 18:3 fatty acids as the leaves expanded. MGDG, DGDG, and PG also showed a similar pattern of decreasing saturation as the leaves expanded and cooled. We plan to further investigate the impact of fatty acid saturation levels on thylakoid membranes by directly measuring fluidity using a spin label and EPR spectrometry. We also intend to study the expression of fatty acid desaturase (FAD) genes during leaf expansion to determine whether alterations in membrane lipid composition are regulated at the transcriptional level. Our results may be important in understanding how young cotton seedlings survive the crucial weeks after planting, and also relevant to how plants will respond to expected rising global temperatures and increased desertification. |
