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منابع طبیعی - سازگاری گیاهان در بیابان
منابع طبیعی
یکشنبه 20 تیر 1389

سازگاری گیاهان در بیابان

یکشنبه 20 تیر 1389

نوع مطلب :
نویسنده :مهدی صامتی

گیاهان بر خلاف جانوران برای گریز از مخاطرات طبیعی از جمله خشكی، تشعشع خورشید و گرمای زیاد ، بادهای سوزناك سرد، یخبندان و امثال آن  قادر به حركت نیستند و نمی توانند جابجا شوند لذا برای مقابله با این گونه مسایل سازگاریهای خاصی پیدا كرده اند. مهمترین این مكانیزمها به قرار زیر است :

- در گیاهان بیابانی غالباً برگها به رنگ روشن دیده می شوند این سازگاری برای جلوگیری از شدت تعرق و انعكاس بیشتر تشعشات خورشید است .

- برای جلوگیری از تعرق بیشتر و حفظ آب موجود برگها كوچك یا فلسی شكل و گاهی بكلی از بین رفته اند و عمل جذب كلروفیل به وسیله ساقه انجام می شود .

- بسیاری از درختان بیابانی برای كم كردن گستره خود در مقابل تشعشع خورشید دارای تاجی سرتخت و چتری شکل هستند .

- دارای اپیدرم ضخیم ، روزنه های مخفی یا فرو رفته ، پوشش واكسی یا حفاظتی مانند كرک و امثال آن هستند

-سیستم ریشه ای وسیع ، گسترده و عمیق است بطوریكه بتوانند از رطوبت خاك ناشی از بارندگی ناچیز بیابان و همچنین رطوبت اعماق خاك استفاده كنند

- در گیاهان بیابانی بافتهای ذخیره آب در اندامهای هوایی یا ریشه دیده می شود. اینگونه گیاهان مهمترین تیپ رستنیهای بیایانی به حساب می آیند .

- قدرت تحمل آب كشیدگی بافتها در برخی از گونه های گیاهی مانند گونه های Zygophyllum که مقادیر زیادی از آب خود را در ایام گرم سال از دست می دهند بسیار زیاد است مثلاً در Zygophyllum qatarens  مقدار آب بافتهای گیاه به طور معمول 80 درصد است در حالی كه این مقدار در خشك ترین و گرم ترین ماه سال به 20 درصد می رسد یعنی گیاه 60 در صد آب خود را از دست می دهد.

- نسبت وزن خشك ریشه به وزن خشك ساقه در اغلب رستنی های بیابانی چندین برابر بیش  از بیوم های دیگر است .

- سازش با شرایط محیط از طریق توقف فتوسنتز درساعات گرم روز و تشدید عمل فتوسنتز در ساعات نسبتاً خنك روز نوع دیگری از سازگاری این گیاهان با بیابان است.  مثلاً در درخت چوج Salvadora persica  عمل جذب كلروفیل در ایام گرم سال بین ساعت 7 تا 9 صبح و 4 تا 7 بعد از ظهر انجام می گیرد و حدوداً 5 ساعت توقف جذب كلروفیلی توام با بسته شدن روزنه ها در طول روز دارد. در این حالت حرارت برگ بالا می رود كه بسیاری از گیاهان اكوسیستم های دیگر قادر به تحمل چنین حرارتی نمی باشند . در عین حال بعضی از گیاهان بیابانی مانندCapparis spinosa  قسمتی از روزنه های خود را در تمام طول روز باز نگه می دارند و لذا تعرق در حد لزوم برای خنك كردن برگها انجام می گیرد ولی جذب كلروفیل در ساعات گرم روز  قطع می شود .

- در مناطق بیابانی گاهی خشكی فیزیولوژیك جانشین خشكی فیزیكی می گردد یعنی محیط از نظر فیزیكی مرطوب است ولی شوری ناشی از املاح به خصوص سدیم گیاه را در معرض خشكی فیزیولوژیك قرار  می دهد و تیپی از رستنی ها به وجود می آید كه به نام هالوفیت مشهورند . در بین هالوفیتها ممكن است  هم گیاهان تیپ گوشتی و هم غیر گوشتی وجود داشته باشد . هالوفیتها قادرند فشارهای اسمزی خیلی شدید خاك را تحمل كنند .

- مطالعه فنولوژی گیاه به خصوص برگ در تشخیص نوع اكوسیستم های بیابانی كمك فراوانی می كند . از این طریق می توان به تغییرات تولید  اكوسیستم بر حسب فصل پی برد در این رابطه رستنی ها دارای انواع مختلف می باشند برخی از گیاهان در طول گرم ترین و خشكترین ماه سال برگهای خود را به كلی از دست داده و لخت می شود و پس از بارندگی گیاه پوشیده از برگهای توسعه یافته مجدد می گردد و یا این كه گیاه در طول گرمترین و خشك ترین ماه سال مقدار زیادی از برگهای خود را از دست می دهد ولی هرگز كاملاً لخت نمی شود .  




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